Publications scientifiques

Abstract: Most Of The Greenland And Antarctic Ice Sheets Are Covered With Firn – The Transitional Material Between Snow And Glacial Ice. Firn Is Vital For Understanding Ice-Sheet Mass Balance And Hydrology, And Palaeoclimate. In This Review, We Synthesize Knowledge Of Firn, Including Its Formation, Observation, Modelling And Relevance To Ice Sheets. The Refreezing Of Meltwater In The Pore Space Of Firn Currently Prevents 50% Of Meltwater In Greenland From Running Off Into The Ocean And Protects Antarctic Ice Shelves From Catastrophic Collapse. Continued Atmospheric Warming Could Inhibit Future Protection Against Mass Loss. For Example, Warming In Greenland Has Already Contributed To A 5% Reduction In Firn Pore Space Since 1980. All Projections Of Future Firn Change Suggest That Surface Meltwater Will Have An Increasing Impact On Firn, With Melt Occurring Tens To Hundreds Of Kilometres Further Inland In Greenland, And More Extensively On Antarctic Ice Shelves. Although Progress In Observation And Modelling Techniques Has Led To A Well-Established Understanding Of Firn, The Large Uncertainties Associated With Meltwater Percolation Processes (Refreezing, Ice-Layer Formation And Storage) Must Be Reduced Further. A Tighter Integration Of Modelling Components (Firn, Atmosphere And Ice-Sheet Models) Will Also Be Needed To Better Simulate Ice-Sheet Responses To Anthropogenic Warming And To Quantify Future Sea-Level Rise. A Firn Layer Covers The Earth'S Ice Sheets. This Review Outlines Techniques To Observe And Model Changes In Firn Properties And Meltwater Retention To Understand How This Firn Layer Will Respond To Climate Change.

Abstract: Due To Its Size And Geographical Features, Different Average Annual Rainfall Regimes Co-Exist In The Amazon Basin, With Distinct Year-To-Year Variability Dependent On Regions Within The Basin. In This Study, We Define And Explain The Seasonal Regional Types Of Annual Regimes, That Is, Years With Similar Seasonal Anomalies. Our Work Is Based On A 205 Rain Gauge Network Distributed Over Five Amazonian Countries, Spanning A Period Over 30 Years. Using A Spectral Clustering Method, We Identified Seven Sub-Regions Within The Basin In Which Annual Rainfall Regimes Are Spatially Homogenous. For Each Sub-Domain, We Estimated Specific Parameters That Characterize The Rainy Season (Onset And Demise Dates, Sign And Duration Of Rainfall Anomalies). Finally, Using Spectral Analysis We Identified Between Two And Four 'Seasonal Type' Of Precipitation In These Seven Sub-Domains. Most Of These Seasonal Types Are In Phase With The Large-Scale Atmospheric Circulation, Which Explains The Temporal Link With Rainfall Anomalies. The Seasonal Types Result Of The Superposition Of Inter-Annual And Intra-Seasonal Variability Whose Factors Are Then Difficult To Identify And Attribute. Part Of The Rainfall Anomalies Characterizing Seasonal Types Is Related To The Inter-Annual Variability Of The Sea Surface Temperature In The Atlantic Or The Pacific Oceans, Especially In The Northeast And Southeast Part Of The Amazon Basin, Whereas In Other Parts, Strong Intra-Seasonal And Local Factors Have A Larger Impact. The Same Sign And Duration Of Anomalies Do Not Concomitantly Affect The Various Regions Of The Amazon Basin, Confirming That One Mode Of Variability Does Not Homogeneously Affect Precipitation In Different Parts Of The Basin. Based On Rain Gauges Data And Spectral Analysis, We Identified Between Two And Four 'Seasonal Type' Of Precipitation, Compared With The Historical Mean Seasonal Regime, In Seven Homogeneous Rainfall Regions Of The Amazon Basin. Seasonal Types Result Of The Superposition Of Inter-Annual And Intra-Seasonal Variability. Part Of The Rainfall Anomalies Is Related To The Inter-Annual Variability Of The Sea Surface Temperature In The Surrounding Oceans, Especially In The Northeast And Southeast Part Of The Amazon Basin. Image

Abstract: The Northwestern Peruvian Amazon (Nwpa) Basin (78.4-75.8 Degrees W, 7.9-5.4 Degrees S) Is An Important Region For Coffee And Rice Production In Peru. Currently, No Prediction Models Are Available For Estimating Rainfall In Advance During The Wet Season (January-February-March, Jfm). Hence, We Developed Multiple Linear Regression (Mlr) Models Using Predictors Derived From Sea Surface Temperature (Sst) Indices Of The Pacific, Atlantic, And Indian Oceans, Including Central El Nino (C), Eastern El Nino (E), Tropical South Atlantic (Tsatl), Tropical North Atlantic (Tnatl), Extratropical North Atlantic (Enatl), And Indian Ocean Basin-Wide With E And C Removed (Iobw*) Indices. Additionally, We Utilized Large-Scale Convection Indices, Namely, The Eastern Pacific Intertropical Convergence Zone (Itcze) And South American Monsoon System (Samsi) Indices, For The 1981-2018 Period. Rainfall In The Lowland Nwpa Exhibits A Bimodal Annual Cycle, Whereas Rainfall In The Highland Nwpa Exhibits A Unimodal Annual Cycle. The Mlr Model Can Be Used To Accurately Capture The Interannual Variability During The Wet Season In The Highland Nwpa By Utilizing Predictors Derived From The C And Samsi Indices. In Contrast, Regarding Rainfall In The Lowland Nwpa, The Pacific Sst Variability, Sams And Tropical North Atlantic Index Were Relevant. For Long Lead Times, The Mlr Model Provided Reliable Forecasts Of Jfm Rainfall Anomalies In The Highlands (R3, Approximately 2700 M Asl) As These Regions Are Governed By Pacific Variability. However, The Mlr Model Exhibited Limitations In Accurately Estimating The Wettest Jfm Season In The Highlands Due To The Absence Of A Predictor For The Amplified Effect Of The Madden-Julian Oscillation On Rainfall.

