2023 |
Chung, A., Parrenin, F., Steinhage, D., Mulvaney, R., Martín, C., Cavitte, M., et al. (2023). Stagnant Ice And Age Modelling In The Dome C Region, Antarctica. Cryosphere, 171(8), 3461–3483.
Abstract: The European Beyond Epica Project Aims To Extract A Continuous Ice Core Of Up To 1.5 Ma, With A Maximum Age Density Of 20 Kyr M(-1) At Little Dome C (Ldc). We Present A 1D Numerical Model Which Calculates The Age Of The Ice Around Dome C. The Model Inverts For Basal Conditions And Accounts Either For Melting Or For A Layer Of Stagnant Ice Above The Bedrock. It Is Constrained By Internal Reflecting Horizons Traced In Radargrams And Dated Using The Epica Dome C (Edc) Ice Core Age Profile. We Used Three Different Radar Datasets Ranging From A 10 000 Km(2) Airborne Survey Down To 5 Km Long Ground-Based Radar Transects Over Ldc. We Find That Stagnant Ice Exists In Many Places, Including Above The Ldc Relief Where The New Beyond Epica Drill Site (Beldc) Is Located. The Modelled Thickness Of This Layer Of Stagnant Ice Roughly Corresponds To The Thickness Of The Basal Unit Observed In One Of The Radar Surveys And In The Autonomous Phase-Sensitive Radio-Echo Sounder (Apres) Dataset. At Beldc, The Modelled Stagnant Ice Thickness Is 198 +/- 44 M And The Modelled Oldest Age Of Ice Is 1.45 +/- 0.16 Ma At A Depth Of 2494 +/- 30 M. This Is Very Similar To All Sites Situated On The Ldc Relief, Including That Of The Million Year Ice Core Project Being Conducted By The Australian Antarctic Division. The Model Was Also Applied To Radar Data In The Area 10-15 Km North Of Edc (North Patch), Where We Find Either A Thin Layer Of Stagnant Ice (Generally <60 M) Or A Negligible Melt Rate (<0.1 Mm Yr(-1)). The Modelled Maximum Age At North Patch Is Over 2 Ma In Most Places, With Ice At 1.5 Ma Having A Resolution Of 9-12 Kyr M(-1), Making It An Exciting Prospect For A Future Oldest Ice Drill Site.
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Lecavalier, B., Tarasov, L., Balco, G., Spector, P., Hillenbrand, C., Buizert, C., et al. (2023). Antarctic Ice Sheet Paleo-Constraint Database. Earth System Science Data, 151(8), 3573–3596.
Abstract: We Present A Database Of Observational Constraints On Past Antarctic Ice Sheet Changes During The Last Glacial Cycle Intended To Consolidate The Observations That Represent Our Understanding Of Past Antarctic Changes And For State-Space Estimation And Paleo-Model Calibrations. The Database Is A Major Expansion Of The Initial Work Of Briggs And Tarasov (2013). It Includes New Data Types And Multi-Tier Data Quality Assessment. The Updated Constraint Database, Antice2 (Https://Theghub.Org/Resources/4884, Lecavalier Et Al., 2022), Consists Of Observations Of Past Grounded- And Floating-Ice-Sheet Extent, Past Ice Thickness, Past Relative Sea Level, Borehole Temperature Profiles, And Present-Day Bedrock Displacement Rates. In Addition To Paleo-Observations, The Present-Day Ice Sheet Geometry And Surface Ice Velocities Are Incorporated To Constrain The Present-Day Ice Sheet Configuration. The Method By Which The Data Are Curated Using Explicitly Defined Criteria Is Detailed. Moreover, The Observational Uncertainties Are Specified. The Methodology By Which The Constraint Database Can Be Applied To Evaluate A Given Ice Sheet Reconstruction Is Discussed. The Implementation Of The Antice2 Database For Antarctic Ice Sheet Model Calibrations Will Improve Antarctic Ice Sheet Predictions During Past Warm And Cold Periods And Yield More Robust Paleo-Model Spin Ups For Forecasting Future Ice Sheet Changes.
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2022 |
Ershadi, M. R., Drews, R., Martin, C., Eisen, O., Ritz, C., Corr, H., et al. (2022). Polarimetric Radar Reveals The Spatial Distribution Of Ice Fabric At domes and divides in East Antarctica. Cryosphere, 161(5), 1719–1739.
Abstract: Ice crystals are mechanically and dielectrically anisotropic. They progressively align under cumulative deformation, forming an ice-crystal-orientation fabric that, in turn, impacts ice deformation. However, almost all the observations of ice fabric are from ice core analysis, and its influence on the ice flow is unclear. Here, we present a non-linear inverse approach to process co- and cross-polarized phase-sensitive radar data. We estimate the continuous depth profile of georeferenced ice fabric orientation along with the reflection ratio and horizontal anisotropy of the ice column. Our method approximates the complete second-order orientation tensor and all the ice fabric eigenvalues. As a result, we infer the vertical ice fabric anisotropy, which is an essential factor to better understand ice deformation using anisotropic ice flow models. The approach is validated at two Antarctic ice core sites (EPICA (European Project for Ice Coring in Antarctica) Dome C and EPICA Dronning Maud Land) in contrasting flow regimes. Spatial variability in ice fabric characteristics in the dome-to-flank transition near Dome C is quantified with 20 more sites located along with a 36 km long cross-section. Local horizontal anisotropy increases under the dome summit and decreases away from the dome summit We suggest that this is a consequence of the nonlinear rheology of ice, also known as the Raymond effect. On larger spatial scales, horizontal anisotropy increases with increasing distance from the dome. At most of the sites, the main driver of ice fabric evolution is vertical compression, yet our data show that the horizontal distribution of the ice fabric is consistent with the present horizontal flow. This method uses polarimetric-radar data, which are suitable for profiling radar applications and are able to constrain ice fabric distribution on a spatial scale comparable to ice flow observations and models.
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Stokes, C., Abram, N., Bentley, M., Edwards, T., England, M., Foppert, A., et al. (2022). Response Of The East Antarctic Ice Sheet To Past And Future Climate Change. Nature, 6086(79227), 275–+.
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2021 |
Buizert, C., Fudge, T., Roberts, W., Steig, E., Sherriff-Tadano, S., Ritz, C., et al. (2021). Antarctic surface temperature and elevation during the Last Glacial Maximum. Science, 372(6546), 1097–+.
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Cavitte, M., Young, D., Mulvaney, R., Ritz, C., Greenbaum, J., Ng, G., et al. (2021). A detailed radiostratigraphic data set for the central East Antarctic Plateau spanning from the Holocene to the mid-Pleistocene. Earth System Science Data, 13(10), 4759–4777.
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Lilien, D., Steinhage, D., Taylor, D., Parrenin, F., Ritz, C., Mulvaney, R., et al. (2021). Brief communication: New radar constraints support presence of ice older than 1.5 Myr at Little Dome C. Cryosphere, 15(4), 1881–1888.
Abstract: The area near Dome C, East Antarctica, is thought to be one of the most promising targets for recovering a continuous ice-core record spanning more than a million years. The European Beyond EPICA consortium has selected Little Dome C (LDC), an area similar to 35 km southeast of Concordia Station, to attempt to recover such a record. Here, we present the results of the final ice-penetrating radar survey used to refine the exact drill site. These data were acquired during the 2019-2020 austral summer using a new, multi-channel high-resolution very high frequency (VHF) radar operating in the frequency range of 170-230 MHz. This new instrument is able to detect reflectors in the near-basal region, where previous surveys were largely unable to detect horizons. The radar stratigraphy is used to transfer the timescale of the EPICA Dome C ice core (EDC) to the area of Little Dome C, using radar isochrones dating back past 600 ka. We use these data to derive the expected depth-age relationship through the ice column at the now-chosen drill site, termed BELDC (Beyond EPICA LDC). These new data indicate that the ice at BELDC is considerably older than that at EDC at the same depth and that there is about 375m of ice older than 600 kyr at BELDC. Stratigraphy is well preserved to 2565 m, similar to 93% of the ice thickness, below which there is a basal unit with unknown properties. An ice-flow model tuned to the isochrones suggests ages likely reach 1.5 Myr near 2500 m, similar to 65m above the basal unit and similar to 265m above the bed, with sufficient resolution (19 +/- 2 kyrm(-1)) to resolve 41 kyr glacial cycles.
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Matsuoka, K., Skoglund, A., Roth, G., De Pomereu, J., Griffiths, H., Headland, R., et al. (2021). Quantarctica, an integrated mapping environment for Antarctica, the Southern Ocean, and sub-Antarctic islands. Environmental Modelling & Software, 140.
Abstract: Quantarctica (https://www.npolar.no/quantarctica) is a geospatial data package, analysis environment, and visualization platform for the Antarctic Continent, Southern Ocean (>40oS), and sub-Antarctic islands. Quantarctica works with the free, cross-platform Geographical Information System (GIS) software QGIS and can run without an Internet connection, making it a viable tool for fieldwork in remote areas. The data package includes basemaps, satellite imagery, terrain models, and scientific data in nine disciplines, including physical and biological sciences, environmental management, and social science. To provide a clear and responsive user experience, cartography and rendering settings are carefully prepared using colour sets that work well for typical data combinations and with consideration of users with common colour vision deficiencies. Metadata included in each dataset provides brief abstracts for non-specialists and references to the original data sources. Thus, Quantarctica provides an integrated environment to view and analyse multiple Antarctic datasets together conveniently and with a low entry barrier.
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Quiquet, A., Dumas, C., Paillard, D., Ramstein, G., Ritz, C., & Roche, D. (2021). Deglacial Ice Sheet Instabilities Induced by Proglacial Lakes. Geophysical Research Letters, 48(9).
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2020 |
Hanna, E., Pattyn, F., Navarro, F., Favier, V., Goelzer, H., Van Den Broeke, M., et al. (2020). Mass balance of the ice sheets and glaciers – Progress since AR5 and challenges. Earth-Science Reviews, 201.
Abstract: Recent research shows increasing decadal ice mass losses from the Greenland and Antarctic Ice Sheets and more generally from glaciers worldwide in the light of continued global warming. Here, in an update of our previous ISMASS paper (Hanna et al., 2013), we review recent observational estimates of ice sheet and glacier mass balance, and their related uncertainties, first briefly considering relevant monitoring methods. Focusing on the response to climate change during 1992-2018, and especially the post-IPCC AR5 period, we discuss recent changes in the relative contributions of ice sheets and glaciers to sea-level change. We assess recent advances in understanding of the relative importance of surface mass balance and ice dynamics in overall ice-sheet mass change. We also consider recent improvements in ice-sheet modelling, highlighting data-model linkages and the use of updated observational datasets in ice-sheet models. Finally, by identifying key deficiencies in the observations and models that hamper current understanding and limit reliability of future ice-sheet projections, we make recommendations to the research community for reducing these knowledge gaps. Our synthesis aims to provide a critical and timely review of the current state of the science in advance of the next Intergovernmental Panel on Climate Change Assessment Report that is due in 2021.
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Robinson, A., Alvarez-Solas, J., Montoya, M., Goelzer, H., Greve, R., & Ritz, C. (2020). Description and validation of the ice-sheet model Yelmo (version 1.0). Geoscientific Model Development, 13(6), 2805–2823.
Abstract: We describe the physics and features of the ice-sheet model Yelmo, an open-source project intended for collaborative development. Yelmo is a thermomechanical model, solving for the coupled velocity and temperature solutions of an ice sheet simultaneously. The ice dynamics are currently treated via a “hybrid” approach combining the shallow-ice and shallow-shelf/shelfy-stream approximations, which makes Yelmo an apt choice for studying a wide variety of problems. Yelmo's main innovations lie in its flexible and user-friendly infrastructure, which promotes portability and facilitates long-term development. In particular, all physics subroutines have been designed to be self-contained, so that they can be easily ported from Yelmo to other models, or easily replaced by improved or alternate methods in the future. Furthermore, hard-coded model choices are eschewed, replaced instead with convenient parameter options that allow the model to be adapted easily to different contexts. We show results for different ice-sheet benchmark tests, and we illustrate Yelmo's performance for the Antarctic ice sheet.
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Talalay, P., Li, Y., Augustin, L., Clow, G., Hong, J., Lefebvre, E., et al. (2020). Geothermal heat flux from measured temperature profiles in deep ice boreholes in Antarctica. Cryosphere, 14(11), 4021–4037.
Abstract: The temperature at the Antarctic Ice Sheet bed and the temperature gradient in subglacial rocks have been directly measured only a few times, although extensive thermodynamic modeling has been used to estimate the geothermal heat flux (GHF) under the ice sheet. During the last 5 decades, deep ice-core drilling projects at six sites – Byrd, WAIS Divide, Dome C, Kohnen, Dome F, and Vostok – have succeeded in reaching or nearly reaching the bed at inland locations in Antarctica. When temperature profiles in these boreholes and steady-state heat flow modeling are combined with estimates of vertical velocity, the heat flow at the ice-sheet base is translated to a geothermal heat flux of 57.9 +/- 6.4mW m(-2) at Dome C, 78.9 +/- 5.0mW m(-2) at Dome F, and 86.9 +/- 16.6mW m(-2) at Kohnen, all higher than the predicted values at these sites. This warm base under the East Antarctic Ice Sheet (EAIS) could be caused by radiogenic heat effects or hydrothermal circulation not accounted for by the models. The GHF at the base of the ice sheet at Vostok has a negative value of -3.6 +/- 5.3mW m(-2), indicating that water from Lake Vostok is freezing onto the ice-sheet base. Correlation analyses between modeled and measured depth-age scales at the EAIS sites indicate that all of them can be adequately approximated by a steady-state model. Horizontal velocities and their variation over ice-age cycles are much greater for the West Antarctic Ice Sheet than for the interior EAIS sites; a steady-state model cannot precisely describe the temperature distribution here. Even if the correlation factors for the best fitting age-depth curve are only moderate for the West Antarctic sites, the GHF values estimated here of 88.4 +/- 7.6mW m(-2) at Byrd and 113.3 +/- 16.9mW m(-2) at WAIS Divide can be used as references before more precise estimates are made on the subject.
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2019 |
Edwards, T., Brandon, M., Durand, G., Edwards, N., Golledge, N., Holden, P., et al. (2019). Revisiting Antarctic ice loss due to marine ice-cliff instability. Nature, 566(7742), 58–+.
Abstract: Predictions for sea-level rise this century due to melt from Antarctica range from zero to more than one metre. The highest predictions are driven by the controversial marine ice-cliff instability (MICI) hypothesis, which assumes that coastal ice cliffs can rapidly collapse after ice shelves disintegrate, as a result of surface and sub-shelf melting caused by global warming. But MICI has not been observed in the modern era and it remains unclear whether it is required to reproduce sea-level variations in the geological past. Here we quantify ice-sheet modelling uncertainties for the original MICI study and show that the probability distributions are skewed towards lower values (under very high greenhouse gas concentrations, the most likely value is 45 centimetres). However, MICI is not required to reproduce sea-level changes due to Antarctic ice loss in the mid-Pliocene epoch, the last interglacial period or 1992-2017; without it we find that the projections agree with previous studies (all 95th percentiles are less than 43 centimetres). We conclude that previous interpretations of these MICI projections over-estimate sea-level rise this century; because the MICI hypothesis is not well constrained, confidence in projections with MICI would require a greater range of observationally constrained models of ice-shelf vulnerability and ice-cliff collapse.
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Le Clec'H, S., Charbit, S., Quiquet, A., Fettweis, X., Dumas, C., Kageyama, M., et al. (2019). Assessment of the Greenland ice sheet-atmosphere feedbacks for the next century with a regional atmospheric model coupled to an ice sheet model. Cryosphere, 13(1), 373–395.
Abstract: In the context of global warming, growing attention is paid to the evolution of the Greenland ice sheet (GrIS) and its contribution to sea-level rise at the centennial timescale. Atmosphere-GrIS interactions, such as the temperature-elevation and the albedo feedbacks, have the potential to modify the surface energy balance and thus to impact the GrIS surface mass balance (SMB). In turn, changes in the geometrical features of the ice sheet may alter both the climate and the ice dynamics governing the ice sheet evolution. However, changes in ice sheet geometry are generally not explicitly accounted for when simulating atmospheric changes over the Greenland ice sheet in the future. To account for ice sheet-climate interactions, we developed the first two-way synchronously coupled model between a regional atmospheric model (MAR) and a 3-D ice sheet model (GRISLI). Using this novel model, we simulate the ice sheet evolution from 2000 to 2150 under a prolonged representative concentration pathway scenario, RCP8.5. Changes in surface elevation and ice sheet extent simulated by GRISLI have a direct impact on the climate simulated by MAR. They are fed to MAR from 2020 onwards, i.e. when changes in SMB produce significant topography changes in GRISLI. We further assess the importance of the atmosphere-ice sheet feedbacks through the comparison of the two-way coupled experiment with two other simulations based on simpler coupling strategies: (i) a one-way coupling with no consideration of any change in ice sheet geometry; (ii) an alternative one-way coupling in which the elevation change feedbacks are parameterized in the ice sheet model (from 2020 onwards) without taking into account the changes in ice sheet topography in the atmospheric model. The two-way coupled experiment simulates an important increase in surface melt below 2000m of elevation, resulting in an important SMB reduction in 2150 and a shift of the equilibrium line towards elevations as high as 2500 m, despite a slight increase in SMB over the central plateau due to enhanced snowfall. In relation with these SMB changes, modifications of ice sheet geometry favour ice flux convergence towards the margins, with an increase in ice velocities in the GrIS interior due to increased surface slopes and a decrease in ice velocities at the margins due to decreasing ice thickness. This convergence counteracts the SMB signal in these areas. In the two-way coupling, the SMB is also influenced by changes in fine-scale atmospheric dynamical processes, such as the increase in katabatic winds from central to marginal regions induced by increased surface slopes. Altogether, the GrIS contribution to sea-level rise, inferred from variations in ice volume above floatation, is equal to 20.4 cm in 2150. The comparison between the coupled and the two uncoupled experiments suggests that the effect of the different feedbacks is amplified over time with the most important feedbacks being the SMB-elevation feedbacks. As a result, the experiment with parameterized SMB-elevation feedback provides a sea-level contribution from GrIS in 2150 only 2.5% lower than the two-way coupled experiment, while the experiment with no feedback is 9.3% lower. The change in the ablation area in the two-way coupled experiment is much larger than those provided by the two simplest methods, with an underestimation of 11.7% (14 %) with parameterized feedbacks (no feedback). In addition, we quantify that computing the GrIS contribution to sea-level rise from SMB changes only over a fixed ice sheet mask leads to an overestimation of ice loss of at least 6% compared to the use of a time variable ice sheet mask. Finally, our results suggest that ice-loss estimations diverge when using the different coupling strategies, with differences from the two-way method becoming significant at the end of the 21st century. In particular, even if averaged over the whole GrIS the climatic and ice sheet fields are relatively similar; at the local and regional scale there are important differences, highlighting the importance of correctly representing the interactions when interested in basin scale changes.
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Le Clec'H, S., Quiquet, A., Charbit, S., Dumas, C., Kageyama, M., & Ritz, C. (2019). A rapidly converging initialisation method to simulate the present-day Greenland ice sheet using the GRISLI ice sheet model (version 1.3). Geoscientific Model Development, 12(6), 2481–2499.
Abstract: Providing reliable projections of the ice sheet contribution to future sea-level rise has become one of the main challenges of the ice sheet modelling community. To increase confidence in future projections, a good knowledge of the present-day state of ice flow dynamics, which is critically dependent on basal conditions, is strongly needed. The main difficulty is tied to the scarcity of observations at the ice-bed interface at the scale of the whole ice sheet, resulting in poorly constrained parameterisations in ice sheet models. To circumvent this drawback, inverse modelling approaches can be developed to infer initial conditions for ice sheet models that best reproduce available data. Most often such approaches allow for a good representation of the mean present-day state of the ice sheet but are accompanied with unphysical trends. Here, we present an initialisation method for the Greenland ice sheet using the thermo-mechanical hybrid GRISLI (GRenoble Ice Shelf and Land Ice) ice sheet model. Our approach is based on the adjustment of the basal drag coefficient that relates the sliding velocities at the ice-bed interface to basal shear stress in unfrozen bed areas. This method relies on an iterative process in which the basal drag is periodically adjusted in such a way that the simulated ice thickness matches the observed one. The quality of the method is assessed by computing the root mean square errors in ice thickness changes. Because the method is based on an adjustment of the sliding velocities only, the results are discussed in terms of varying ice flow enhancement factors that control the deformation rates. We show that this factor has a strong impact on the minimisation of ice thickness errors and has to be chosen as a function of the internal thermal state of the ice sheet (e.g. a low enhancement factor for a warm ice sheet). While the method performance slightly increases with the duration of the minimisation procedure, an ice thickness root mean square error (RMSE) of 50.3m is obtained in only 1320 model years. This highlights a rapid convergence and demonstrates that the method can be used for computationally expensive ice sheet models.
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Macelloni, G., Leduc-Leballeur, M., Montomoli, F., Brogioni, M., Ritz, C., & Picard, G. (2019). On the retrieval of internal temperature of Antarctica Ice Sheet by using SMOS observations. Remote Sensing Of Environment, 233.
Abstract: Internal temperature is an essential parameter for understanding ice sheet dynamics. Glaciological models provide estimations of temperature profiles over Antarctica and few boreholes are also available, but, at present, no measurement exists at the scale of the whole continent. The analysis of passive L-band observations from the Soil Moisture and Ocean Salinity (SMOS) satellite shows that, thanks to the high penetration depth (i.e. up to 1500 m), it is possible to infer information on in depth glaciological properties of the ice sheet including temperature. In this study, the temperature profile is retrieved from SMOS observations using jointly glaciological and emission models. The developed methodology is valid in the inner part of Antarctica where the ice sheet is almost stable (i.e. its velocity is limited to 10 m yr(-1)). This analysis points out that in several cases, differences are observed between retrieved temperature profiles and those predicted by glaciological models. In particular, some geophysical parameters, namely the geothermal heat flux and the mean annual accumulation, need to be modified with respect to their prior values in order to simulate SMOS brightness temperatures. Results also clearly show that the reliability of the retrieved profile in depth decreases with increasing ice thickness due to the limited penetration of microwaves in the ice. The obtained results prove the capability of L band (1.4 GHz) passive microwave sensors for investigating the internal temperature of the ice-sheet.
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2018 |
Cavitte, M. G. P., Parrenin, F., Ritz, C., Young, D. A., Van Liefferinge, B., Blankenship, D. D., et al. (2018). Accumulation patterns around Dome C, East Antarctica, in the last 73 kyr. Cryosphere, 12(4), 1401–1414.
Abstract: We reconstruct the pattern of surface accumulation in the region around Dome C, East Antarctica, since the last glacial. We use a set of 18 isochrones spanning all observable depths of the ice column, interpreted from various ice-penetrating radar surveys and a 1-D ice flow model to invert for accumulation rates in the region. The shallowest four isochrones are then used to calculate paleoaccumulation rates between isochrone pairs using a 1-D assumption where horizontal advection is negligible in the time interval of each layer. We observe that the large-scale (100s km) surface accumulation gradient is spatially stable through the last 73 kyr, which reflects current modeled and observed precipitation gradients in the region. We also observe small-scale (10 s km) accumulation variations linked to snow redistribution at the surface, due to changes in its slope and curvature in the prevailing wind direction that remain spatially stationary since the last glacial.
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Passalacqua, O., Cavitte, M., Gagliardini, O., Gillet-Chaulet, F., Parrenin, F., Ritz, C., et al. (2018). Brief communication: Candidate sites of 1.5 Myr old ice 37 km southwest of the Dome C summit, East Antarctica. Cryosphere, 12(6), 2167–2174.
Abstract: The search for ice as old as 1.5 Myr requires the identification of places that maximize our chances to retrieve old, well-resolved, undisturbed and datable ice. One of these locations is very likely southwest of the Dome C summit, where elevated bedrock makes the ice thin enough to limit basal melting. A 3-D ice flow simulation is used to calculate five selection criteria, which together delineate the areas with the most appropriate glaciological properties. These selected areas (a few square kilometers) lie on the flanks of a bedrock high, where a balance is found between risks of basal melting, stratigraphic disturbances and sufficient age resolution. Within these areas, several sites of potential 1.5 Myr old ice are proposed, situated on local bedrock summits or ridges. The trajectories of the ice particles towards these locations are short, and the ice flows over a smoothly undulating bedrock. These sites will help to choose where new high-resolution ground radar surveys should be conducted in upcoming field seasons.
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Passalacqua, O., Picard, G., Ritz, C., Leduc-Leballeur, M., Quiquet, A., Larue, F., et al. (2018). Retrieval of the Absorption Coefficient of L-Band Radiation in Antarctica From SMOS Observations. Remote Sensing, 10(12).
Abstract: Microwave emissions at the L-band (1-2 GHz) in Antarctica are characterized by a significant contribution of ice layers at great depth, from hundreds to a thousand meters. Brightness temperatures, thus, could provide the internal temperature of the ice sheet. However, there are two difficulties to overcome in developing an accurate retrieval algorithm. First, it is difficult to know precisely from which depths waves are emanating because the ice-absorption coefficient is uncertain at the L-band, despite several formulations proposed in the literature over the past few decades. Second, emissivity potentially varies in Antarctica due to remnant scattering in firn (or ice), even at the Brewster angle, and despite the low frequency, limiting the accuracy of the estimate of the physical temperature. Here, we present a retrieval method able to disentangle the absorption and emissivity effects from brightness temperature over the whole continent. We exploit the fact that scattering and absorption are controlled by different physical parameters and phenomena that can be considered as statistically independent. This independence provides a constraint to the retrieval method, that is then well-conditioned and solvable. Our results show that (1) the retrieved absorption agrees with the permittivity model proposed by Matzler et al. (2006), and (2) emissivity shows significant variations, up to 6% over the continent, which are correlated with wind speed and accumulation patterns. A possible cause of this latter point is density heterogeneity and sastrugi buried in the firn. These two results are an important step forward for the accurate retrieval of internal temperature using low-frequency microwave radiometers.
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Pattyn, F., Ritz, C., Hanna, E., Asay-Davis, X., Deconto, R., Durand, G., et al. (2018). The Greenland and Antarctic ice sheets under 1.5 degrees C global warming. Nature Climate Change, 8(12), 1053–1061.
Abstract: Even if anthropogenic warming were constrained to less than 2 degrees C above pre-industrial, the Greenland and Antarctic ice sheets will continue to lose mass this century, with rates similar to those observed over the past decade. However, nonlinear responses cannot be excluded, which may lead to larger rates of mass loss. Furthermore, large uncertainties in future projections still remain, pertaining to knowledge gaps in atmospheric (Greenland) and oceanic (Antarctica) forcing. On millennial timescales, both ice sheets have tipping points at or slightly above the 1.5-2.0 degrees C threshold; for Greenland, this may lead to irreversible mass loss due to the surface mass balance-elevation feedback, whereas for Antarctica, this could result in a collapse of major drainage basins due to ice-shelf weakening.
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Pierret, M., Cotel, S., Ackerer, P., Beaulieu, E., Benarioumlil, S., Boucher, M., et al. (2018). The Strengbach Catchment: A Multidisciplinary Environmental Sentry for 30 Years. Vadose Zone Journal, 17(1).
Abstract: Research activity associated with various observations at the Strengbach catchment in the Vosges Massif (880-1150 m) addresses many questions in the domains of hydrology and geochemistry. The catchment is the observation and experimental site of the Observatoire Hydro-Geochimique de l'Environnement appointed by the Centre National de la Recherche Scientifique. It also belongs to the research facilities that form the French Network of Critical Zone Observatories (OZCAR), which supports a network of critical zone observatories. The catchment is small (0.8 km(2)) with steep slopes (20-30%) on granitic bedrock that mainly allow for forestry (spruce and beech stands) as the main land cover. Meteorological, hydrological, and geochemical data have been monitored since 1986. The first studies conducted were dedicated to the elucidation of acid rain effects on forest ecosystems and particularly on forest decline. Multidisciplinary research studies conducted on the Strengbach catchment enable exploration of the following issues: (i) hydrological functioning at the scale of a small catchment and questions regarding the evolution and preservation of the water resources in mountainous environments (stock, recharge, infiltration, and water pathways), (ii) exchange processes observed at the soil-plant-atmosphere continuum and in particular weathering processes and the evolution of soil mineral fertility (Ca, Mg, K, P), (iii) processes responsible for the export of water and for associated fluxes (dissolved chemicals, suspended materials, bed loads) and their dynamic at the outlet, and (iv) responses of the ecosystems to environmental disturbances (acid rain, forest management, and climate change) and their current and future modeling.
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Quiquet, A., Dumas, C., Ritz, C., Peyaud, V., & Roche, D. (2018). The GRISLI ice sheet model (version 2.0): calibration and validation for multi-millennial changes of the Antarctic ice sheet. Geoscientific Model Development, 11(12), 5003–5025.
Abstract: In this paper, we present the GRISLI (Grenoble ice sheet and land ice) model in its newest revision (version 2.0). Whilst GRISLI is applicable to any given ice sheet, we focus here on the Antarctic ice sheet because it highlights the importance of grounding line dynamics. Important improvements have been implemented in the model since its original version (Ritz et al., 2001). Notably, GRISLI now includes a basal hydrology model and an explicit flux computation at the grounding line based on the analytical formulations of Schoof (2007) or Tsai et al. (2015). We perform a full calibration of the model based on an ensemble of 300 simulations sampling mechanical parameter space using a Latin hypercube method. Performance of individual members is assessed relative to the deviation from present-day observed Antarctic ice thickness. To assess the ability of the model to simulate grounding line migration, we also present glacial-interglacial ice sheet changes throughout the last 400 kyr using the best ensemble members taking advantage of the capacity of the model to perform multi-millennial long-term integrations. To achieve this goal, we construct a simple climatic perturbation of present-day climate forcing fields based on two climate proxies: atmospheric and oceanic. The model is able to reproduce expected grounding line advances during glacial periods and subsequent retreats during terminations with reasonable glacial-interglacial ice volume changes.
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2017 |
Macelloni, G., Montornoli, F., Leduc-Leballeur, M., Brogioni, M., Ritz, C., & Picard, G. (2017). Retrieval Of Ice Sheet Temperature Profile In Antarctica By Using Smos Data: A Combination Of Glaciological And Microwave Emission Models.
Abstract: The internal ice sheet temperature is a key parameter for the understanding of the ice sheet dynamics which, at present, is available only from glaciological models or in the few boreholes where temperature has been measured. From the analysis of space-borne L-band data from SMOS, collected over Antarctica, it was proved that they are sensitive to the ice sheet temperature profile. In this paper it is demonstrated that starting from satellite data and using a combination of glaciological and microwave emission model it is possible to retrieve this important geophysical parameter at continental scale.
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Parrenin, F., Cavitte, M. G. P., Blankenship, D. D., Chappellaz, J., Fischer, H., Gagliardini, O., et al. (2017). Is there 1.5-million-year-old ice near Dome C, Antarctica? Cryosphere, 11(6), 2427–2437.
Abstract: Ice sheets provide exceptional archives of past changes in polar climate, regional environment and global atmospheric composition. The oldest dated deep ice core drilled in Antarctica has been retrieved at EPICA Dome C (EDC), reaching similar to 800 000 years. Obtaining an older paleoclimatic record from Antarctica is one of the greatest challenges of the ice core community. Here, we use internal isochrones, identified from airborne radar coupled to ice-flow modelling to estimate the age of basal ice along transects in the Dome C area. Three glaciological properties are inferred from isochrones: surface accumulation rate, geothermal flux and the exponent of the Lliboutry velocity profile. We find that old ice (> 1.5 Myr, 1.5 million years) likely exists in two regions: one similar to 40 km south-west of Dome C along the ice divide to Vostok, close to a secondary dome that we name “Little Dome C” (LDC), and a second region named “North Patch” (NP) located 10-30 km north-east of Dome C, in a region where the geothermal flux is apparently relatively low. Our work demonstrates the value of combining radar observations with ice flow modelling to accurately represent the true nature of ice flow, and understand the formation of ice-sheet architecture, in the centre of large ice sheets.
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Passalacqua, O., Ritz, C., Parrenin, F., Urbini, S., & Frezzotti, M. (2017). Geothermal flux and basal melt rate in the Dome C region inferred from radar reflectivity and heat modelling. Cryosphere, 11(5), 2231–2246.
Abstract: Basal melt rate is the most important physical quantity to be evaluated when looking for an old-ice drilling site, and it depends to a great extent on the geothermal flux (GF), which is poorly known under the East Antarctic ice sheet. Given that wet bedrock has higher reflectivity than dry bedrock, the wetness of the ice-bed interface can be assessed using radar echoes from the bedrock. But, since basal conditions depend on heat transfer forced by climate but lagged by the thick ice, the basal ice may currently be frozen whereas in the past it was generally melting. For that reason, the risk of bias between present and past conditions has to be evaluated. The objective of this study is to assess which locations in the Dome C area could have been protected from basal melting at any time in the past, which requires evaluating GF. We used an inverse approach to retrieve GF from radar-inferred distribution of wet and dry beds. A 1-D heat model is run over the last 800 ka to constrain the value of GF by assessing a critical ice thickness, i.e. the minimum ice thickness that would allow the present local distribution of basal melting. A regional map of the GF was then inferred over a 80 km x 130 km area, with a N-S gradient and with values ranging from 48 to 60m Wm(-2). The forward model was then emulated by a polynomial function to compute a time-averaged value of the spatially variable basal melt rate over the region. Three main subregions appear to be free of basal melting, two because of a thin overlying ice and one, north of Dome C, because of a low GF.