Abstract: The Availability Of Freshwater From Glaciers And Snowmelt Is Of Vital Importance For People And Ecosystems In The Context Of Global Climate Change. Here, We Focus On 25 Glaciers Located In Different Climates And Latitudes And Investigate Their Recent (1958-2020) And Future Projected Trends (2020-2050 And 2070-2100) In Monthly Precipitation (Pr), Maximum And Minimum Temperatures, Ice Mass Loss, And Their Relationships With Cloud Properties. The Study Sites Are Located In Temperate Europe (France), The Inner (Ecuador, Venezuela, And Colombia) And Outer Tropics (Bolivia And Peru), Central America (Mexico), Tropical Southeast Asia (Indonesia), Equatorial Africa (Uganda), And The Southern Dry And Patagonian Andes (Chile And Argentina). The Climate Analyses Are Based On Terraclimate Data (Monthly Climate And Climatic Water Balance For Global Terrestrial Surfaces) And 28 Cordex (Coordinated Regional Climate Downscaling Experiment) Climate Simulations. Our Findings Reveal That, Extrapolating Current Glacier Volume Change Trends, Almost Half Of The Studied Glaciers Are Likely To Vanish (95%-100% Volume Loss) By 2050, With Widespread Warming And Drying Trends Since 1958. A Shift Toward Wetter Conditions At Pico Humboldt (Venezuela) And Martial Este (Argentina) Identifiable In The Cordex Simulation Will Very Likely Not Have A Limiting Impact On Glacier Mass Loss Owing To Increasing Temperatures, Which Will Raise The Elevation Of The Rain/Snow Limit. Our Results Provide Useful New Information To Better Understand Glacier-Climate Relationships And Future Scenarios Dominated By Ice Mass Loss Trends Across The Globe. These Findings Suggest Serious Consequences For Future Water Availability, Which Exacerbate The Vulnerability Of Local Populations And Ecosystems.

Abstract: The Assimilation Of Data From Earth Observation Satellites Into Numerical Models Is Considered To Be The Path Forward To Estimate Snow Cover Distribution In Mountain Catchments, Providing Accurate Information On The Mountainous Snow Water Equivalent (Swe). The Land Surface Temperature (Lst) Can Be Observed From Space, But Its Potential To Improve Swe Simulations Remains Underexplored. This Is Likely Due To The Insufficient Temporal Or Spatial Resolution Offered By The Current Thermal Infrared (Tir) Missions. However, Three Planned Missions Will Provide Global-Scale Tir Data At Much Higher Spatiotemporal Resolution In The Coming Years.To Investigate The Value Of Tir Data To Improve Swe Estimation, We Developed A Synthetic Data Assimilation (Da) Experiment At Five Snow-Dominated Sites Covering A Latitudinal Gradient In The Northern Hemisphere. We Generated Synthetic True Lst And Swe Series By Forcing An Energy Balance Snowpack Model With The Era5-Land Reanalysis. We Used This Synthetic True Lst To Recover The Synthetic True Swe From A Degraded Version Of Era5-Land. We Defined Different Observation Scenarios To Emulate The Revisiting Times Of Landsat 8 (16 D) And The Thermal Infrared Imaging Satellite For High-Resolution Natural Resource Assessment (Trishna) (3 D) While Accounting For Cloud Cover. We Replicated The Experiments 100 Times At Each Experimental Site To Assess The Robustness Of The Assimilation Process With Respect To Cloud Cover Under Both Revisiting Scenarios. We Performed The Assimilation Using Two Different Approaches: A Sequential Scheme (Particle Filter) And A Smoother (Particle Batch Smoother).The Results Show That Lst Da Using The Smoother Reduced The Normalized Root Mean Square Error (Nrmse) Of The Swe Simulations From 61 % (Open Loop) To 17 % And 13 % For 16 D Revisit And 3 D Revisit Respectively In The Absence Of Clouds. We Found Similar But Higher Nrmse Values By Removing Observations Due To Cloud Cover But With A Substantial Increase In The Standard Deviation Of The Nrmse Of The Replicates, Highlighting The Importance Of Revisiting Times In The Stability Of The Assimilation Performance. The Smoother Largely Outperformed The Particle Filter Algorithm, Suggesting That The Capability Of A Smoother To Propagate The Information Along The Season Is Key To Exploit Lst Information For Snow Modelling. Finally, We Have Compared The Benefit Of Assimilating Lst With Synthetic Observations Of Fractional Snow Cover Area (Fsca). Lst Da Performed Better Than Fsca Da In All The Study Sites, Suggesting That The Information Provided By Lst Is Not Limited To The Duration Of The Snow Season. These Results Suggest That The Lst Data Assimilation Has An Underappreciated Potential To Improve Snowpack Simulations And Highlight The Value Of Upcoming Tir Missions To Advance Snow Hydrology.