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Young, D. A., Roberts, J. L., Ritz, C., Frezzotti, M., Quartini, E., Cavitte, M. G. P., et al. (2017). High-resolution boundary conditions of an old ice target near Dome C, Antarctica. Cryosphere, 11(4), 1897–1911.
Abstract: A high-resolution (1 km line spacing) aerogeophysical survey was conducted over a region near the East Antarctic Ice Sheet's Dome C that may hold a 1.5 Myr climate record. We combined new ice thickness data derived from an airborne coherent radar sounder with unpublished data that was in part unavailable for earlier compilations, and we were able to remove older data with high positional uncertainties. We generated a revised high-resolution digital elevation model (DEM) to investigate the potential for an old ice record in this region, and used laser altimetry to confirm a Cryosat-2 derived DEM for inferring the glaciological state of the candidate area. By measuring the specularity content of the bed, we were able to find an additional 50 subglacial lakes near the candidate site, and by Doppler focusing the radar data, we were able to map out the roughness of the bed at length scales of hundreds of meters. We find that the primary candidate region contains elevated rough topography interspersed with scattered subglacial lakes and some regions of smoother bed. Free subglacial water appears to be restricted from bed overlain by ice thicknesses of less than 3000 m. A site near the ice divide was selected for further investigation. The high resolution of this ice thickness data set also allows us to explore the nature of ice thickness uncertainties in the context of radar geometry and processing
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2016 |
Bazin, L., Landais, A., Capron, E., Masson-Delmotte, V., Ritz, C., Picard, G., et al. (2016). Phase relationships between orbital forcing and the composition of air trapped in Antarctic ice cores. Climate Of The Past, 12(3), 729–748.
Abstract: Orbital tuning is central for ice core chronologies beyond annual layer counting, available back to 60 ka (i.e. thousands of years before 1950) for Greenland ice cores. While several complementary orbital tuning tools have recently been developed using delta O-18(atm), delta O-2/N-2 and air content with different orbital targets, quantifying their uncertainties remains a challenge. Indeed, the exact processes linking variations of these parameters, measured in the air trapped in ice, to their orbital targets are not yet fully understood. Here, we provide new series of delta O-2/N-2 and delta O-18(atm) data encompassing Marine Isotopic Stage (MIS) 5 (between 100 and 160 ka) and the oldest part (340-800 ka) of the East Antarctic EPICA Dome C (EDC) ice core. For the first time, the measurements over MIS 5 allow an inter-comparison of delta O-2/N-2 and delta O-18(atm) records from three East Antarctic ice core sites (EDC, Vostok and Dome F). This comparison highlights some site-specific delta O-2/N-2 variations. Such an observation, the evidence of a 100 ka periodicity in the delta O-2/N-2 signal and the difficulty to identify extrema and mid-slopes in delta O-2/N-2 increase the uncertainty associated with the use of delta O-2/N-2 as an orbital tuning tool, now calculated to be 3-4 ka. When combining records of delta O-18(atm) and delta O-2/N-2 from Vostok and EDC, we find a loss of orbital signature for these two parameters during periods of minimum eccentricity (similar to 400 ka, 720-800 ka). Our data set reveals a time-varying offset between delta O-2/N-2 and delta O-18(atm) records over the last 800 ka that we interpret as variations in the lagged response of delta O-18(atm) to precession. The largest offsets are identified during Terminations II, MIS 8 and MIS 16, corresponding to periods of destabilization of the Northern polar ice sheets. We therefore suggest that the occurrence of Heinrich like events influences the response of delta O-18(atm) to precession.
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Gillet-Chaulet, F., Durand, G., Gagliardini, O., Mosbeux, C., Mouginot, J., Rémy, F., et al. (2016). Assimilation of surface velocities acquired between 1996 and 2010 to constrain the form of the basal friction law under Pine Island Glacier. Geophys. Res. Lett., 43(19), 10,311–10,321.
Abstract: Abstract In ice-sheet models, slip conditions at the base between the ice and the bed are parameterized by a friction law. The most common relation has two poorly constrained parameters, C and m. The basal slipperiness coefficient, C, depends on local unobserved quantities and is routinely inferred using inverse methods. While model results have shown that transient responses to external forcing are highly sensitive to the stress exponent m, no consensus value has emerged, with values commonly used ranging from 1 to ∞ depending on the slip processes. By assimilation of Pine Island Glacier surface velocities from 1996 to 2010, we show that observed accelerations are best reproduced with m>=5. We conclude that basal motion, in much of the fast flowing region, is governed by plastic deformation of the underlying sediments. This implies that the glacier bed in this area cannot deliver resistive stresses higher than today, making the drainage basin potentially more sensitive to dynamical perturbations than predicted with models using standard values m = 1 or 3.
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Macelloni, G., Leduc-Leballeur, M., Brogioni, M., Ritz, C., & Picard, G. (2016). Analyzing and modeling the SMOS spatial variations in the East Antarctic Plateau. Remote Sensing Of Environment, 180, 193–204.
Abstract: The SMOS brightness temperature (T-B) collected on the East Antarctic Plateau revealed spatial signatures at L-band that have never before been observed when only higher-frequency passive microwave observations were available, and this has opened up a new field of research. Because of the much greater penetration depth, modeling the microwave ice sheet emission requires taldng into account not only snow conditions on the surface, but should also include glaciological information. Even if the penetration depth of the L-band is not well known due to the uncertainty on the imaginary part of the ice permittivity, it is likely to be of the order of several hundreds of meters, which means that the temperature of the ice over a depth of nearly 1000 m influences the emission. Over such a depth, the temperature is related to both the surface conditions and to the ice sheet thickness, which in turn depends on the bedrock topography and on other glaciological variables. The present paper aims to provide a thorough theoretical explanation of the observed T-B spatial variation close to the Brewster angle at vertical polarization, in order to limit the effect of surface and vertical density variability in the firn. In order to provide reliable inputs to the microwave emission models used for simulating T-B data, an in-depth analysis of the temperature profiles was performed by means of glaciological models. The comparison between simulated and observed data over three transects totalling 2000 km in East Antarctica pointed out that, whereas the emission models are capable of explaining the T-B spatial variations of several kelvins (0.7 and 2.9 K), they are unable to predict its absolute value correctly. This study also shows that the main limiting factor in simulating low-frequency microwave data is the uncertainty in the currently available imaginary part of the ice permittivity. (C) 2016 Elsevier Inc All rights reserved.
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Passalacqua, O., Gagliardini, O., Parrenin, F., Todd, J., Gillet-Chaulet, F., & Ritz, C. (2016). Performance and applicability of a 2.5-D ice-flow model in the vicinity of a dome. Geoscientific Model Development, 9(7), 2301–2313.
Abstract: Three-dimensional ice flow modelling requires a large number of computing resources and observation data, such that 2-D simulations are often preferable. However, when there is significant lateral divergence, this must be accounted for (2.5-D models), and a flow tube is considered (volume between two horizontal flowlines). In the absence of velocity observations, this flow tube can be derived assuming that the flowlines follow the steepest slope of the surface, under a few flow assumptions. This method typically consists of scanning a digital elevation model (DEM) with a moving window and computing the curvature at the centre of this window. The ability of the 2.5-D models to account properly for a 3-D state of strain and stress has not clearly been established, nor their sensitivity to the size of the scanning window and to the geometry of the ice surface, for example in the cases of sharp ridges. Here, we study the applicability of a 2.5-D ice flow model around a dome, typical of the East Antarctic plateau conditions. A twin experiment is carried out, comparing 3-D and 2.5-D computed velocities, on three dome geometries, for several scanning windows and thermal conditions. The chosen scanning window used to evaluate the ice surface curvature should be comparable to the typical radius of this curvature. For isothermal ice, the error made by the 2.5-D model is in the range 0-10aEuro-% for weakly diverging flows, but is 2 or 3 times higher for highly diverging flows and could lead to a non-physical ice surface at the dome. For non-isothermal ice, assuming a linear temperature profile, the presence of a sharp ridge makes the 2.5-D velocity field unrealistic. In such cases, the basal ice is warmer and more easily laterally strained than the upper one, the walls of the flow tube are not vertical, and the assumptions of the 2.5-D model are no longer valid.
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2015 |
Beghin, P., Charbit, S., Dumas, C., Kageyama, M., & Ritz, C. (2015). How might the North American ice sheet influence the northwestern Eurasian climate? Climate of the Past, 11(10), 1467–1490.
Abstract: It is now widely acknowledged that past Northern Hemisphere ice sheets covering Canada and northern Europe at the Last Glacial Maximum (LGM) exerted a strong influence on climate by causing changes in atmospheric and oceanic circulations. In turn, these changes may have impacted the development of the ice sheets themselves through a combination of different feedback mechanisms. The present study is designed to investigate the potential impact of the North American ice sheet on the surface mass balance (SMB) of the Eurasian ice sheet driven by simulated changes in the past glacial atmospheric circulation. Using the LMDZ5 atmospheric circulation model, we carried out 12 experiments under constant LGM conditions for insolation, greenhouse gases and ocean. In these experiments, the Eurasian ice sheet is removed. The 12 experiments differ in the North American ice-sheet topography, ranging from a white and flat (present-day topography) ice sheet to a full-size LGM ice sheet. This experimental design allows the albedo and the topographic impacts of the North American ice sheet onto the climate to be disentangled. The results are compared to our baseline experiment where both the North American and the Eurasian ice sheets have been removed. In summer, the sole albedo effect of the American ice sheet modifies the pattern of planetary waves with respect to the no-ice-sheet case, resulting in a cooling of the northwestern Eurasian region. By contrast, the atmospheric circulation changes induced by the topography of the North American ice sheet lead to a strong decrease of this cooling. In winter, the Scandinavian and the Barents-Kara regions respond differently to the American ice-sheet albedo effect: in response to atmospheric circulation changes, Scandinavia becomes warmer and total precipitation is more abundant, whereas the Barents-Kara area becomes cooler with a decrease of convective processes, causing a decrease of total precipitation. The gradual increase of the altitude of the American ice sheet leads to less total precipitation and snowfall and to colder temperatures over both the Scandinavian and the Barents and Kara sea sectors. We then compute the resulting annual surface mass balance over the Fennoscandian region from the simulated temperature and precipitation fields used to force an ice-sheet model. It clearly appears that the SMB is dominated by the ablation signal. In response to the summer cooling induced by the American ice-sheet albedo, high positive SMB values are obtained over the Eurasian region, leading thus to the growth of an ice sheet. On the contrary, the gradual increase of the American ice-sheet altitude induces more ablation over the Eurasian sector, hence limiting the growth of Fennoscandia. To test the robustness of our results with respect to the Eurasian ice sheet state, we carried out two additional LMDZ experiments with new boundary conditions involving both the American (flat or full LGM) and high Eurasian ice sheets. The most striking result is that the Eurasian ice sheet is maintained under full-LGM North American ice-sheet conditions, but loses similar to 10% of its mass compared to the case in which the North American ice sheet is flat. These new findings qualitatively confirm the conclusions from our first series of experiments and suggest that the development of the Eurasian ice sheet may have been slowed down by the growth of the American ice sheet, offering thereby a new understanding of the evolution of Northern Hemisphere ice sheets throughout glacial-interglacial cycles.
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Ritz, C., Edwards, T. L., Durand, G., Payne, A. J., Peyaud, V., & Hindmarsh, R. C. A. (2015). Potential sea-level rise from Antarctic ice-sheet instability constrained by observations. Nature, 528(7580), 115–+.
Abstract: Large parts of the Antarctic ice sheet lying on bedrock below sea level may be vulnerable to marine-ice-sheet instability (MISI)(1), a self-sustaining retreat of the grounding line triggered by oceanic or atmospheric changes. There is growing evidence(2-4) that MISI may be underway throughout the Amundsen Sea embayment (ASE), which contains ice equivalent to more than a metre of global sea-level rise. If triggered in other regions(5-8), the centennial to millennial contribution could be several metres. Physically plausible projections are challenging(9): numerical models with sufficient spatial resolution to simulate grounding-line processes have been too computationally expensive(2,3,10) to generate large ensembles for uncertainty assessment, and lower-resolution model projections(11) rely on parameterizations that are only loosely constrained by present day changes. Here we project that the Antarctic ice sheet will contribute up to 30 cm sea-level equivalent by 2100 and 72 cm by 2200 (95% quantiles) where the ASE dominates. Our process-based, statistical approach gives skewed and complex probability distributions (single mode, 10 cm, at 2100; two modes, 49 cm and 6 cm, at 2200). The dependence of sliding on basal friction is a key unknown: nonlinear relationships favour higher contributions. Results are conditional on assessments of MISI risk on the basis of projected triggers under the climate scenario A1B (ref. 9), although sensitivity to these is limited by theoretical and topographical constraints on the rate and extent of ice loss. We find that contributions are restricted by a combination of these constraints, calibration with success in simulating observed ASE losses, and low assessed risk in some basins. Our assessment suggests that upper-bound estimates from low-resolution models and physical arguments9 (up to a metre by 2100 and around one and a half by 2200) are implausible under current understanding of physical mechanisms and potential triggers.
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2014 |
Beghin, P., Charbit, S., Dumas, C., Kageyama, M., Roche, D. M., & Ritz, C. (2014). Interdependence of the growth of the Northern Hemisphere ice sheets during the last glaciation: the role of atmospheric circulation. Climate Of The Past, 10(1), 345–358.
Abstract: The development of large continental-scale ice sheets over Canada and northern Europe during the last glacial cycle likely modified the track of stationary waves and influenced the location of growing ice sheets through changes in accumulation and temperature patterns. Although they are often mentioned in the literature, these feedback mechanisms are poorly constrained and have never been studied throughout an entire glacial-interglacial cycle. Using the climate model of intermediate complexity CLIMBER-2 coupled with the 3-D ice-sheet model GRISLI (GRenoble Ice Shelf and Land Ice model), we investigate the impact of stationary waves on the construction of past Northern Hemisphere ice sheets during the past glaciation. The stationary waves are not explicitly computed in the model but their effect on sea-level pressure is parameterized. We tested different parameterizations to study separately the effect of surface temperature (thermal forcing) and topography (orographic forcing) on sea-level pressure, and therefore on atmospheric circulation and ice-sheet surface mass balance. Our model results suggest that the response of ice sheets to thermal and/or orographic forcings is rather different. At the beginning of the glaciation, the orographic effect favors the growth of the Laurentide ice sheet, whereas Fennoscandia appears rather sensitive to the thermal effect. Using the ablation parameterization as a trigger to artificially modify the size of one ice sheet, the remote influence of one ice sheet on the other is also studied as a function of the stationary wave parameterizations. The sensitivity of remote ice sheets is shown to be highly sensitive to the choice of these parameterizations with a larger response when orographic effect is accounted for. Results presented in this study suggest that the various spatial distributions of ice sheets could be partly explained by the feedback mechanisms occurring between ice sheets and atmospheric circulation.
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Bonan, B., Nodet, M., Ritz, C., & Peyaud, V. (2014). An ETKF approach for initial state and parameter estimation in ice sheet modelling. Nonlinear Processes In Geophysics, 21(2), 569–582.
Abstract: Estimating the contribution of Antarctica and Greenland to sea-level rise is a hot topic in glaciology. Good estimates rely on our ability to run a precisely calibrated ice sheet evolution model starting from a reliable initial state. Data assimilation aims to provide an answer to this problem by combining the model equations with observations. In this paper we aim to study a state-of-the-art ensemble Kalman filter (ETKF) to address this problem. This method is implemented and validated in the twin experiments framework for a shallow ice flowline model of ice dynamics. The results are very encouraging, as they show a good convergence of the ETKF (with localisation and inflation), even for small-sized ensembles.
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Edwards, T. L., Fettweis, X., Gagliardini, O., Gillet-Chaulet, F., Goelzer, H., Gregory, J. M., et al. (2014). Effect of uncertainty in surface mass balance-elevation feedback on projections of the future sea level contribution of the Greenland ice sheet. Cryosphere, 8(1), 195–208.
Abstract: We apply a new parameterisation of the Greenland ice sheet (GrIS) feedback between surface mass balance (SMB: the sum of surface accumulation and surface ablation) and surface elevation in the MAR regional climate model (Edwards et al., 2014) to projections of future climate change using five ice sheet models (ISMs). The MAR (Modele Atmospherique Regional: Fettweis, 2007) climate projections are for 2000-2199, forced by the ECHAM5 and HadCM3 global climate models (GCMs) under the SRES A1B emissions scenario. The additional sea level contribution due to the SMB-elevation feedback averaged over five ISM projections for ECHAM5 and three for HadCM3 is 4.3% (best estimate; 95% credibility interval 1.8-6.9 %) at 2100, and 9.6% (best estimate; 95% credibility interval 3.6-16.0 %) at 2200. In all results the elevation feedback is significantly positive, amplifying the GrIS sea level contribution relative to the MAR projections in which the ice sheet topography is fixed: the lower bounds of our 95% credibility intervals (CIs) for sea level contributions are larger than the “no feedback” case for all ISMs and GCMs. Our method is novel in sea level projections because we propagate three types of modelling uncertainty – GCM and ISM structural uncertainties, and elevation feedback parameterisation uncertainty – along the causal chain, from SRES scenario to sea level, within a coherent experimental design and statistical framework. The relative contributions to uncertainty depend on the timescale of interest. At 2100, the GCM uncertainty is largest, but by 2200 both the ISM and parameterisation uncertainties are larger. We also perform a perturbed parameter ensemble with one ISM to estimate the shape of the projected sea level probability distribution; our results indicate that the probability density is slightly skewed towards higher sea level contributions.
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Edwards, T. L., Fettweis, X., Gagliardini, O., Gillet-Chaulet, F., Goelzer, H., Gregory, J. M., et al. (2014). Probabilistic parameterisation of the surface mass balance-elevation feedback in regional climate model simulations of the Greenland ice sheet. Cryosphere, 8(1), 181–194.
Abstract: We present a new parameterisation that relates surface mass balance (SMB: the sum of surface accumulation and surface ablation) to changes in surface elevation of the Greenland ice sheet (GrIS) for the MAR (Modele Atmospherique Regional: Fettweis, 2007) regional climate model. The motivation is to dynamically adjust SMB as the GrIS evolves, allowing us to force ice sheet models with SMB simulated by MAR while incorporating the SMB-elevation feedback, without the substantial technical challenges of coupling ice sheet and climate models. This also allows us to assess the effect of elevation feedback uncertainty on the GrIS contribution to sea level, using multiple global climate and ice sheet models, without the need for additional, expensive MAR simulations. We estimate this relationship separately below and above the equilibrium line altitude (ELA, separating negative and positive SMB) and for regions north and south of 77 degrees N, from a set of MAR simulations in which we alter the ice sheet surface elevation. These give four “SMB lapse rates”, gradients that relate SMB changes to elevation changes. We assess uncertainties within a Bayesian framework, estimating probability distributions for each gradient from which we present best estimates and credibility intervals (CI) that bound 95% of the probability. Below the ELA our gradient estimates are mostly positive, because SMB usually increases with elevation: 0.56 (95% CI: -0.22 to 1.33) kg m(-3) a(-1) for the north, and 1.91 (1.03 to 2.61) kg m(-3) a(-1) for the south. Above the ELA, the gradients are much smaller in magnitude: 0.09 (-0.03 to 0.23) kg m(-3) a(-1) in the north, and 0.07 (-0.07 to 0.59) kg m(-3) a(-1) in the south, because SMB can either increase or decrease in response to increased elevation. Our statistically founded approach allows us to make probabilistic assessments for the effect of elevation feedback uncertainty on sea level projections (Edwards et al., 2014).
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Roche, D. M., Dumas, C., Bugelmayer, M., Charbit, S., & Ritz, C. (2014). Adding a dynamical cryosphere to iLOVECLIM (version 1.0): coupling with the GRISLI ice-sheet model. Geoscientific Model Development, 7(4), 1377–1394.
Abstract: We present a coupling approach to and the first results of the GRISLI ice-sheet model within the iLOVECLIM-coupled climate model. The climate component is a relatively low-resolution earth system model of intermediate complexity, well suited for long-term integrations and thus for coupled climate-cryosphere studies. We describe the coupling procedure with emphasis on the down-scaling scheme and the methods to compute the snow fraction from total precipitation fields. We then present results for the Greenland ice sheet under pre-industrial climate conditions at the end of a 14 000 yr long integration. The simulated ice sheet presents too large a thickness in its central part owing to the overestimation of precipitation in the atmospheric component. We find that including downscaling procedures for temperature improves the temperature distributions over Greenland for both the summer and annual means. We also find an ice-sheet areal extent that is overestimated with respect to the observed Greenland ice sheet.
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2013 |
Alvarez-Solas, J., Robinson, A., Montoya, M., & Ritz, C. (2013). Iceberg discharges of the last glacial period driven by oceanic circulation changes. Proceedings Of The National Academy Of Sciences Of The United States Of America, 110(41), 16350–16354.
Abstract: Proxy data reveal the existence of episodes of increased deposition of ice-rafted detritus in the North Atlantic Ocean during the last glacial period interpreted as massive iceberg discharges from the Laurentide Ice Sheet. Although these have long been attributed to self-sustained ice sheet oscillations, growing evidence of the crucial role that the ocean plays both for past and future behavior of the cryosphere suggests a climatic control of these ice surges. Here, we present simulations of the last glacial period carried out with a hybrid ice sheet-ice shelf model forced by an oceanic warming index derived from proxy data that accounts for the impact of past ocean circulation changes on ocean temperatures. The model generates a time series of iceberg discharge that closely agrees with ice-rafted debris records over the past 80 ka, indicating that oceanic circulation variations were responsible for the enigmatic ice purges of the last ice age.
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Bazin, L., Landais, A., Lemieux-Dudon, B., Kele, H. T. M., Veres, D., Parrenin, F., et al. (2013). An optimized multi-proxy, multi-site Antarctic ice and gas orbital chronology (AICC2012): 120-800 ka. Climate Of The Past, 9(4), 1715–1731.
Abstract: An accurate and coherent chronological framework is essential for the interpretation of climatic and environmental records obtained from deep polar ice cores. Until now, one common ice core age scale had been developed based on an inverse dating method (Datice), combining glaciological modelling with absolute and stratigraphic markers between 4 ice cores covering the last 50 ka (thousands of years before present) (Lemieux-Dudon et al., 2010). In this paper, together with the companion paper of Veres et al. (2013), we present an extension of this work back to 800 ka for the NGRIP, TALDICE, EDML, Vostok and EDC ice cores using an improved version of the Datice tool. The AICC2012 (Antarctic Ice Core Chronology 2012) chronology includes numerous new gas and ice stratigraphic links as well as improved evaluation of background and associated variance scenarios. This paper concentrates on the long timescales between 120-800 ka. In this framework, new measurements of delta O-18(atm) over Marine Isotope Stage (MIS) 11-12 on EDC and a complete delta O-18(atm) record of the TALDICE ice cores permit us to derive additional orbital gas age constraints. The coherency of the different orbitally deduced ages (from delta O-18(atm), delta O-2/N-2 and air content) has been verified before implementation in AICC2012. The new chronology is now independent of other archives and shows only small differences, most of the time within the original uncertainty range calculated by Datice, when compared with the previous ice core reference age scale EDC3, the Dome F chronology, or using a comparison between speleothems and methane. For instance, the largest deviation between AICC2012 and EDC3 (5.4 ka) is obtained around MIS 12. Despite significant modifications of the chronological constraints around MIS 5, now independent of speleothem records in AICC2012, the date of Termination II is very close to the EDC3 one.
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Charbit, S., Dumas, C., Kageyama, M., Roche, D. M., & Ritz, C. (2013). Influence of ablation-related processes in the build-up of simulated Northern Hemisphere ice sheets during the last glacial cycle. Cryosphere, 7(2), 681–698.
Abstract: Since the original formulation of the positive-degree-day (PDD) method, different PDD calibrations have been proposed in the literature in response to the increasing number of observations. Although these formulations generally provide a satisfactory description of the present-day Greenland geometry, they have not all been tested for paleo ice sheets. Using the climate-ice sheet model CLIMBER-GRISLI coupled with different PDD models, we evaluate how the parameterisation of the ablation may affect the evolution of Northern Hemisphere ice sheets in the transient simulations of the last glacial cycle. Results from fully coupled simulations are compared to time-slice experiments carried out at different key periods of the last glacial period. We find large differences in the simulated ice sheets according to the chosen PDD model. These differences occur as soon as the onset of glaciation, therefore affecting the subsequent evolution of the ice system. To further investigate how the PDD method controls this evolution, special attention is given to the role of each PDD parameter. We show that glacial inception is critically dependent on the representation of the impact of the temperature variability from the daily to the inter-annual time scale, whose effect is modulated by the refreezing scheme. Finally, an additional set of sensitivity experiments has been carried out to assess the relative importance of melt processes with respect to initial ice sheet configuration in the construction and the evolution of past Northern Hemisphere ice sheets. Our analysis reveals that the impacts of the initial ice sheet condition may range from quite negligible to explaining about half of the LGM ice volume depending on the representation of stochastic temperature variations which remain the main driver of the evolution of the ice system. The main findings of this paper underline the need for conducting studies with high resolution climate models coupled to detailed snow models to better constrain the temporal and spatial variations of the PDD parameters. The development of such approaches could improve the calibration of the PDD formulation which is still widely used in climate-ice sheet studies.
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Dahl-Jensen, D., Albert, M. R., Aldahan, A., Azuma, N., Balslev-Clausen, D., Baumgartner, M., et al. (2013). Eemian interglacial reconstructed from a Greenland folded ice core. Nature, 493(7433), 489–494.
Abstract: Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling ('NEEM') ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 +/- 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 +/- 250 metres, reaching surface elevations 122,000 years ago of 130 +/- 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.
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Fischer, H., Severinghaus, J., Brook, E., Wolff, E., Albert, M., Alemany, O., et al. (2013). Where to find 1.5 million yr old ice for the IPICS “Oldest-Ice” ice core. Climate Of The Past, 9(6), 2489–2505.
Abstract: The recovery of a 1.5 million yr long ice core from Antarctica represents a keystone of our understanding of Quaternary climate, the progression of glaciation over this time period and the role of greenhouse gas cycles in this progression. Here we tackle the question of where such ice may still be found in the Antarctic ice sheet. We can show that such old ice is most likely to exist in the plateau area of the East Antarctic ice sheet (EAIS) without stratigraphic disturbance and should be able to be recovered after careful pre-site selection studies. Based on a simple ice and heat flow model and glaciological observations, we conclude that positions in the vicinity of major domes and saddle position on the East Antarctic Plateau will most likely have such old ice in store and represent the best study areas for dedicated reconnaissance studies in the near future. In contrast to previous ice core drill site selections, however, we strongly suggest significantly reduced ice thickness to avoid bottom melting. For example for the geothermal heat flux and accumulation conditions at Dome C, an ice thickness lower than but close to about 2500m would be required to find 1.5 Myr old ice (i.e., more than 700m less than at the current EPICA Dome C drill site). Within this constraint, the resolution of an Oldest-Ice record and the distance of such old ice to the bedrock should be maximized to avoid ice flow disturbances, for example, by finding locations with minimum geothermal heat flux. As the geothermal heat flux is largely unknown for the EAIS, this parameter has to be carefully determined beforehand. In addition, detailed bedrock topography and ice flow history has to be reconstructed for candidates of an Oldest-Ice ice coring site. Finally, we argue strongly for rapid access drilling before any full, deep ice coring activity commences to bring datable samples to the surface and to allow an age check of the oldest ice.
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Hanna, E., Navarro, F. J., Pattyn, F., Domingues, C. M., Fettweis, X., Ivins, E. R., et al. (2013). Ice-sheet mass balance and climate change. Nature, 498(7452), 51–59.
Abstract: Since the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report, new observations of ice-sheet mass balance and improved computer simulations of ice-sheet response to continuing climate change have been published. Whereas Greenland is losing ice mass at an increasing pace, current Antarctic ice loss is likely to be less than some recently published estimates. It remains unclear whether East Antarctica has been gaining or losing ice mass over the past 20 years, and uncertainties in ice-mass change for West Antarctica and the Antarctic Peninsula remain large. We discuss the past six years of progress and examine the key problems that remain.
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Quiquet, A., Ritz, C., Punge, H. J., & Melia, D. S. Y. (2013). Greenland ice sheet contribution to sea level rise during the last interglacial period: a modelling study driven and constrained by ice core data. Climate Of The Past, 9(1), 353–366.
Abstract: As pointed out by the forth assessment report of the Intergovernmental Panel on Climate Change, IPCC-AR4 (Meehl et al., 2007), the contribution of the two major ice sheets, Antarctica and Greenland, to global sea level rise, is a subject of key importance for the scientific community. By the end of the next century, a 3-5 degrees C warming is expected in Greenland. Similar temperatures in this region were reached during the last interglacial (LIG) period, 130-115 ka BP, due to a change in orbital configuration rather than to an anthropogenic forcing. Ice core evidence suggests that the Greenland ice sheet (GIS) survived this warm period, but great uncertainties remain about the total Greenland ice reduction during the LIG. Here we perform long-term simulations of the GIS using an improved ice sheet model. Both the methodologies chosen to reconstruct palaeoclimate and to calibrate the model are strongly based on proxy data. We suggest a relatively low contribution to LIG sea level rise from Greenland melting, ranging from 0.7 to 1.5m of sea level equivalent, contrasting with previous studies. Our results suggest an important contribution of the Antarctic ice sheet to the LIG highstand.
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2012 |
Alvarez-Solas, J., Robinson, A., & Ritz, C. (2012). Brief communication 'Can recent ice discharges following the Larsen-B ice-shelf collapse be used to infer the driving mechanisms of millennial-scale variations of the Laurentide ice sheet?'. Cryosphere, 6(3), 687–693.
Abstract: The effects of an ice-shelf collapse on inland glacier dynamics have recently been widely studied, especially since the breakup of the Antarctic Peninsula's Larsen-B ice shelf in 2002. Several studies have documented acceleration of the ice streams that were flowing into the former ice shelf. The mechanism responsible for such a speed-up lies with the removal of the ice-shelf backforce. Independently, it is also well documented that during the last glacial period, the Northern Hemisphere ice sheets experienced large discharges into the ocean, likely reflecting ice flow acceleration episodes on the millennial time scale. The classic interpretation of the latter is based on the existence of an internal thermo-mechanical feedback with the potential to generate oscillatory behavior in the ice sheets. Here we would like to widen the debate by considering that Larsen-B-like glacial analog episodes could have contributed significantly to the registered millennial-scale variablity.
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Gillet-Chaulet, F., Gagliardini, O., Seddik, H., Nodet, M., Durand, G., Ritz, C., et al. (2012). Greenland ice sheet contribution to sea-level rise from a new-generation ice-sheet model. Cryosphere, 6(6), 1561–1576.
Abstract: Over the last two decades, the Greenland ice sheet (GrIS) has been losing mass at an increasing rate, enhancing its contribution to sea-level rise (SLR). The recent increases in ice loss appear to be due to changes in both the surface mass balance of the ice sheet and ice discharge (ice flux to the ocean). Rapid ice flow directly affects the discharge, but also alters ice-sheet geometry and so affects climate and surface mass balance. Present-day ice-sheet models only represent rapid ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, we present a new-generation prognostic ice-sheet model which reproduces the current patterns of rapid ice flow. This requires three essential developments: the complete solution of the full system of equations governing ice deformation; a variable resolution unstructured mesh to resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled ice discharge is in good agreement with observations on the continental scale and for individual outlets. From this initial state, we investigate possible bounds for the next century ice-sheet mass loss. We run sensitivity experiments of the GrIS dynamical response to perturbations in climate and basal lubrication, assuming a fixed position of the marine termini. We find that increasing ablation tends to reduce outflow and thus decreases the ice-sheet imbalance. In our experiments, the GrIS initial mass (im)balance is preserved throughout the whole century in the absence of reinforced forcing, allowing us to estimate a lower bound of 75 mm for the GrIS contribution to SLR by 2100. In one experiment, we show that the current increase in the rate of ice loss can be reproduced and maintained throughout the whole century. However, this requires a very unlikely perturbation of basal lubrication. From this result we are able to estimate an upper bound of 140 mm from dynamics only for the GrIS contribution to SLR by 2100.
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Gusmeroli, A., Pettit, E. C., Kennedy, J. H., & Ritz, C. (2012). The crystal fabric of ice from full-waveform borehole sonic logging. Journal Of Geophysical Research-Earth Surface, 117, F03021.