Abstract: This Study Assessed The Climate Conditions That Caused The Tropical Zongo Glacier (16 Degrees S, Bolivia) To Reach Its Little Ice Age (Lia) Maximum Extent In The Late 17Th Century. We Carried Out Sensitivity Analyses Of The Annual Surface Mass Balance To Different Physically Coherent Climate Scenarios Constrained By Information Taken From Paleoclimate Proxies And Sensitivity Studies Of Past Glacier Advances. These Scenarios Were Constrained By A 1.1 K Cooling And A 20% Increase In Annual Precipitation Compared To The Current Climate. Seasonal Precipitation Changes Were Constructed Using Shuffled Input Data For The Model: Measurements Of Air Temperature And Relative Humidity, Precipitation, Wind Speed, Incoming Short And Longwave Radiation Fluxes, And Assessed Using A Distributed Energy Balance Model. They Were Considered Plausible If Conditions Close To Equilibrium Glacier-Wide Mass Balance Were Obtained. Results Suggest That On Top Of A 1.1 K Cooling And Similar To 20% Increase In Annual Precipitation, Only Two Seasonal Precipitation Patterns Allow Lia Equilibrium: Evenly Distributed Precipitation Events Across The Year And An Early Wet Season Onset.

Abstract: Glaciers Distinct From The Greenland And Antarctic Ice Sheets Are Currently Losing Mass Rapidly With Direct And Severe Impacts On The Habitability Of Some Regions On Earth As Glacier Meltwater Contributes To Sea-Level Rise And Alters Regional Water Resources In Arid Regions. In This Review, We Present The Different Techniques Developed During The Last Two Decades To Measure Glacier Mass Change From Space: Digital Elevation Model (Dem) Differencing From Stereo-Imagery And Synthetic Aperture Radar Interferometry, Laser And Radar Altimetry And Space Gravimetry. We Illustrate Their Respective Strengths And Weaknesses To Survey The Mass Change Of A Large Arctic Ice Body, The Vatnajokull Ice Cap (Iceland) And For The Steep Glaciers Of The Everest Area (Himalaya). For Entire Regions, Mass Change Estimates Sometimes Disagree When A Similar Technique Is Applied By Different Research Groups. At Global Scale, These Discrepancies Result In Mass Change Estimates Varying By 20%-30%. Our Review Confirms The Need For More Thorough Inter-Comparison Studies To Understand The Origin Of These Differences And To Better Constrain Regional To Global Glacier Mass Changes And, Ultimately, Past And Future Glacier Contribution To Sea-Level Rise.

Abstract: The South Col Glacier Is A Small Body Of Ice And Snow (Approx. 0.2 Km(2)) Located At The Very High Elevation Of 8000Ma.S.L. (Above Sea Level) On The Southern Ridge Of Mt. Everest. A Recent Study By Potocki Et Al. (2022) Proposed That South Col Glacier Is Rapidly Losing Mass. This Is In Contradiction To Our Comparison Of Two Digital Elevation Models Derived From Aerial Photographs Taken In December 1984 And A Stereo Pleiades Satellite Acquisition From March 2017, From Which We Estimate A Mean Elevation Change Of 0.01 +/- 0.05M A(-1). To Reconcile These Results, We Investigate Some Aspects Of The Surface Energy And Mass Balance Of South Col Glacier. From Satellite Images And A Simple Model Of Snow Compaction And Erosion, We Show That Wind Erosion Has A Major Impact On The Surface Mass Balance Due To The Strong Seasonality In Precipitation And Wind And That It Cannot Be Neglected. Additionally, We Show That The Melt Amount Predicted By A Surface Energy And Mass Balance Model Is Very Sensitive To The Model Structure And Implementation. Contrary To Previous Findings, Melt Is Likely Not A Dominant Ablation Process On This Glacier, Which Remains Mostly Snow-Covered During The Monsoon.

Abstract: The Copernicus Sentinel-3 Surface Topography Mission (Stm) Land Altimetry Provides Valuable Surface Elevation Information Over Inland Waters, Sea Ice, And Land Ice, Thanks To Its Synthetic Aperture Radar (Sar) Altimeter And Its Orbit That Covers High-Latitude Polar Regions. To Ensure That These Measurements Are Reliable And To Maximise The Return On Investment, Adequate Validation Of The Geophysical Retrieval Methods, Processing Algorithms, And Corrections Must Be Performed Using Independent Observations. The Eu-Esa Project St3Tart (Started July 2021) Aims To Generalise The Concept Of Fiducial Reference Measurements (Frms) For The Copernicus Sentinel-3 Stm. This Work Has Gathered Existing Data, Made New Observations During Field Campaigns, And Ensured That These Observations Meet The Criteria Of Frm Standards So That They Can Be Used To Validate Sentinel-3 Stm Land Altimetry Products Operationally. A Roadmap For The Operational Provision Of The Frm, Including The Definition, Consolidation, And Identification Of The Most Relevant And Cost-Effective Methods And Protocols To Be Maintained, Supported, Or Implemented, Has Been Developed. The Roadmap Includes Guidelines For Si Traceability, Definitions Of Frm Measurement Procedures, Processing Methods, And Uncertainty Budget Estimations.