Abstract: In an ice sheet, a preferred crystal orientation fabric affects deformation rates because ice crystals are strongly anisotropic: shear along the basal plane is significantly easier than shear perpendicular to the basal plane. The effect of fabric can be as important as temperature in defining deformation rates. Fabric is typically measured using analysis of thin sections under the microscope with co-polarized light. Due to the time-consuming and destructive nature of these measurements, however, it is difficult to capture the spatial variation in fabric necessary for evincing ice sheet flow patterns. Because an ice crystal is similarly elastically anisotropic, the speed of elastic waves through ice can be used as a proxy for quantify anisotropy. We use borehole sonic logging measurements and thin section data from Dome C, East Antarctica to define the relations between apparent fabric and borehole measured elastic speeds (compressional V-P and vertically polarized shear V-SV). These relations, valid for single maximum fabrics, allow in-situ, depth-continuous fabric estimates of unimodal fabric strength from borehole sonic logging. We describe the single maximum fabric using a(1): the largest eigenvalue of the second-order orientation tensor. For ice at -16 degrees C and a(1) in the 0.7-1 range the relations are V-P = 248 a(1)(3.7) + 3755 m s(-1) and V-SV = -210a(1)(7.3) + 1968 m s(-1).
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Masson-Delmotte, V., Swingedouw, D., Landais, A., Seidenkrantz, M. S., Gauthier, E., Bichet, V., et al. (2012). Greenland climate change: from the past to the future. Wiley Interdisciplinary Reviews-Climate Change, 3(5), 427–449.
Abstract: Climate archives available from deep sea and marine shelf sediments, glaciers, lakes, and ice cores in and around Greenland allow us to place the current trends in regional climate, ice sheet dynamics, and land surface changes in a broader perspective. We show that, during the last decade (2000s), atmospheric and sea surface temperatures are reaching levels last encountered millennia ago, when northern high latitude summer insolation was higher due to a different orbital configuration. Records from lake sediments in southern Greenland document major environmental and climatic conditions during the last 10,000 years, highlighting the role of soil dynamics in past vegetation changes, and stressing the growing anthropogenic impacts on soil erosion during the recent decades. Furthermore, past and present changes in atmospheric and oceanic heat advection appear to strongly influence both regional climate and ice sheet dynamics. Projections from climate models are investigated to quantify the magnitude and rates of future changes in Greenland temperature, which may be faster than past abrupt events occurring under interglacial conditions. Within one century, in response to increasing greenhouse gas emissions, Greenland may reach temperatures last time encountered during the last interglacial period, approximately 125,000 years ago. We review and discuss whether analogies between the last interglacial and future changes are reasonable, because of the different seasonal impacts of orbital and greenhouse gas forcings. Over several decades to centuries, future Greenland melt may act as a negative feedback, limiting regional warming albeit with global sea level and climatic impacts. WIREs Clim Change 2012 doi: 10.1002/wcc.186 For further resources related to this article, please visit the WIREs website.
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Quiquet, A., Punge, H. J., Ritz, C., Fettweis, X., Gallee, H., Kageyama, M., et al. (2012). Sensitivity of a Greenland ice sheet model to atmospheric forcing fields. Cryosphere, 6(5), 999–1018.
Abstract: Predicting the climate for the future and how it will impact ice sheet evolution requires coupling ice sheet models with climate models. However, before we attempt to develop a realistic coupled setup, we propose, in this study, to first analyse the impact of a model simulated climate on an ice sheet. We undertake this exercise for a set of regional and global climate models. Modelled near surface air temperature and precipitation are provided as upper boundary conditions to the GRISLI (GRenoble Ice Shelf and Land Ice model) hybrid ice sheet model (ISM) in its Greenland configuration. After 20 kyrs of simulation, the resulting ice sheets highlight the differences between the climate models. While modelled ice sheet sizes are generally comparable to the observed one, there are considerable deviations among the ice sheets on regional scales. These deviations can be explained by biases in temperature and precipitation near the coast. This is especially true in the case of global models. But the deviations between the climate models are also due to the differences in the atmospheric general circulation. To account for these differences in the context of coupling ice sheet models with climate models, we conclude that appropriate down-scaling methods will be needed. In some cases, systematic corrections of the climatic variables at the interface may be required to obtain realistic results for the Greenland ice sheet (GIS).
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2011 |
Alvarez-Solas, J., Charbit, S., Ramstein, G., Paillard, D., Dumas, C., Ritz, C., et al. (2011). Millennial-scale oscillations in the Southern Ocean in response to atmospheric CO2 increase. Global And Planetary Change, 76(3-4), 128–136.
Abstract: A coupled climate-ice-sheet model is used to investigate the response of climate at the millennial time scale under several global warming long-term scenarios, stabilized at different levels ranging from 2 to 7 times the pre-industrial CO2 level. The climate response is mainly analyzed in terms of changes in temperature, oceanic circulation, and ice-sheet behaviour. For the 4 x CO2 scenario, the climate response appears to be highly non-linear: abrupt transitions occur in the Southern Ocean deep water formation strength with a period of about 1200 yr. These millennial oscillations do not occur for both lower and larger CO2 levels. We show that these transitions are associated with internal oscillations of the Southern Ocean, triggered by the Antarctic freshwater budget. We first analyse the oscillatory mechanism. Secondly, through a series of 420 sensitivity experiments we also explore the range of temperature and freshwater flux for which such oscillations can be triggered. (C) 2010 Elsevier B.V. All rights reserved.
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Alvarez-Solas, J., Montoya, M., Ritz, C., Ramstein, G., Charbit, S., Dumas, C., et al. (2011). Heinrich event 1: an example of dynamical ice-sheet reaction to oceanic changes. Climate Of The Past, 7(4), 1297–1306.
Abstract: Heinrich events, identified as enhanced ice-rafted detritus (IRD) in North Atlantic deep sea sediments (Heinrich, 1988; Hemming, 2004) have classically been attributed to Laurentide ice-sheet (LIS) instabilities (MacAyeal, 1993; Calov et al., 2002; Hulbe et al., 2004) and assumed to lead to important disruptions of the Atlantic meridional overturning circulation (AMOC) and North Atlantic deep water (NADW) formation. However, recent paleoclimate data have revealed that most of these events probably occurred after the AMOC had already slowed down or/and NADW largely collapsed, within about a thousand years (Hall et al., 2006; Hemming, 2004; Jonkers et al., 2010; Roche et al., 2004), implying that the initial AMOC reduction could not have been caused by the Heinrich events themselves. Here we propose an alternative driving mechanism, specifically for Heinrich event 1 (H1; 18 to 15 ka BP), by which North Atlantic ocean circulation changes are found to have strong impacts on LIS dynamics. By combining simulations with a coupled climate model and a three-dimensional ice sheet model, our study illustrates how reduced NADW and AMOC weakening lead to a subsurface warming in the Nordic and Labrador Seas resulting in rapid melting of the Hudson Strait and Labrador ice shelves. Lack of buttressing by the ice shelves implies a substantial ice-stream acceleration, enhanced ice-discharge and sea level rise, with peak values 500-1500 yr after the initial AMOC reduction. Our scenario modifies the previous paradigm of H1 by solving the paradox of its occurrence during a cold surface period, and highlights the importance of taking into account the effects of oceanic circulation on ice-sheets dynamics in order to elucidate the triggering mechanism of Heinrich events.
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Masson-Delmotte, V., Buiron, D., Ekaykin, A., Frezzotti, M., Gallee, H., Jouzel, J., et al. (2011). A comparison of the present and last interglacial periods in six Antarctic ice cores. Climate Of The Past, 7(2), 397–423.
Abstract: We compare the present and last interglacial periods as recorded in Antarctic water stable isotope records now available at various temporal resolutions from six East Antarctic ice cores: Vostok, Taylor Dome, EPICA Dome C (EDC), EPICA Dronning Maud Land (EDML), Dome Fuji and the recent TALDICE ice core from Talos Dome. We first review the different modern site characteristics in terms of ice flow, meteorological conditions, precipitation intermittency and moisture origin, as depicted by meteorological data, atmospheric reanalyses and Lagrangian moisture source diagnostics. These different factors can indeed alter the relationships between temperature and water stable isotopes. Using five records with sufficient resolution on the EDC3 age scale, common features are quantified through principal component analyses. Consistent with instrumental records and atmospheric model results, the ice core data depict rather coherent and homogenous patterns in East Antarctica during the last two interglacials. Across the East Antarctic plateau, regional differences, with respect to the common East Antarctic signal, appear to have similar patterns during the current and last interglacials. We identify two abrupt shifts in isotopic records during the glacial inception at TALDICE and EDML, likely caused by regional sea ice expansion. These regional differences are discussed in terms of moisture origin and in terms of past changes in local elevation histories, which are compared to ice sheet model results. Our results suggest that elevation changes may contribute significantly to inter-site differences. These elevation changes may be underestimated by current ice sheet models.
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Stenni, B., Buiron, D., Frezzotti, M., Albani, S., Barbante, C., Bard, E., et al. (2011). Expression of the bipolar see-saw in Antarctic climate records during the last deglaciation. Nat. Geosci., 4(1), 46–49.
Abstract: Ice-core records of climate from Greenland and Antarctica show asynchronous temperature variations on millennial timescales during the last glacial period(1). The warming during the transition from glacial to interglacial conditions was markedly different between the hemispheres, a pattern attributed to the thermal bipolar see-saw(2). However, a record from the Ross Sea sector of East Antarctica has been suggested to be synchronous with Northern Hemisphere climate change(3). Here we present a temperature record from the Talos Dome ice core, also located in the Ross Sea sector. We compare our record with ice-core analyses from Greenland, based on methane synchronization(4), and find clearly asynchronous temperature changes during the deglaciation. We also find distinct differences in Antarctic records, pointing to differences in the climate evolution of the Indo-Pacific and Atlantic sectors of Antarctica. In the Atlantic sector, we find that the rate of warming slowed between 16,000 and 14,500 years ago, parallel with the deceleration of the rise in atmospheric carbon dioxide concentrations and with a slight cooling over Greenland. In addition, our chronology supports the hypothesis that the cooling of the Antarctic Cold Reversal is synchronous with the Bolling-Allerod warming in the northern hemisphere 14,700 years ago(5).
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2010 |
Albergel, C., Rudiger, C., Calvet, J. C., Carrer, D., Fritz, N. E. D., & Pellarin, T. (2010). Use of in-situ soil moisture measurements to evaluate microwave remote sensing products in south-western France.
Abstract: A long term data acquisition effort of profile soil moisture is under way in southwestern France at 12 automated weather stations. In this paper, both those in situ measurements and a synthetic data set covering continental France are used to test a simple method to retrieve root zone soil moisture from a time series of surface soil moisture information. A recursive exponential filter using a time constant, T, is used to compute a soil water index. A unique T value equal to 6 days, estimated after comparing the retrieved soil water index with the observed values at a depth of 30 cm, presents a satisfactory correlation for all stations. The surface soil moisture observed at 5 cm is used to evaluate the normalized surface soil moisture estimates derived from coarse resolution (25 km) active microwave data of the ASCAT C-band scatterometer instrument (onboard METOP) for a period of 6 months (April-September) in 2007. For 10 stations, significant correlation levels are found for either the whole pooled data set or for a part of them function of the localisation. The best correlation between a soil water index derived from ASCAT and the in-situ observations is obtained for a T-value of 14 days.
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Alvarez-Solas, J., Charbit, S., Ritz, C., Paillard, D., Ramstein, G., & Dumas, C. (2010). Links between ocean temperature and iceberg discharge during Heinrich events. Nat. Geosci., 3(2), 122–126.
Abstract: Palaeoclimate records have revealed the presence of millennial-scale climate oscillations throughout the last glacial period(1). Six periods of extreme cooling in the Northern Hemisphere-known as Heinrich events-were marked by an enhanced discharge of icebergs into the North Atlantic Ocean(2,3), increasing the deposition of ice-rafted debris(2). Increased sliding at the base of ice sheets as a result of basal warming has been proposed to explain the iceberg pulses(4-6), but recent observations(7,8) suggest that iceberg discharge is related to a strong coupling between ice sheets, ice shelves and ocean conditions. Here we use a conceptual numerical model to simulate the effect of ocean temperature on ice-shelf width, as well as the impact of the resultant changes in ice-shelf geometry on ice-stream velocities. Our results demonstrate that ocean temperature oscillations affect the basal melting of the ice shelf and will generate periodic pulses of iceberg discharge in an ice sheet with a fringing shelf. We also find that the irregular occurrence of Heinrich events seen in the palaeoclimate records can be simulated by periodic ocean forcing combined with varying accumulation rates of the ice sheet. Our model simulations support a link between millennial-scale ocean temperature variability and Heinrich events during the last glacial period.
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Calov, R., Greve, R., Abe-Ouchi, A., Bueler, E., Huybrechts, P., Johnson, J. V., et al. (2010). Results from the Ice-Sheet Model Intercomparison Project-Heinrich Event INtercOmparison (ISMIP HEINO). J. Glaciol., 56(197), 371–383.
Abstract: Results from the Heinrich Event INtercOmparison (HEINO) topic of the Ice-Sheet Model Intercomparison Project (ISMIP) are presented. ISMIP HEINO was designed to explore internal large-scale ice-sheet instabilities in different contemporary ice-sheet models. These instabilities are of interest because they are a possible cause of Heinrich events. A simplified geometry experiment reproduces the main characteristics of the Laurentide ice sheet, including the sedimented region over Hudson Bay and Hudson Strait. The model experiments include a standard run plus seven variations. Nine dynamic/thermodynamic ice-sheet models were investigated; one of these models contains a combination of the shallow-shelf (SSA) and shallow-ice approximation (SIA), while the remaining eight models are of SIA type only. Seven models, including the SIA-SSA model, exhibit oscillatory surges with a period of similar to 1000 years for a broad range of parameters, while two models remain in a permanent state of streaming for most parameter settings. In a number of models, the oscillations disappear for high surface temperatures, strong snowfall and small sediment sliding parameters. In turn, low surface temperatures and low snowfall are favourable for the ice-surge cycles. We conclude that further improvement of ice-sheet models is crucial for adequate, robust simulations of cyclic large-scale instabilities.
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Lemieux-Dudon, B., Blayo, E., Petit, J. R., Waelbroeck, C., Svensson, A., Ritz, C., et al. (2010). Consistent dating for Antarctic and Greenland ice cores. Quat. Sci. Rev., 29(1-2), 8–20.
Abstract: We are hereby presenting a new dating method based on inverse techniques, which aims at calculating consistent gas and ice chronologies for several ice cores. The proposed method yields new dating scenarios simultaneously for several cores by making a compromise between the chronological information brought by glaciological modeling (i.e.. ice flow model, firn densification model, accumulation rate model), and by gas and ice stratigraphic constraints. This method enables us to gather widespread chronological information and to use regional or global markers (i.e., methane, volcanic sulfate. Beryllium-10, tephra layers, etc.) to link the core chronologies stratigraphically. Confidence intervals of the new dating scenarios can be calculated thanks to the probabilistic formulation of the new method, which takes into account both modeling and data uncertainties. We apply this method simultaneously to one Greenland (NGRIP) and three Antarctic (EPICA Dome C, EPICA Dronning Maud Land, and Vostok) ices cores, and refine existent chronologies. Our results show that consistent ice and gas chronologies can be derived for depth intervals that are well-constrained by relevant glaciological data. In particular, we propose new and consistent dating of the last deglaciation for Greenland and Antarctic ice and gas records. (C) 2009 Elsevier Ltd. All rights reserved.
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Ma, Y., Gagliardini, O., Ritz, C., Gillet-Chaulet, F., Durand, G., & Montagnat, M. (2010). Enhancement factors for grounded ice and ice shelves inferred from an anisotropic ice-flow model. J. Glaciol., 56(199), 805–812.
Abstract: Polar ice is known to be one of the most anisotropic natural materials. For a given fabric the polycrystal viscous response is strongly dependent on the actual state of stress and strain rate. Within an ice sheet, grounded-ice parts and ice shelves have completely different stress regimes, so one should expect completely different impacts of ice anisotropy on the flow. The aim of this work is to quantify, through the concept of enhancement factors, the influence of ice anisotropy on the flow of grounded ice and ice shelves. For this purpose, a full-Stokes anisotropic marine ice-sheet flowline model is used to compare isotropic and anisotropic diagnostic velocity fields on a fixed geometry. From these full-Stokes results, we propose a definition of enhancement factors for grounded ice and ice shelves, coherent with the asymptotic models used for these regions. We then estimate realistic values for the enhancement factors induced by ice anisotropy for grounded ice and ice shelves.
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Pol, K., Masson-Delmotte, V., Johnsen, S., Bigler, M., Cattani, O., Durand, G., et al. (2010). New MIS 19 EPICA Dome C high resolution deuterium data: Hints for a problematic preservation of climate variability at sub-millennial scale in the “oldest ice”. Earth Planet. Sci. Lett., 298(1-2), 95–103.
Abstract: Marine Isotope Stage 19 (MIS 19) is the oldest interglacial period archived in the EPICA Dome C ice core (similar to 780 ky BP) and the closest “orbital analogue” to the Holocene – albeit with a different obliquity amplitude and phase with precession. New detailed deuterium measurements have been conducted with a depth resolution of 11 cm (corresponding time resolution of similar to 130 years). They confirm our earlier low resolution profile (55 cm), showing a relatively smooth shape over the MIS 20 to MIS 18 time period with a lack of sub-millennial climate variability, first thought to be due to this low resolution. The MIS 19 high resolution profile actually reveals a strong isotopic diffusion process leading to a diffusion length of at least similar to 40 cm erasing sub-millennial climate variability. We suggest that this diffusion is caused by water-veins associated with large ice crystals at temperatures above -10 degrees C, temperature conditions in which the MIS 19 ice has spent more than 200 Icy. This result has implications for the selection of the future “oldest ice” drilling site. (C) 2010 Elsevier B.V. All rights reserved.
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2009 |
Colleoni, F., Krinner, G., Jakobsson, M., Peyaud, V., & Ritz, C. (2009). Influence of regional parameters on the surface mass balance of the Eurasian ice sheet during the peak Saalian (140 kya). Glob. Planet. Change, 68(1-2), 132–148.
Abstract: Recent geologically-based reconstructions of the Eurasian ice sheet show that during the peak Saalian (approximate to 140 kya) the ice sheet was larger over Eurasia than during the Last Glacial Maximum (LGM) at approximate to 21 kya. To address this problem we use the LMDZ4 atmospheric general circulation model to evaluate the impact on the Saalian ice sheet's surface mass balance (SMB) from proglacial lakes. dust deposition on snow, vegetation and sea surface temperatures (SST) since geological records suggest that these environmental parameters were different during the two glacial periods. Seven model simulations have been carried out. Dust deposition decreases the mean SMB by intensifying surface melt during summer while proglacial lakes cool the summer climate and reduce surface melt on the ice sheet. A simulation including both proglacial lakes and dust shows that the presence of the former parameter reduces the impact of the latter, in particular, during summer. A switch from needle-leaf to tundra vegetation affects the regional climate but not enough to significantly influence the SMB of the nearby ice margin. However, a steady-state vegetation in equilibrium with the climate should be computed to improve the boundary conditions for further evaluations of the vegetation impact on the ice sheet's SMB. Finally, changes of the SST broadly affect the regional climate with significant consequences for the SMB. (C) 2009 Elsevier B.V. All rights reserved.
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2008 |
Albergel, C., Rudiger, C., Pellarin, T., Calvet, J. C., Fritz, N., Froissard, F., et al. (2008). From near-surface to root-zone soil moisture using an exponential filter: an assessment of the method based on in-situ observations and model simulations. Hydrology And Earth System Sciences, 12(6), 1323–1337.
Abstract: A long term data acquisition effort of profile soil moisture is under way in southwestern France at 13 automated weather stations. This ground network was developed in order to validate remote sensing and model soil moisture estimates. In this paper, both those in situ observations and a synthetic data set covering continental France are used to test a simple method to retrieve root zone soil moisture from a time series of surface soil moisture information. A recursive exponential filter equation using a time constant, T, is used to compute a soil water index. The Nash and Sutcliff coefficient is used as a criterion to optimise the T parameter for each ground station and for each model pixel of the synthetic data set. In general, the soil water indices derived from the surface soil moisture observations and simulations agree well with the reference root-zone soil moisture. Overall, the results show the potential of the exponential filter equation and of its recursive formulation to derive a soil water index from surface soil moisture estimates. This paper further investigates the correlation of the time scale parameter T with soil properties and climate conditions. While no significant relationship could be determined between T and the main soil properties ( clay and sand fractions, bulk density and organic matter content), the modelled spatial variability and the observed inter-annual variability of T suggest that a weak climate effect may exist.
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2007 |
Charbit, S., Ritz, C., Philippon, G., Peyaud, V., & Kageyama, M. (2007). Numerical reconstructions of the Northern Hemisphere ice sheets through the last glacial-interglacial cycle. Clim. Past., 3(1), 15–37.
Abstract: A 3-dimensional thermo-mechanical ice-sheet model is used to simulate the evolution of the Northern Hemisphere ice sheets through the last glacial-interglacial cycle. The ice-sheet model is forced by the results from six different atmospheric general circulation models (AGCMs). The climate evolution over the period under study is reconstructed using two climate equilibrium simulations performed for the Last Glacial Maximum (LGM) and for the present-day periods and an interpolation through time between these snapshots using a glacial index calibrated against the GRIP delta O-18 record. Since it is driven by the timing of the GRIP signal, the temporal evolution of the ice volume and the ice-covered area is approximately the same from one simulation to the other. However, both ice volume curves and spatial distributions of the ice sheets present some major differences from one AGCM forcing to the other. The origin of these differences, which are most visible in the maximum amplitude of the ice volume, is analyzed in terms of differences in climate forcing. This analysis allows for a partial evaluation of the ability of GCMs to simulate climates consistent with the reconstructions of past ice sheets. Although some models properly reproduce the advance or retreat of ice sheets in some specific areas, none of them is able to reproduce both North American or Eurasian ice complexes in full agreement with observed sea-level variations and geological data. These deviations can be attributed to shortcomings in the climate forcing and in the LGM ice-sheet reconstruction used as a boundary condition for GCM runs, but also to missing processes in the ice-sheet model itself.
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Parrenin, F., Dreyfus, G., Durand, G., Fujita, S., Gagliardini, O., Gillet, F., et al. (2007). 1-D-ice flow modelling at EPICA Dome C and Dome Fuji, East Antarctica. Clim. Past., 3(2), 243–259.
Abstract: One-dimensional ( 1-D) ice flow models are used to construct the age scales at the Dome C and Dome Fuji drilling sites ( East Antarctica). The poorly constrained glaciological parameters at each site are recovered by fitting independent age markers identified within each core. We reconstruct past accumulation rates, that are larger than those modelled using the classical vapour saturation pressure relationship during glacial periods by up to a factor 1.5. During the Early Holocene, changes in reconstructed accumulation are not linearly related to changes in ice isotopic composition. A simple model of past elevation changes is developed and shows an amplitude variation of 110-120m at both sites. We suggest that there is basal melting at Dome C ( 0.56 +/- 0.19 mm/yr). The reconstructed velocity profile is highly non-linear at both sites, which suggests complex ice flow effects. This induces a non-linear thinning function in both drilling sites, which is also characterized by bumps corresponding to variations in ice thickness with time.
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Peyaud, V., Ritz, C., & Krinner, G. (2007). Modelling the Early Weichselian Eurasian Ice Sheets: role of ice shelves and influence of ice-dammed lakes. Clim. Past., 3(3), 375–386.
Abstract: During the last glaciation, a marine ice sheet repeatedly appeared in Eurasia. The floating part of this ice sheet was essential to its rapid extension over the seas. During the earliest stage (90 kyr BP), large ice-dammed lakes formed south of the ice sheet. These lakes are believed to have cooled the climate at the margin of the ice. Using an ice sheet model, we investigated the role of ice shelves during the inception and the influence of ice-dammed lakes on the ice sheet evolution. Inception in Barents sea seems due to thickening of a large ice shelf. We observe a substantial impact of the lakes on the evolution of the ice sheets. Reduced summer ablation enhances ice extent and thickness, and the deglaciation is delayed by 2000 years.
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2006 |
Barbante, C., Barnola, J. M., Becagli, S., Beer, J., Bigler, M., Boutron, C., et al. (2006). One-to-one coupling of glacial climate variability in Greenland and Antarctica. Nature, 444(7116), 195–198.
Abstract: Precise knowledge of the phase relationship between climate changes in the two hemispheres is a key for understanding the Earth's climate dynamics. For the last glacial period, ice core studies(1,2) have revealed strong coupling of the largest millennial-scale warm events in Antarctica with the longest Dansgaard – Oeschger events in Greenland(3-5) through the Atlantic meridional overturning circulation(6-8). It has been unclear, however, whether the shorter Dansgaard – Oeschger events have counterparts in the shorter and less prominent Antarctic temperature variations, and whether these events are linked by the same mechanism. Here we present a glacial climate record derived from an ice core from Dronning Maud Land, Antarctica, which represents South Atlantic climate at a resolution comparable with the Greenland ice core records. After methane synchronization with an ice core from North Greenland(9), the oxygen isotope record from the Dronning Maud Land ice core shows a one-to-one coupling between all Antarctic warm events and Greenland Dansgaard – Oeschger events by the bipolar seesaw(6). The amplitude of the Antarctic warm events is found to be linearly dependent on the duration of the concurrent stadial in the North, suggesting that they all result from a similar reduction in the meridional overturning circulation.
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Durand, G., Weiss, J., Lipenkov, V., Barnola, J. M., Krinner, G., Parrenin, F., et al. (2006). Effect of impurities on grain growth in cold ice sheets. J. Geophys. Res.-Earth Surf., 111(F1), 18 pp.
Abstract: [1] On the basis of a detailed study of the ice microstructure of the European Project for Ice Coring in Antarctica (EPICA) ice core at Dome Concordia, Antarctica, we analyze the effect of impurities (solubles, and insolubles, that is, dust particles) on the grain growth process in cold ice sheets. As a general trend, the average grain size increases with depth. This global increase, induced by the normal grain growth process, is punctuated by several sharp decreases that can be associated with glacial-interglacial climatic transitions. To explain the modifications of the microstructure with climatic changes, we discuss the role of soluble and insoluble impurities on the grain growth process, coupled with an analysis of the pinning of grain boundaries by microparticles. Our data indicate that high soluble impurity content does not necessarily imply a slowdown of grain growth kinetics, whereas the pinning of grain boundaries by dust explains all the observed modifications of the microstructure. We propose a numerical model of the evolution of the average grain size in deep ice cores that takes into account recrystallization processes such as normal grain growth and rotation recrystallization as well as the pinning effect induced by dust particles, bubbles, and clathrates on the grain boundaries. Applied to the first 2135 m of the Dome Concordia core, the model reproduces accurately the measured mean grain radius. This indicates a major role of dust in the modification of polar ice microstructure and shows that the average grain size is not a true paleothermometer, as it is correlated with climatic transitions through the dust content of the ice.
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Masson-Delmotte, V., Kageyama, M., Braconnot, P., Charbit, S., Krinner, G., Ritz, C., et al. (2006). Past and future polar amplification of climate change: climate model intercomparisons and ice-core constraints. Clim. Dyn., 26(5), 513–529.
Abstract: Climate model simulations available from the PMIP1, PMIP2 and CMIP (IPCC-AR4) intercomparison projects for past and future climate change simulations are examined in terms of polar temperature changes in comparison to global temperature changes and with respect to pre-industrial reference simulations. For the mid-Holocene (MH, 6,000 years ago), the models are forced by changes in the Earth's orbital parameters. The MH PMIP1 atmosphere-only simulations conducted with sea surface temperatures fixed to modern conditions show no MH consistent response for the poles, whereas the new PMIP2 coupled atmosphere-ocean climate models systematically simulate a significant MH warming both for Greenland (but smaller than ice-core based estimates) and Antarctica (consistent with the range of ice-core based range). In both PMIP1 and PMIP2, the MH annual mean changes in global temperature are negligible, consistent with the MH orbital forcing. The simulated last glacial maximum (LGM, 21,000 years ago) to pre-industrial change in global mean temperature ranges between 3 and 7 degrees C in PMIP1 and PMIP2 model runs, similar to the range of temperature change expected from a quadrupling of atmospheric CO2 concentrations in the CMIP simulations. Both LGM and future climate simulations are associated with a polar amplification of climate change. The range of glacial polar amplification in Greenland is strongly dependent on the ice sheet elevation changes prescribed to the climate models. All PMIP2 simulations systematically underestimate the reconstructed glacial-interglacial Greenland temperature change, while some of the simulations do capture the reconstructed glacial-interglacial Antarctic temperature change. Uncertainties in the prescribed central ice cap elevation cannot account for the temperature change underestimation by climate models. The variety of climate model sensitivities enables the exploration of the relative changes in polar temperature with respect to changes in global temperatures. Simulated changes of polar temperatures are strongly related to changes in simulated global temperatures for both future and LGM climates, confirming that ice-core-based reconstructions provide quantitative insights on global climate changes.
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Philippon, G., Ramstein, G., Charbit, S., Kageyama, M., Ritz, C., & Dumas, C. (2006). Evolution of the Antarctic ice sheet throughout the last deglaciation: A study with a new coupled climate – north and south hemisphere ice sheet model. Earth Planet. Sci. Lett., 248(3-4), 750–758.
Abstract: The aim of this paper is to assess, through the understanding of deglaciation processes, the contribution of the Antarctic ice sheet to sea-level rise during the last deglaciation. To achieve this goal, we use an Earth System model in which the interactions between the atmosphere, the ocean, the vegetation and the northern and Antarctic ice sheets are represented. This new tool allows the simulation of the evolution of the Antarctic ice volume, which starts to decrease at around 15 ka. At the end of deglaciation, the melting of the Antarctic ice sheet contributes to an ice-equivalent sea-level rise of 9.5 m in the standard experiment and 17.5 m in a more realistic sensitivity experiment accounting for a different bathymetry in the Weddell Sea which succeeds in producing both major ice shelves (Ross and Ronne-Filchner). In both experiments, the melting of all ice sheets contributes to 121.5 m and 129.5 m, respectively, which is very consistent with data. The new coupled model provides a timing and amplitude of the Antarctic deglaciation different from those previously obtained by prescribing the temperature record from the Vostok Antarctic ice core (78 degrees 27'S 106 degrees 52'E) as a uniform temperature forcing. Sensitivity experiments have also been performed to analyse the impact of the parameters at the origin of the deglaciation process: insolation changes, atmospheric CO2 variation, basal melting and sea-level rise. All those parameters have an influence on the timing of the deglaciation. The prescribed global sea level rise is shown to be a major forcing factor for the evolution of the Antarctic ice volume during the last deglaciation. We quantify the direct effect of the sea-level rise due to the northern hemisphere ice sheet melting on the grounding line retreat which, in turn, favours enhancement of grounded ice flow by lowering the buttressing effect of ice shelves. (c) 2006 Elsevier B.V. All rights reserved.
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2005 |
Charbit, S., Kageyama, M., Roche, D., Ritz, C., & Ramstein, G. (2005). Investigating the mechanisms leading to the deglaciation of past continental northern hemisphere ice sheets with the CLIMBER-GREMLINS coupled model. Glob. Planet. Change, 48(4), 253–273.
Abstract: A coupling procedure between a climate model of intermediate complexity (CLIMBER-2.3) and a 3-dimensional thermomechanical ice-sheet model (GREMLINS) has been elaborated. The resulting coupled model describes the evolution of atmosphere, ocean, biosphere, cryosphere and their Mutual interactions. It is used to perform several simulations of the Last Deglaciation period to identify the physical mechanisms at the origin of the deglaciation process. Our baseline experiment, forced by insolation and atmospheric CO2, produces almost complete deglaciation of past northern hemisphere continental ice sheets, although ice remains over the Cordilleran region at the end of the Simulation and also in Alaska and Eastern Siberia. Results clearly demonstrate that, in this Study, the melting of the North American ice sheet is critically dependent on the deglaciation of Fennoscandia through processes involving switches of the thermohaline circulation from a glacial mode to a modern one and associated warming of the northern hemisphere. A set of sensitivity experiments has been carried out to test the relative importance of both forcing factors and internal processes in the deglaciation mechanism. It appears that the deglaciation is primarily driven by insolation. However, the atmospheric CO2 modulates the timing of the melting of the Fennoscandian ice sheet, and results relative to Laurentide illustrate the existence of threshold CO2 values, that can be translated in terms of critical temperature, below which the deglaciation is impeded. Finally, we show that the beginning of the deglaciation process of the Laurentide ice sheet may be influenced by the time at which the shift of the thermohaline circulation from one mode to the other occurs. (c) 2005 Elsevier B.V. All rights reserved.
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Lhomme, N., Clarke, G. K. C., & Ritz, C. (2005). Global budget of water isotopes inferred from polar ice sheets. Geophys. Res. Lett., 32(20), 4 pp.
Abstract: Water isotope ratios in ice cores and marine sediments are a key indicator of past temperature and global ice volume. Quantitative interpretation of these ratios requires understanding of the storage capacity and exchanges among the ocean, atmosphere, and cryosphere. We combine numerical models of ice dynamics and tracer transport to predict bulk ice properties by simulating the fine layering of ice sheets locally validated at ice core sites. The O-18/O-16 content of ice sheets is found to vary between the present and 20 kyr ago from -34 parts per thousand to -37 parts per thousand for Greenland, from -41 parts per thousand to -42.5 parts per thousand for West Antarctica, and always remained near -56.5 parts per thousand for East Antarctica. Their combined effect on sea-water O-18/O-16 is a 0.08-0.12 parts per thousand increase 20 kyr ago, a 1.11 parts per thousand decrease if ice sheets were to vanish. We confirm that ice volume changes in Antarctica and Greenland linearly affect ocean composition, though at different rates.