Abstract: Climate Change Impacts Social-Ecological Systems In Mountainous Areas By Increasing Certain Natural Disasters And Changing Nature'S Contributions To People (Ncp). Nature-Based Solutions (Nbs) Are Increasingly Implemented To Help Local Communities Adapt To Climatic Hazards. However, The Relevance Of Their Location In Relation To Those Hazards And Local Ncp Has Hardly Been Addressed. In The Portal (Pathways Of Transformation In The Alps) Project, We Identified And Mapped A Portfolio Of 97 Nbs For Climate Change Adaptation In The European Alps. Most Nbs Addressed Drought Or Soil Instability And Aimed To Provide Multiple Ncp Simultaneously Such As Wood Production And Protective Function Against Landslides. We Analysed Whether Nbs Are Located Where They Are The Most Needed, According To Both Current And Future Intensity Of The Hazards They Aim To Address And To Supply-Flow-Demand Indicators Of The Ncp They Aim To Provide. We Found That The Location Of Nbs Is Not Overall Related To Current Supply-Flow-Demand Indicators Of Most Ncp, Nor To The Intensity Of Hazards. Nevertheless, Nbs Addressing Droughts And Floods Are Located In Areas Where These Hazards Are More Intense, But Do Not Match Higher Values For Ncp Indicators. Conversely, Nbs Aiming To Produce Wood And To Provide Protective Function Against Landslides Are Located In Areas With Greater Levels Of These Ncp, Regardless Of The Intensity Of Hazards. These Results Suggest That Hazards And Ncp Indicators Are Not The Main Drivers Of Nbs Implementation. We Argue That Integrating Local Climate Conditions And Current Ncp Flows Is Needed To Underpin A Macro-Regional Strategy For Planning Nbs Implementation.

Abstract: Extensive Floodplains Throughout The Amazon Basin Support Important Ecosystem Services And Influence Global Water And Carbon Cycles. A Recent Change In The Hydroclimatic Regime Of The Region, With Increased Rainfall In The Northern Portions Of The Basin, Has Produced Record-Breaking High Water Levels On The Amazon River Mainstem. Yet, The Implications For The Magnitude And Duration Of Floodplain Inundation Across The Basin Remain Unknown. Here We Leverage State-Of-The-Art Hydrological Models, Supported By In-Situ And Remote Sensing Observations, To Show That The Maximum Annual Inundation Extent Along The Central Amazon Increased By 26% Since 1980. We Further Reveal Increased Flood Duration And Greater Connectivity Among Open Water Areas In Multiple Amazon Floodplain Regions. These Changes In The Hydrological Regime Of The World'S Largest River System Have Major Implications For Ecology And Biogeochemistry, And Require Rapid Adaptation By Vulnerable Populations Living Along Amazonian Rivers.

Abstract: In The Context Of Global Change, Social-Ecological Perspectives For Organisations And Territorial Planning Schemes Are Key To Facilitate A Transition Towards Sustainability. Consequently, Ecosystem Services (Es) Assessment And Mapping Are Being Increasingly Used To Enhance Knowledge Co-Production For More Sustainable Territorial Management. Higher Education Organizations Have The Potential To Catalyse The Sustainability Transition Of So-Ciety, But There Is A Lack Of Studies Integrating Novel And Current Sustainable Development Approaches In Spatial Planning At University Campuses. Here, We Proposed An Integrated Approach For Developing An Ecosystem Services-Based Plan To Manage The Campus Of The Autonomous University Of Madrid (Spain). We Developed A Participatory Process Integrating All The Various Actors And The Decision Making-Bodies Of The University. This Participatory Process Involved Meetings With Different Groups And 2,126 Surveys Administrated To Students, Professors, Researchers And Administration/Services Staff. We Also Developed A Technical Process For Assessing Es Including Eight Es Maps (Four Regulating, One Provisioning And Three Cultural), Consulted With The Experts And Stakeholders Involved, And Built The Final Version Of The Planning Guide Integrating All Perspectives To Promote Es-Based Strategic Lines. Our Results Showed That Es Maps Are Useful For Creating Management Units And For Multi -Scale Analysis. Es Maps Highlighted Important Values Of The Campus, Allowing The Integration Of More Social Actors That Allow A Stronger Participative Process And The Creation Of A Long-Term Planning Scheme. However, We Identified Certain Aspects Of The Es Approach That Hinder Real-World Application, Including Inherent Problems Associated With Shifting Politics.

Abstract: This Paper Critically Examines The Current Political Context In Which Valuation Studies Of Nature Are Undertaken. It Challenges The Belief That Somehow, More And Technically Better Valuation Will Drive The Societal Change Toward More Just And Sustainable Futures. Instead, We Argue That Current And Proposed Valuation Practices Risk To Continue To Overrepresent The Values Of Those Who Hold Power And Dominate The Valuation Space, And To Perpetuate The Discrimination Of The Views And Values Of Nondominant Stakeholders. In Tackling This Politically Sensitive Issue, We Define A Political Typology Of Valuations, Making Explicit The Roles Of Power And Discrimination. This Is Done To Provide Valuation Professionals And Other Actors With A Simple Framework To Determine If Valuation Actions And Activities Are Constructive, Inclusive, Resolve Injustices And Enable Systemic Change, Or Rather Entrench The Status Quo Or Aggravate Existing Injustices. The Objective Is To Buttress Actors In Their Decisions To Support, Accept, Improve, Oppose, Or Reject Such Valuations.