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2004 |
Andersen, K. K., Azuma, N., Barnola, J. M., Bigler, M., Biscaye, P., Caillon, N., et al. (2004). High-resolution record of Northern Hemisphere climate extending into the last interglacial period. Nature, 431(7005), 147–151.
Abstract: Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 degreesC warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.
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Augustin, L., Barbante, C., Barnes, P. R. F., Barnola, J. M., Bigler, M., Castellano, E., et al. (2004). Eight glacial cycles from an Antarctic ice core. Nature, 429(6992), 623–628.
Abstract: The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago ( Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long – 28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.
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Kageyama, M., Charbit, S., Ritz, C., Khodri, M., & Ramstein, G. (2004). Quantifying ice-sheet feedbacks during the last glacial inception. Geophys. Res. Lett., 31(24), 4 pp.
Abstract: The last glacial inception (similar to116 ky ago) has long been used to test the sensitivity of climate models to insolation. From these simulations, atmospheric, oceanic and vegetation feedbacks have been shown to amplify the initial insolation signal into a rapid growth of ice-sheets over the northern hemisphere. However, due to the lack of comprehensive atmosphere-ocean-vegetation-northern hemisphere ice-sheet models, the impact of all these feedbacks acting concurrently has not yet been evaluated. Here we present the results from such a model, which simulates significant ice-sheet growth over North America, but none over Eurasia. Our analyses focus on the different behaviours over these regions, and the quantification of the ice-sheet feedbacks on climate.
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Krinner, G., Mangerud, J., Jakobsson, M., Crucifix, M., Ritz, C., & Svendsen, J. I. (2004). Enhanced ice sheet growth in Eurasia owing to adjacent ice-dammed lakes. Nature, 427(6973), 429–432.
Abstract: Large proglacial lakes cool regional summer climate because of their large heat capacity, and have been shown to modify precipitation through mesoscale atmospheric feedbacks, as in the case of Lake Agassiz(1). Several large ice-dammed lakes, with a combined area twice that of the Caspian Sea, were formed in northern Eurasia about 90,000 years ago, during the last glacial period when an ice sheet centred over the Barents and Kara seas(2) blocked the large northbound Russian rivers(3). Here we present high-resolution simulations with an atmospheric general circulation model that explicitly simulates the surface mass balance of the ice sheet. We show that the main influence of the Eurasian proglacial lakes was a significant reduction of ice sheet melting at the southern margin of the Barents – Kara ice sheet through strong regional summer cooling over large parts of Russia. In our simulations, the summer melt reduction clearly outweighs lake-induced decreases in moisture and hence snowfall, such as has been reported earlier for Lake Agassiz1. We conclude that the summer cooling mechanism from proglacial lakes accelerated ice sheet growth and delayed ice sheet decay in Eurasia and probably also in North America.
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Parrenin, F., Remy, F., Ritz, C., Siegert, M. J., & Jouzel, J. (2004). New modeling of the Vostok ice flow line and implication for the glaciological chronology of the Vostok ice core. J. Geophys. Res.-Atmos., 109(D20), 14 pp.
Abstract: [1] We have used new spaceborne ( elevation) and airborne ( ice thickness) data to constrain a 2D1/2 model of snow accumulation and ice flow along the Ridge B-Vostok station ice flow line ( East Antarctica). We show that new evaluations of the ice flow line geometry ( from the surface elevation), ice thickness ( from low-frequency radar data), and basal melting and sliding change significantly the chronology of the Vostok ice core. This new Vostok dating model reconciles orbital and glaciological timescales and is in good agreement with the Dome Fuji glaciological timescale. At the same time, the new model shows significantly older ages than the previous GT4 timescale for the last glacial part, being thus in better agreement with the GRIP and GISP2 chronologies.
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Rodgers, K. B., Charbit, S., Kageyama, M., Philippon, G., Ramstein, G., Ritz, C., et al. (2004). Sensitivity of Northern Hemispheric continental ice sheets to tropical SST during deglaciation. Geophys. Res. Lett., 31(2), 4 pp.
Abstract: A thermomechanical ice sheet model ( ISM) is used to investigate the sensitivity of the Laurentide and Fennoscandian ice sheets to tropical sea surface temperature (SST) perturbations during deglaciation. The ISM is driven by surface temperature and precipitation fields from three different atmospheric general circulation models (AGCMs). For each AGCM, the responses in temperature and precipitation over the ice sheets nearly compensate, such that ice sheet mass balance is not strongly sensitive to tropical SST boundary conditions. It was also found that there is significant variation in the response of the ISM to the different AGCM output fields.
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2003 |
Donnadieu, Y., Fluteau, F., Ramstein, G., Ritz, C., & Besse, J. (2003). Is there a conflict between the Neoproterozoic glacial deposits and the snowball Earth interpretation: an improved understanding with numerical modeling. Earth Planet. Sci. Lett., 208(1-2), 101–112.
Abstract: The behavior of the terrestrial glacial regime during the Neoproterozoic glaciations is still a matter of debate. Some papers claim that the glacial sequences cannot be explained with the snowball Earth scenario. Indeed, the near shutdown of the hydrological cycle simulated by climatic models, once the Earth is entirely glaciated, has been put in contrast with the need for active, wet-based continental ice sheets to produce the observed thick glacial deposits. A climate ice-sheet model is applied to the older extreme Neoproterozoic glaciation (around 750 Ma) with a realistic paleogeographic reconstruction of Rodinia. Our climate model shows that a small quantity of precipitation remains once the ocean is completely ice-covered, thanks to sublimation processes over the sea-ice at low latitudes acting as a water vapor source. After 10 ka of the ice-sheet model, the ice volume in the tropics is small and confined as separate ice caps on coastal areas where water vapor condenses. However, after 180 ka, large ice sheets can extend over most of the supercontinent Rodinia. Several areas of basal melting appear while ice sheets reach their ice-volume equilibrium state, at 400 ka, they are located either under the two single-domed ice sheets covering the Antarctica and the Laurentia cratons, or near the ice-sheet margins where fast flow occurs. Only the isolated and high-latitude cratons stay cold-based. Finally, among the simulated ice sheets, most have a dynamic behavior, in good agreement with the needs inferred by the preserved thick formations of diamictite, and share the features of the Antarctica present-day ice sheet. Therefore, our conclusion is that a global glaciation would not have hindered the formation of the typical glacial structures seen everywhere in the rock record of Neoproterozoic times. (C) 2003 Elsevier Science B.V. All rights reserved.
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Goujon, C., Barnola, J. M., & Ritz, C. (2003). Modeling the densification of polar firn including heat diffusion: Application to close-off characteristics and gas isotopic fractionation for Antarctica and Greenland sites. J. Geophys. Res.-Atmos., 108(D24), 18 pp.
Abstract: Modeling the densification of polar firn is essential to estimate variations of close-off characteristics (density, close-off depth, delta age) in relation with past climate parameters (temperature and accumulation rates). Furthermore, the air composition in the firn is modified by gravitational and thermal fractionation, and stable isotope measurements of permanent gases like nitrogen or argon can provide information on the amplitude of these fractionations. In this work, we propose a new model coupling firn densification and heat diffusion. In addition to the determination of the firn thickness and gas-ice age differences, the model allows a reconstruction of the time evolution of the temperature for different sites in Antarctica (Vostok) and Greenland (GISP2) and therefore the evolution of gravitational and thermal isotopic fractionations in firn. Under present-day conditions, the modeled profiles are in good agreement with the available temperature measurements in firn. For sites with low accumulation rates such as Vostok, the results show the existence of temperature gradients in the firn column even when no rapid climatic changes occur. The comparison of the modeled delta(15)N results to measurements allows to better constrain the delta(18)O-temperature relationship, used to infer the surface temperature history, and for GISP2, the model validates the long-term borehole-based temperature.
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2002 |
Charbit, S., Ritz, C., & Ramstein, G. (2002). Simulations of Northern Hemisphere ice-sheet retreat: sensitivity to physical mechanisms involved during the Last Deglaciation. Quat. Sci. Rev., 21(1-3), 243–265.
Abstract: A 3-dimensional thermomechanical ice-sheet model is used to simulate the evolution of the geometry of Northern Hemisphere ice sheets through the Last Deglaciation. The ice-sheet model is forced by a time-evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the LMD5.3 atmospheric general circulation model (AGCM) at different periods of the Last Deglaciation (21, 15, 9, 6 and 0 kyr BP). The AGCM is driven by insolation, atmospheric CO2 content, ice-sheet configuration and sea surface temperatures. Although our approach is able to produce the complete continental ice retreat, our simulated deglaciation presents a phase-lag with reconstructions based on observational evidences. This suggests that physical mechanisms related to climate forcing and/or ice-sheet internal dynamics are not properly represented. The influence of millennial-scale forcing, feedback mechanisms between ice-sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments. The rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature-mass balance relationships. Fennoscandia appears to be strongly sensitive to the small-scale ice-sheet instability. Both ice sheets are to some extent sensitive to an increased basal sliding. (C) 2001 Elsevier Science Ltd. All rights reserved.
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Fritzsche, D., Wilhelms, F., Savatyugin, L. M., Pinglot, J. F., Meyer, H., Hubberten, H. W., et al. (2002). A new deep ice core from Akademii Nauk ice cap, Severnaya Zemlya, Eurasian Arctic: first results (Vol. 35). Int Glaciological Soc.
Abstract: The paper presents first results from the upper 54 m of a 723.91 in ice core drilled on Akademii Nauk ice cap, Severnaya Zemlya, Eurasian Arctic, in 1999-2001, supplemented by data from shallow ice cores. The glaciers peculiarity is the Infiltration and refreezing of meltwater, which changes the original isotopic and chemical signals. Therefore, stratigraphical observations in these ice cores are more difficult than in those from central Greenland or Antarctica. However, the 1963 maximum of artificial radioactivity from atmospheric nuclear tests is clearly detectable in the deep ice core, and the delta(18)O profile of a 12.82 m shallow core shows annual variations. Consequently, at least for the upper part of the main core, an almost seasonal time resolution of palaeoclimate record could be expected. The Chernobyl layer is detected by increased Cs-137 activity at depths of 11.81-12.51 in related to the AD 2000 surf ace. The resulting mean annual net mass balance is 53 +/- 2 g cm(-1). Data froin dielectric profiling oftlic rnain core show considerable peaks in conductivity; one of them is interpreted as a volcano event. According to the resulting chronology, this part of the core represents approximately the last 100 years.
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2001 |
Barrie, L. A., Yi, Y., Leaitch, W. R., Lohmann, U., Kasibhatla, P., Roelofs, G. J., et al. (2001). A comparison of large-scale atmospheric sulphate aerosol models (COSAM): overview and highlights. Tellus Ser. B-Chem. Phys. Meteorol., 53(5), 615–645.
Abstract: The comparison of large-scale sulphate aerosol models study (COSAM) compared the performance of atmospheric models with each other and observations. It involved: (i) design of a standard model experiment for the world wide web, (ii) 10 model simulations of the cycles of sulphur and Rn-222 Pb-210 conforming to the experimental design, (iii) assemblage of the best available observations of atmospheric SO4=, SO2 and MSA and (iv) a workshop in Halifax, Canada to analyze model performance and future model development needs. The analysis presented in this paper and two companion papers by Roelofs, and Lohmann and co-workers examines the variance between models and observations. discusses the sources of that variance and suggests ways to improve models. Variations between models in the export of SOx from Europe or North America are not sufficient to explain an order of magnitude variation in spatial distributions of SOx downwind in the northern hemisphere. On average, models predicted surface level seasonal mean SO4= aerosol mixing ratios better (most within 20%) than SO2 mixing ratios (over-prediction by factors of 2 or more). Results suggest that vertical mixing from the planetary boundary layer into the free troposphere in source regions is a major source of uncertainty in predicting the global distribution of SO aerosols in climate models today. For improvement. it is essential that globally coordinated research efforts continue to address emissions of all atmospheric species that affect the distribution and optical properties of ambient aerosols in models and that a global network of observations be established that will ultimately produce a world aerosol chemistry climatology.
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Parrenin, F., Jouzel, J., Waelbroeck, C., Ritz, C., & Barnola, J. M. (2001). Dating the Vostok ice core by an inverse method. J. Geophys. Res.-Atmos., 106(D23), 31837–31851.
Abstract: Using the chronological information available in the Vostok records, we apply an inverse method to assess the quality of the Vostok glaciological timescale. The inversion procedure provides not only an optimized glaciological timescale and its confidence interval but also a reliable estimate of the duration of successive events. Our results highlight a disagreement between orbitally tuned and glaciological timescales below similar to2700 m (i.e., similar to250 kyr B.P., thousands of years before present). This disagreement could be caused by some discontinuity in the spatial variation of accumulation upstream of Vostok. Moreover, the stratigraphic datings of central Greenland ice cores (GRIP and GISP2) appear older than our optimized timescale for the late glacial. This underlines an unconsistency between the physical assumptions used to construct the Vostok glaciological timescale and the stratigraphic datings. The inverse method allows the first assessment of the evolution of the phase between Vostok climatic records and insolation. This phase significantly varies with time which gives a measure of the nonlinear character of the climatic system and suggests that the climatic response to orbital forcing is of different nature for glacial and interglacial periods. We confirm that the last interglacial, as recorded in the Vostok deuterium record, was long (16.2 +/- 2 kyr, thousands of years). However, midtransition of termination 11 occurred at 133.4 +/- 2.5 kyr BP, which does not support the recent claim for an earlier deglaciation. Finally, our study suggests that temperature changes are correctly estimated when using the spatial present-day deuterium-temperature relationship to interpret the Vostok deuterium record.
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Ritz, C., Rommelaere, V., & Dumas, C. (2001). Modeling the evolution of Antarctic ice sheet over the last 420,000 years: Implications for altitude changes in the Vostok region. J. Geophys. Res.-Atmos., 106(D23), 31943–31964.
Abstract: A new thermomechanical three-dimensional model designed to simulate the evolution of the Antarctic ice sheet over long time periods is presented. This model incorporates the various types of ice flow found in Antarctica: relatively slow inland ice flow that is essentially due to ice deformation, fast ice flow in the regions with ice streams, and ice shelf flow. By coupling these three types of flow, it is possible to predict grounding line migration. Simulations covering four glacial-interglacial cycles have been conducted by forcing this model with a temperature record from Vostok and a sea level record from marine cores. Our findings indicate that grounding line migration induced by sea level changes is the primary factor governing the evolution of the Antarctic ice volume. On the other hand, the altitude of the ice sheet surface at Vostok is driven by accumulation rate variations. The amplitude of the altitude change does not exceed 150 m and is very similar for all the sites located on the Antarctic Plateau.
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2000 |
Payne, A. J., Huybrechts, P., Abe-Ouchi, A., Calov, R., Fastook, J. L., Greve, R., et al. (2000). Results from the EISMINT model intercomparison: the effects of thermomechanical coupling. J. Glaciol., 46(153), 227–238.
Abstract: This paper discusses results from the second phase of the European Ice Sheet Modelling Initiative (EISMINT). It reports the intercomparison of ten operational ice-sheet models and uses a series of experiments to examine the implications of thermomechanical coupling for model behaviour. A schematic, circular ice sheet is used in the work which investigates both steady states and the response to stepped changes in climate. The major finding is that the radial symmetry implied in the experimental design can, under certain circumstances, break down with the formation of distinct, regularly spaced spokes of cold ice which extended from the interior of the ice sheet outward to the surrounding zone of basal melt. These features also manifest themselves in the thickness and velocity distributions predicted by the models. They appear to be a common feature to all of the models which took part in the intercomparison, and may stem from interactions between ice temperature, flow and surface form. The exact nature of these features varies between models, and their existence appears to be controlled by the overall thermal regime of the ice sheet. A second result is that there is considerable agreement between the models in their predictions of global-scale response to imposed climate change.
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Rasch, P. J., Feichter, J., Law, K., Mahowald, N., Penner, J., Benkovitz, C., et al. (2000). A comparison of scavenging and deposition processes in global models: results from the WCRP Cambridge Workshop of 1995. Tellus Ser. B-Chem. Phys. Meteorol., 52(4), 1025–1056.