Abstract: Measuring Surface Mass-Balance In The Accumulation Areas Of Glaciers Is Challenging Because Of The High Spatial Variability Of Snow Accumulation And The Difficulty Of Conducting Annual Field Glaciological Measurements. Here, We Propose A Method That Can Solve Both These Problems For Many Locations. Ground-Penetrating Radar Measurements And Firn Cores Extracted From A Site In The French Alps Were First Used To Reconstruct The Topography Of A Buried End-Of-Summer Snow Horizon From A Past Year. Using These Data And Surface Elevation Observations From Lidar And Global Navigation Satellite System Instruments, We Calculated The Submergence Velocities Over The Period Between The Buried Horizon And More Recent Surface Elevation Observations. The Differences Between The Changes In Surface Elevation And The Submergence Velocities Were Then Used To Calculate The Annual Surface Mass-Balances With An Accuracy Of +/- 0.34 M W.E. Assuming That The Submergence Velocities Remain Stable Over Several Years, The Surface Mass-Balance Can Be Reconstructed For Subsequent Years From The Differences In Surface Elevation Alone. As Opposed To The Glaciological Method That Requires Substantial Fieldwork Year After Year To Provide Only Point Observations, This Method, Once Submergence Velocities Have Been Calculated, Requires Only Remote-Sensing Data To Provide Spatially Distributed Annual Mass-Balances In Accumulation Areas.

Abstract: Precisely Measuring The Earth'S Changing Surface On Decadal To Centennial Time Scales Is Critical For Many Science And Engineering Applications, Yet Long-Term Records Of Quantitative Landscape Change Are Often Temporally And Geographically Sparse. Archives Of Scanned Historical Aerial Photographs Provide An Opportunity To Augment These Records With Accurate Elevation Measurements That Capture The Historical State Of The Earth Surface. Structure From Motion (Sfm) Photogrammetry Workflows Produce High-Quality Digital Elevation Models (Dems) And Orthoimage Mosaics From These Historical Images, But Time-Intensive Tasks Like Manual Image Preprocessing (E.G., Fiducial Marker Identification) And Ground Control Point (Gcp) Selection Impede Processing At Scale. We Developed An Automated Method To Process Historical Images And Generate Self-Consistent Time Series Of High-Resolution (0.5-2 M) Dems And Orthomosaics, Without Manual Gcp Selection. The Method Relies On Sfm To Correct Camera Interior And Exterior Orientation And A Robust Multi-Stage Co-Registration Approach Using Modern Reference Terrain Datasets For Geolocation Refinement. We Demonstrate The Method Using Scanned Images From The North American Glacier Aerial Photography (Nagap) Archive Collected Between 1967 And 1997. We Present Results For Two Sites With Variable Photo Acquisition Geometry And Overlap – Mount Baker And South Cascade Glacier In Washington State, Usa. The Automated Method Corrects Initial Camera Position Errors Of Several Kilometers And Produces Accurately Georeferenced, High-Resolution Dems And Orthoimages, Regardless Of Camera Configuration, Acquisition Geometry, Terrain Characteristics, And Reference Dem Properties. The Average Rms Reprojection Error Following Bundle Adjustment Optimization Was 0.67 Px (0.15 M) For The 261 Images Contributing To 10 Final Dem Mosaics Between 1970 And 1992 At Mount Baker, And 0.65 Px (0.13 M) For The 243 Images Contributing To 18 Individual Dems Between 1967 And 1997 At South Cascade Glacier. The Relative Accuracy Of Elevation Values In The Historical Time Series Stacks Was 0.68 M At Mount Baker And 0.37 M At South Cascade Glacier. Our Products Have Reduced Systematic Error And Improved Accuracy Compared To Dem Products Generated Using Sfm With Manual Gcp Selection. Final Elevation Change Measurement Precision Was Similar To 0.7-1.0 M Over A 30-Year Period, Enabling The Study Of Processes With Rates As Low As Similar To 1-3 Cm/Yr. Our Results Demonstrate The Potential Of This Scalable Method To Rapidly Process Archives Of Historical Imagery And Deliver New Quantitative Insights On Long-Term Geodetic Change And Earth Surface Processes.

Abstract: This Study Investigates The Impact Of Topography On Five Snow Cover Fraction (Scf) Parameterizations Developed For Global Climate Models (Gcms), Including Two Novel Ones. The Parameterization Skill Is First Assessed With The High Mountain Asia Snow Reanalysis (Hmasr), And Three Of Them Are Implemented In The Orchidee Land Surface Model (Lsm) And Tested In Global Land-Atmosphere Coupled Simulations. Hmasr Includes Snow Depth (Sd) Uncertainties, Which May Be Due To The Elevation Differences Between In Situ Stations And Hmasr Grid Cells. Nevertheless, The Scf-Sd Relationship Varies Greatly Between Mountainous And Flat Areas In Hmasr, Especially During The Snow-Melting Period. The New Parameterizations That Include A Dependency On The Subgrid Topography Allow A Significant Scf Bias Reduction, Reaching 5 % To 10 % On Average In The Global Simulations Over Mountainous Areas, Which In Turn Leads To A Reduction Of The Surface Cold Bias From – 1.8 Circle C To About – 1 Circle C In High Mountain Asia (Hma). Furthermore, The Seasonal Hysteresis Between Scf And Sd Found In Hmasr Is Better Captured In The Parameterizations That Split The Accumulation And The Depletion Curves Or That Include A Dependency On The Snow Density. The Deep-Learning Scf Parameterization Is Promising But Exhibits More Resolution-Dependent And Region-Dependent Features. Persistent Snow Cover Biases Remain In Global Land-Atmosphere Experiments. This Suggests That Other Model Biases May Be Intertwined With The Snow Biases And Points Out The Need To Continue Improving Snow Models And Their Calibration. Increasing The Model Resolution Does Not Consistently Reduce The Simulated Scf Biases, Although Biases Get Narrower Around Mountain Areas. This Study Highlights The Complexity Of Calibrating Scf Parameterizations Since They Affect Various Land-Atmosphere Feedbacks. In Summary, This Research Spots The Importance Of Considering Topography In Scf Parameterizations And The Challenges In Accurately Representing Snow Cover In Mountainous Regions. It Calls For Further Efforts To Improve The Representation Of Subgrid-Scale Processes Affecting Snowpack In Climate Models.

Abstract: During The Austral Winter (June-August) Of 2021, The Meteorological Services Of Brazil, Argentina, Peru, Paraguay, Bolivia, And Chile All Issued Forecasts For Unusually Cold Conditions. Record-Low Minimum Temperatures And Cold Spells Were Documented, Including One Strong Cold Wave Episode That Affected 5 Countries. In This Study, We Define A Cold Wave As A Period In Which Daily Maximum And Minimum Air Temperatures Are Below The Corresponding Climatological 10Th Percentile For Three Or More Consecutive Days. The Intense Cold Wave Event In The Last Week Of June, 2021, Resulted In Record-Breaking Minimum Daily Temperatures In Several Places In Central South America And Chile. Several Locations Had Temperatures About 10 Degrees C Below Average, Central South America Had Freezing Conditions, And Southern Brazil Even Saw Snow. The Cold Air Surge Was Characterized By An Intense Upper-Air Trough Located Close To 35 Degrees S And 70 Degrees W. The Southerly Flow To The West Of This Trough Brought Very Cold Air Northward Into Subtropical And Tropical South America. A Northward Flow Between The Lower-Level Cyclonic And Anticyclonic Perturbations Caused The Intense Southerly Flow Between The Upper-Level Ridge And Trough. This Condition Facilitated The Inflow Of Near-Surface Cold Air From Southern Argentina Into Southeastern Brazil And Tropical South America East Of The Andes. In The City Of Sao Paulo, The Cold Wave Caused The Death Of 13 Homeless People From Hypothermia. Frost And Snow Across Southern And Southeastern Brazil Caused Significant Damage To Coffee, Sugarcane, Oranges, Grapes, And Other Fruit And Vegetable Crops. Wine And Coffee Production Fell, The Latter By 30%, And Prices Of Food And Commodities In The Region Rose.

Abstract: Global Warming Has Already Substantially Altered The Arctic Cryosphere. Due To The Arctic Warming Amplification, The Temperature Is Increasing More Strongly, Leading To Pervasive Changes In This Area. Recent Years Were Notably Marked By Melt Records Over The Greenland Ice Sheet, While Other Regions Such As Svalbard Seem To Remain Less Influenced. This Raises The Question Of The Current State Of The Greenland Ice Sheet And The Various Ice Caps In The Arctic For Which Few Studies Are Available. Here, We Run The Regional Climate Model (Rcm) Modele Atmospherique Regional (Mar) At A Resolution Of 6 Km Over Four Different Domains Covering All Arctic Land Ice To Produce A Unified Surface Mass Balance Product From 1950 To The Present Day. We Also Compare Our Results To Large-Scale Indices To Better Understand The Heterogeneity Of The Evolutions Across The Arctic And Their Links To Recent Climate Change. We Find A Sharp Decrease Of Surface Mass Balance (Smb) Over The Western Arctic (Canada And Greenland) In Relationship With The Atmospheric Blocking Situations That Have Become More Frequent In Summer, Resulting In A 41 % Increase Of The Melt Rate Since 1950. This Increase Is Not Seen Over The Russian Arctic Permanent Ice Areas, Where Melt Rates Have Increased By Only 3 % On Average, Illustrating A Heterogeneity In The Arctic Smb Response To Global Warming.

Abstract: Identifying Rainfall Regions Associated With Specific Modes Of Variability Is Of Practical Interest For Water Resources Management, Seasonal Forecasting, And Mitigation Of Weather-Related Risks. This Study Aims To Identify Homogeneous Rainfall Regions Within The Similar To 1 Million Km(2) Upper Madeira River Basin-Southwestern Amazon-By Their Interannual And Decadal Variability And Relates This Variability To Ocean Indices. An Observed Dataset Of 146 Groundbased Rainfall Stations, Distributed Throughout The Andes And The Amazon, And Homogenized At The Monthly Time-Step For The Period 1980-2016, Was Used For The Analysis. With No Spatial Constraints, Hierarchical Cluster Analysis And Principal Component Analysis (Pca) Optimally Grouped Stations Into 10 Rainfall Homogenous Regions. The Value Of The Regionalization For Interpreting The Rain-Fall Variability Was Evaluated By Relating The Seasonal Rainfall Time Series Of The Regions With Ocean Indices. Then, By Applying Pca To Seasonal Rainfall Series And Linking The Principal Components To Sea Surface Temperature And Ocean Indices, An Insight Into The Main Large-Scale Drivers Of The Rainfall Spatio-Temporal Variability In This Basin At Interannual And Decadal Scales Is Provided. This Analysis Identified Differences In The Year-Round Influences Of The Tropical Pacific And/Or Atlantic Oceans On The 10 Homogenous Regions.