Abstract: We report on results from a World Climate Research Program workshop on representations of scavenging and deposition processes in global transport models of the atmosphere. 15 models were evaluated by comparing simulations of radon, lead, sulfur dioxide, and sulfate against each other, and against observations of these constituents. This paper provides a survey on the simulation differences between models. It identifies circumstances where models are consistent with observations or with each other, and where they differ from observations or with each other. The comparison shows that most models are able to simulate seasonal species concentrations near the surface over continental sites to within a factor of 2 over many regions of the globe. Models tend to agree more closely over source (continental) regions than for remote (polar and oceanic) regions. Model simulations differ most strongly in the upper troposphere for species undergoing wet scavenging processes. There are not a sufficient number of observations to characterize the climatology (long-term average) of species undergoing wet scavenging in the upper troposphere. This highlights the need for either a different strategy for model evaluation (e.g., comparisons on an event by event basis) or many more observations of a few carefully chosen constituents.
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1999 |
Brzoska, J. B., Lesaffre, B., Coleou, C., Xu, K., & Pieritz, R. A. (1999). Computation of 3D curvatures on a wet snow sample. European Physical Journal-Applied Physics, 7(1), 45–57.
Abstract: The map of 3D curvatures of a porous medium characterizes most of its capillary properties. A model for directly computing curvatures from a three-dimensional image of the solid matrix of a porous medium is presented. A precise distance map of the object is built using the “chamfer” distance of discrete geometry. The set of local maxima of the distance map is used for quick location of the normal to each point P of the object's surface. The normal being known, principal radii of curvature are computed in 2D and lead to 3D curvature. This model was validated on geometric shapes of known curvature, then applied on a natural snow sample. The snow image was obtained from a serial cut (performed in cold laboratory) observed under specularly reflected light. Views of both fresh and sublimated sections were taken for each of the 64 section planes: this allowed easier distinction between snow and filling medium and made possible automatic contouring of section plane images. Curvature maps computed from pore and grain phases respectively were found to be in excellent agreement for each tested object shape, including the snow sample.
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Calvet, J. C., Bessemoulin, P., Noilhan, J., Berne, C., Braud, I., Courault, D., et al. (1999). MUREX: a land-surface field experiment to study the annual cycle of the energy and water budgets. Annales Geophysicae-Atmospheres Hydrospheres And Space Sciences, 17(6), 838–854.
Abstract: The MUREX (monitoring the usable soil reservoir experimentally) experiment was designed to provide continuous time series of field data over a long period, in order to improve and validate the Soil-vegetation-Atmosphere Transfer (SVAT) parameterisations employed in meteorological models. Intensive measurements were performed for more than three years over fallow farmland in southwestern France. To capture the main processes controlling land-atmosphere exchanges, the local climate was fully characterised, and surface water and energy fluxes, vegetation biomass, soil moisture profiles, surface soil moisture and surface and soil temperature were monitored. Additional physiological measurements were carried out during selected periods to describe the biological control of the fluxes. The MUREX data of 1995, 1996, and 1997 are presented. Four SVAT models are applied to the annual cycle of 1995. In general, they succeed in simulating the main features of the fallow functioning, although some shortcomings are revealed.
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Netto, A. M., Pieritz, R. A., & Gaudet, J. P. (1999). Field study on the local variability of soil water content and solute concentration. Journal Of Hydrology, 215(1-4), 23–37.
Abstract: We characterized temporal and spatial variables describing hydraulic and solute transport in a heterogeneous soil. Under field conditions, bulk density, water content and concentration of three applied tracers (conservative Br-, nitrate NO3-, and a pesticide BENTAZON) were measured after destructive sampling and compared to continuous measurements. A comparison of destructive sampling and continuous measurements were used to investigate the held. Soil volumetric samples were obtained from trenches on a grid of 1.2 x 1.1 x 0.1 m in a fallow area of approximately 1300 m(2) to generate two dimensional (2D) interpolated maps. Bulk density, water content, and solute concentration data sets were evaluated by geostatistical analysis in order to choose the best method to build interpolations. Semivariogram analyses for volumetric water content (theta), bulk density (rho(d)) and solute concentrations (NO3- and BENTAZON) showed no autocorrelation possessing only a pure nugget effect. 2D interpolated maps of these soil parameters were constructed using the Radial Basis Function method that allowed calculation of vertical water content and solute concentration profiles and the associated mean (mu) and variance (sigma) distributions. Vertical neutron and TDR water content profiles were successfully simulated with a discrete geometry approach using the 2D interpolated maps. Destructive sampling for water content and continuous measurements using neutron and TDR probes, revealed consistent results. Comparison of solute concentration from destructive sampling and ceramic solution samplers is less satisfactory at this site owing to both the heterogeneity and the inadequacy of ceramic solution samplers to sample the total soil concentration. Finally, preferential flow pathways can be visualized on the 2D interpolated maps of the concentration of Br-, BENTAZON, and NO3-. The observed distribution of tracers provides clear evidence of the influence of both soil hydraulic properties and geochemical heterogeneities which must be incorporated to predict water and solute fluxes. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
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Petit, J. - R., Jouzel, J., Raynaud, D., Barkov, N. I., Barnola, J. - M., Basile, I., et al. (1999). Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature, 399(6735), 429–436.
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1998 |
Fabre, A., Ramstein, G., Ritz, C., Pinot, S., & Fournier, N. (1998). Coupling an AGCM with an ISM to investigate the ice sheets mass balance at the last glacial maximum. Geophysical Research Letters, 25(4), 531–534.
Abstract: The aim of this paper is to investigate the consistency between the ice sheets reconstruction at the LGM and the climate simulated by AGCM. In particular, to investigate whether the laurentian and the fennoscandian ice sheets have an equilibrated mass balance, we use two complementary approaches. First we analyze the changes of snow and water budgets over the ice sheets, only using the model results of LGM runs. Second, we use a thermomechanical ice sheet model [Ritz ei al., 1997] forced with the AGCM simulated climate, to perform long term runs at finer spatial resolution. Analyzing the results obtained with both approaches, we show that the ice sheet mass balance computed directly from AGCM results may be biaised. Moreover we show, using two versions of the LMD AGCM, that the ice sheet mass balance computed by the ISM is drastically sensitive to the summer surface temperatures simulated by the AGCM.
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1997 |
Fabre, A., Ritz, C., & Ramstein, G. (1997). Modelling of last glacial maximum ice sheets using different accumulation parameterizations. Annals of Glaciology, 24, 223–229.
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Jacob, D. J., Prather, M. J., Rasch, P. J., Shia, R. L., Balkanski, Y. J., Beagley, S. R., et al. (1997). Evaluation and intercomparison of global atmospheric transport models using Rn-222 and other short-lived tracers. Journal Of Geophysical Research-Atmospheres, 102(D5), 5953–5970.
Abstract: Simulations of Rn-222 and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean Rn-222 concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of Rn-222 concentrations in the upper troposphere, and none reproduces the high Rn-222 concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high- Rn-222 episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of Rn-222 in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.
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Ramstein, G., Fabre, A., Pinot, S., Ritz, C., & Joussaume, S. (1997). Ice-sheet mass balance during the last glacial maximum. Annals of Glaciology, 25, 145–152.
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Raynaud, D., Chappellaz, J., Ritz, C., & Martinerie, P. (1997). Air content along the Greenland Ice Core Project core: A record of surface climatic parameters and elevation in central Greenland. Journal Of Geophysical Research-Oceans, 102(C12), 26607–26613.
Abstract: We present here measurements of the air content of the ice, V, performed along the Greenland Ice Core Project (GRIP) ice core. The main features of the longterm trends are (1) a decrease of 13% between the last glacial maximum (LGM) and the earliest part of the Holocene, and (2) an increase of 8% during the Holocene. The results are discussed in terms of changes in atmospheric pressure, surface elevation and porosity at close-off. The V record contains a significant signal of past changes of surface elevation in qualitative agreement with ice sheet modeling simulations. It suggests a thickening of central Greenland during the transition from the LGM to the early Holocene, and a significant thinning through the Holocene period. It also stresses the large influence on past V variations of changes in ice porosity, which are not explained by the present-day spatial relationship with temperature and may reflect changes in other surface climatic parameters (like precipitation seasonality or wind stress). The potential role of temporal variations of atmospheric pressure patterns is also evaluated. Air content results in the GRIP ice older than 110 ka indicate values approximately in the same range as those observed during the last 40,000 years, with generally higher air content corresponding to isotopically warmer ice.
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1996 |
Huybrechts, P., Payne, T., Abe-Ouchi, A., Calov, R., Fabre, A., Fastook, J. L., et al. (1996). The EISMINT benchmarks for testing ice-sheet models. Annals of Glaciology, 23, 1–12.
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Jouzel, J., Waelbroeck, C., Malaize, B., Bender, M., Petit, J. R., Stievenard, M., et al. (1996). Climatic interpretation of the recently extended Vostok ice records. Climate Dynamics, 12(8), 513–521.
Abstract: A new ice core drilled at the Russian station of Vostok in Antarctica reached 2755 m depth in September 1993, At this depth, the glaciological time scale provides an age of 260 ky BP (+/-25). We refine this estimate using records of dust and deuterium in the ice and of delta(18)O of O-2 in the entrapped air. delta(18)O of O-2 is highly correlated with insolation over the last two climatic cycles if one assumes that the EGT chronology overestimates the increase of age with depth by 12% for ages older than 112 ky BP. This modified age-depth scale gives an age of 244 ky BP at 2755 m depth and agrees well with the age-depth scale of Walbroeck et al. (in press) derived by orbital tuning of the Vostok delta D record. We discuss the temperature interpretation of this latter record accounting for the influence of the origin of the ice and using information derived from deuterium-excess data. We conclude that the warmest period of stage 7 was likely as warm as today in Antarctica. A remarkable feature of the Vostok record is the high level of similarity of proxy temperature records for the last two climatic cycles (stages 6 and 7 versus stages 1-5). This similarity has no equivalent in other paleorecords.
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MacAyeal, D. R., Rommelaere, V., Huybrechts, P., Hulbe, C. L., Determan, J., & Ritz, C. (1996). An ice-shelf model test based on the Ross Ice Shelf, Antarctica. Annals of Glaciology, 23, 46–51.
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Rémy, F., Ritz, C., & Brisset, L. (1996). Ice-sheet flow features and rheological parameters derived from precise altimetric topography. Annals of Glaciology, 23, 277–283.
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Ritz, C., Fabre, A., & Letreguilly, A. (1996). Sensitivity of a Greenland ice sheet model to ice flow and ablation parameters: Consequences for the evolution through the last climatic cycle. Climate Dynamics, 13(1), 11–24.
Abstract: Sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the Greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice. Two types of experiments are performed: steady-state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the GRIP record. The experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters (deformation and sliding law coefficients, geothermal flux) and that it is low when the ice flow is fast. On the other hand, the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate. The ice sheet extension shows the opposite sensitivity: it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients. For colder climates, when there is no ablation, the ice sheet extension depends on the sea level. An interesting result is that the variations with time of the altitude at the ice divide (Summit) do not depend on the parameters we tested. The present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry. It seems that, compared to the parameter set most commonly used, higher ablation rate coefficients must be used.
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Rommelaere, V., & Ritz, C. (1996). Thermomechanical model of ice-shelf flow. Annals of Glaciology, 23, 13–20.
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Salamatin, A. N., & Ritz, C. (1996). A simplified multi-scale model for predicting climatic variations of the ice-sheet surface elevation in central Antarctica. Annals of Glaciology, 23, 28–35.
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1995 |
Abysov, S. S., de Angelis, M., Barkov, N. I., Barnola, J. - M., Bender, M., Chappellaz, J., et al. (1995). International effort helps decipher mysteries of paleoclimate from Antarctic ice cores. Eos, 76(17), 169 et 179.
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Fabre, A., Letreguilly, A., Ritz, C., & Mangeney, A. (1995). Greenland under changing climates : sensitivity experiments with a new three-dimensional ice-sheet model. Annals of Glaciology, 21, 1–7.
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Fabre, A., Letreguilly, A., Ritz, C., & Mangeney, A. (1995). Modeling Of The Greenland Ice Cap – Sensitivity Test And Development Experiment Using A New 3d Model. Houille Blanche-Revue Internationale De L Eau, 50(7), 94–99.
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1994 |
Boaretto, E., Berkovits, D., Delmas, R., Johnson, R., Kaufman, A., Magaritz, M., et al. (1994). Measurements Of Anthropogenic Radionuclides In Environmental-Samples. Nuclear Instruments & Methods In Physics Research Section B-Beam Interactions With Materials And Atoms, 92(1-4), 350–356.
Abstract: Measurements of anthropogenic I-129 and Ca-41 in two Greenland firn cores spanning the years 1935 to 1989 have been performed by accelerator mass spectrometry. The two sets of data points measured for I-129 show a discrepancy and prevent definitive conclusions. One of the sets indicates no large increase of I-129 concentrations in contrast to precipitation at lower latitudes. Data on the Ca-41 profile are preliminary and show a few points markedly above background. The feasibility of detection of Sr-90 in the environment by accelerator mass spectrometry is studied and first results are reported.
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Jouzel, J., Lorius, C., Petit, J. - R., Ritz, C., Stievenard, M., Yiou, P., et al. (1994). The climatic record from Antarctic ice now extends back to 220 ky BP. NATO ASI Series, 122, 213–237.
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1993 |
Dahljensen, D., Johnsen, S., Paterson, W., & Ritz, C. (1993). Paleothermometry By Control Methods – Comment. Journal Of Glaciology, 39(132), 421–423.
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Jouzel, J., Barkov, N., Barnola, J., Bender, M., Chappellaz, J., Genthon, C., et al. (1993). Extending The Vostok Ice-Core Record Of Paleoclimate To The Penultimate Glacial Period. Nature, 364(6436), 407–412.
Abstract: The ice-core record of local temperature, dust accumulation and air composition at Vostok station, Antarctica, now extends back to the penultimate glacial period (approximately 140-200 kyr ago) and the end of the preceding interglacial. This yields a new glaciological timescale for the whole record, which is consistent with ocean records. Temperatures at Vostok appear to have been more uniformly cold in the penultimate glacial period than in the most recent one. Concentrations of CO2 and CH4 Correlate well with temperature throughout the record.
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1992 |
Jouzel, J., Petit, J. - R., Barkov, N. I., Barnola, J. - M., Chappellaz, J., Ciais, P., et al. (1992). The last deglaciation in Antarctica : further evidence of a “younger Dryas” type climatic event.
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1991 |
Espinasse, S., Klinger, J., Ritz, C., & Schmitt, B. (1991). Modeling Of The Thermal-Behavior And Of The Chemical Differentiation Of Cometary Nuclei. Icarus, 92(2), 350–365.
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1989 |
Espinasse, S., Klinger, J., Ritz, C., & Schmitt, B. (1989). A method of estimating temperature profiles and chemical differentiation in th near surface layers of porous comet nuclei – first results for comet P/Churuymov-Gerasimnko. In International Workshop on Physics and Mechanics of Cometary Materials (pp. 185–190). Esa Sp-302. Munster, FRG.
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Jouzel, J., Barkov, N. I., Barnola, J. - M., Genthon, C., Korotkevich, Y. S., Kotlyakov, V. M., et al. (1989). Global change over the last climatic cycle from the Vostok ice core record (Antarctica). Quaternary International, 2, 15–24.
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Lorius, C., Barnola, J. - M., Petit, J. - R., Raynaud, D., Ritz, C., Barkov, N. I., et al. (1989). Long-term climatic and environmental records from Antarctic ice. International Union of Geodesy and Geophysics and American Geophysical Union, , 11–16.
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Ritz, C. (1989). Interpretation of the temperature profile measured at Vostok, East Antarctica. Annals of Glaciology, 12, 138–144.
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1988 |
Ritz, C. (1988). Dating of the Vostok ice core by flow modelling. Antarctic Climate Research, 3, 16.
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1987 |
Ritz, C. (1987). Time dependent boundary conditions for calculation of temperature fields in ice sheets. (pp. 207–216). in Physical Basis of Ice Sheet Modelling, 170. Vancouver: I.A.H.S.
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1985 |
Lorius, C., Jouzel, J., Ritz, C., Merlivat, L., Barkov, N., Korotkevich, Y., et al. (1985). A 150,000-Year Climatic Record From Antarctic Ice. Nature, 316(6029), 591–596.
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1982 |
Ritz, C., Lliboutry, L., & Rado, C. (1982). Analysis of a 870 m deep temperature profile at Dome C. Annals of Glaciology, 3, 284–289.
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1980 |
Ritz, C. (1980). Exploitation du profil de températures mesuré dans la calotte glaciaire au Dôme CThèse de 3e cycle de l'Université Scientifique et Médicale de Grenoble. Ph.D. thesis, , .
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1978 |
Lliboutry, L., & Ritz, C. (1978). Steady Flow Of A Nonlinear Viscous-Fluid (Glen Body) Around A Perfectly Smooth Sphere. Annales De Geophysique, 34(2), 133–146.
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