Abstract: Lake Ice Provides Important Social And Economic Services To Local Communities, In Addition To Being A Sensitive Indicator Of Climate Change. The Reduction Of Ground Observations Of Freshwater Ice Has Led To An Increased Reliance On The Use Of Satellite Remote Sensing Data. There Is Currently Interest In The Retrieval Of Lake Ice Properties (E.G., Ice Thickness, Bubble Radius, Roughness) Using Synthetic Aperture Radar (Sar). Roughness At The Ice-Water Interface Is Particularly Important As It Has Been Identified As The Dominant Mechanism For Increasing Sar Backscatter Throughout The Ice Season And Must Be Considered In Numerical Radiative Transfer Models. Therefore, This Study Determines Optimal Ice-Water Interface Roughness Height For Two Subarctic Lakes In Northern Canada And Models Backscatter Throughout Two Ice Seasons Using The Snow Microwave Radiative Transfer (Smrt) Model. The Two Lakes For This Study Are Noell Lake And Malcolm Ramsay Lake. Field Observations Of Ice Thickness, Snow Depth, Snow Density, And The Canadian Lake Ice Model (Climo) Are Used To Parameterize Smrt. Modelled L, C, And X-Band Backscatter At Different Incidence Angles Is Assessed Using Sar Imagery From Multiple Satellite Missions. Root Mean Square Errors Ranged From 0.38 To 1.45 Db For Noell Lake And 0.70 To 2.33 Db For Malcolm Ramsay Lake. Discrepancies Between Modelled And Observed Backscatter Were Found To Be Connected To The Representation Of Roughness At Different Interfaces Within The Ice Column And Changes That Occurred During Freeze-Melt Events. These Results Provide Insight Into How Changes In Ice Properties Impact Backscatter Throughout The Ice Season. Smrt Is Valuable For Modelling Backscatter From Lake Ice During The Cold Season And Could Be Used To Develop Retrieval Algorithms For Estimating Ice-Water Interface Roughness. This Would Allow For The Development Of Other Inversion Models For Retrieval Of Surface Ice Conditions And Ice Thickness.

Abstract: The Reflection Of Sunlight Off The Snow Is A Major Driver Of The Earth'S Climate. This Reflection Is Governed By The Shape And Arrangement Of Ice Crystals At The Micrometer Scale, Called Snow Microstructure. However, Snow Optical Models Overlook The Complexity Of This Microstructure By Using Simple Shapes, And Mainly Spheres. The Use Of These Various Shapes Leads To Large Uncertainties In Climate Modeling, Which Could Reach 1.2 K In Global Air Temperature. Here, We Accurately Simulate Light Propagation In Three-Dimensional Images Of Natural Snow At The Micrometer Scale, Revealing The Optical Shape Of Snow. This Optical Shape Is Neither Spherical Nor Close To The Other Idealized Shapes Commonly Used In Models. Instead, It More Closely Approximates A Collection Of Convex Particles Without Symmetry. Besides Providing A More Realistic Representation Of Snow In The Visible And Near-Infrared Spectral Region (400 To 1400 Nm), This Breakthrough Can Be Directly Used In Climate Models, Reducing By 3 The Uncertainties In Global Air Temperature Related To The Optical Shape Of Snow. Micrometre Scale Simulation Of The Trajectory Of Sunlight As It Reaches The Snowpack Shows What Snow Looks Like From The Photon'S Perspective, Providing A More Universal Representation Of Snow In Optical Models.

Abstract: Antarctica, The Coldest And Driest Continent, Is Home To The Largest Ice Sheet, Whose Mass Is Predominantly Recharged By Snowfall. A Common Feature Of Polar Regions Is The Warming Associated With Snowfall, As Moist Oceanic Air And Cloud Cover Increase The Surface Temperature. Consequently, Snow That Accumulates On The Ice Sheet Is Deposited Under Unusually Warm Conditions. Here We Use A Polar-Oriented Regional Atmospheric Model To Study The Statistical Difference Between Average And Snowfall-Weighted Temperatures. During Snowfall, The Warm Anomaly Scales With Snowfall Amount, With The Strongest Sensitivity Occurring At Low-Accumulation Sites. Heavier Snowfall In Winter Helps To Decrease The Annual Snowfall-Weighted Temperature, But This Effect Is Overwritten By The Event-Scale Warming Associated With Precipitating Atmospheric Systems, Which Particularly Contrast With The Extremely Cold Conditions That Occur In Winter. Consequently, The Seasonal Range Of Snowfall-Weighted Temperature Is Reduced By 20 %. On The Other Hand, The Annual Snowfall-Weighted Temperature Shows 80 % More Interannual Variability Than The Annual Temperature Due To The Irregularity Of Snowfall Occurrence And Its Associated Temperature Anomaly. Disturbances Of The Apparent Annual Temperature Cycle And Interannual Variability Have Important Consequences For The Interpretation Of Water Isotopes In Precipitation, Which Are Deposited With Snowfall And Commonly Used For Paleotemperature Reconstructions From Ice Cores.

Abstract: Despite Knowledge Of The Presence Of The Tibetan Plateau (Tp) In Reorganizing Large-Scale Atmospheric Circulation, It Remains Unclear How Surface Albedo Darkening Over Tp Will Impact Local Glaciers And Remote Asian Monsoon Systems. Here, We Use A Coupled Land-Atmosphere Global Climate Model And A Glacier Model To Address These Questions. Under A High-Emission Scenario, Tp Surface Albedo Darkening Will Increase Local Temperature By 0.24 K By The End Of This Century. This Warming Will Strengthen The Elevated Heat Pump Of Tp, Increasing South Asian Monsoon Precipitation While Exacerbating The Current “South Flood-North Drought” Pattern Over East Asia. The Albedo Darkening-Induced Climate Change Also Leads To An Accompanying Tp Glacier Volume Loss Of 6.9%, Which Further Increases To 25.2% At The Equilibrium, With A Notable Loss In Western Tp. Our Findings Emphasize The Importance Of Land-Surface Change Responses In Projecting Future Water Resource Availability, With Important Implications For Water Management Policies. Impacts Of Tibetan Plateau Darkening Remain Unclear. Here Authors Show That Darkening Under The Rcp8.5 Scenario Will Increase South Asian Monsoon Precipitation And The “South Flood-North Drought” Pattern Over East Asia, While Lead To Local Glacier Loss.

Abstract: The Values Assessment (Va) Of The Intergovernmental Science Policy Platform On Biodiversity And Ecosystem Services Shows That While A Wide Range Of Valuation Methods Exist To Include Nature'S Values In Diverse Decision-Making Contexts, Uptake Of These Methods Remains Limited. Building On The Va, This Paper Reviews Five Critical Steps In The Evaluation Of Project Or Policy Proposals That Can Improve The Inclusion Of Nature'S Values In Decisions. Furthermore, Improving Valuation Practice Requires Guidelines That Utilise Quality Criteria For Valuation Of Nature And Ensure A Balance Between Them. This Paper Proposes Three Such Quality Criteria: Relevance, Robustness And Resource Efficiency. The Paper Argues That The Five Steps And Three Rs Can Generate A Practical Checklist To Support Commissioning, Evaluation And Performance Of More Plural Valuations. Such Guidelines Can Provide The Next Steps Needed To Improve Uptake Of Nature Valuation In Decision-Making.

Abstract: During The Resolve Project (“High-Resolution Imaging In Subsurface Geophysics: Development Of A Multi-Instrument Platform For Interdisciplinary Research”), Continuous Surface Displacement And Seismic Array Observations Were Obtained On Glacier D'Argentiere In The French Alps For 35 Days In May 2018. The Data Set Is Used To Perform A Detailed Study Of Targeted Processes Within The Highly Dynamic Cryospheric Environment. In Particular, The Physical Processes Controlling Glacial Basal Motion Are Poorly Understood And Remain Challenging To Observe Directly. Especially In The Alpine Region For Temperate Based Glaciers Where The Ice Rapidly Responds To Changing Climatic Conditions And Thus, Processes Are Strongly Intermittent In Time And Heterogeneous In Space. Spatially Dense Seismic And Global Positioning System (Gps) Measurements Are Analyzed Applying Machine Learning To Gain Insight Into The Processes Controlling Glacial Motions Of Glacier D'Argentiere. Using Multiple Bandpass-Filtered Copies Of The Continuous Seismic Waveforms, We Compute Energy-Based Features, Develop A Matched Field Beamforming Catalog And Include Meteorological Observations. Features Describing The Data Are Analyzed With A Gradient Boosting Decision Tree Model To Directly Estimate The Gps Displacements From The Seismic Noise. We Posit That Features Of The Seismic Noise Provide Direct Access To The Dominant Parameters That Drive Displacement On The Highly Variable And Unsteady Surface Of The Glacier. The Machine Learning Model Infers Daily Fluctuations And Longer Term Trends. The Results Show On-Ice Displacement Rates Are Strongly Modulated By Activity At The Base Of The Glacier. The Techniques Presented Provide A New Approach To Study Glacial Basal Sliding And Discover Its Full Complexity.

Abstract: The Lake Level Dynamics Of The Qinghai-Tibetan Plateau (Qtp, Also Called The 'Third Pole') Are A Crucial Indicator Of Climate Change And Human Activities; However, They Remain Poorly Measured Due To Extremely High Elevation And Cold Climate. The Existing Satellite Altimeters Also Suffer From Relatively Coarse Temporal Resolution Or Low Spatial Coverage, Preventing Effective Monitoring Of Lake Level Change At Such A Large Spatial Scale. The Recently Launched Surface Water And Ocean Topography (Swot) Mission Is Expected To Greatly Enhance The Current Lake Level Monitoring Capabilities. However, A Systematic Evaluation Is Still Lacking In The Region. To Elucidate This Potential, Here, We Generated Swot-Like Lake Products For 38 Major Lakes (>150 Km(2)) Over The Qtp During 2000-2018 Using A Large-Scale Swot Hydrology Simulator With The Input Of Satellite Altimetry And Water Mask Databases. The Comparative Assessments Between The Satellite Altimetry Data And Swot Simulations Using Various Statistical Metrics And Decomposed Time Series Components Demonstrate That Swot Can Successfully Monitor Both Short-Term Dynamics And Long-Term Trends. Extended Experiments To Derive Swot-Like Data Of 783 Lakes (>1 Km(2)) Based On The Synthetic Lake Level Series Present The Spatial Pattern Of Swot Performance That Tends To Improve With The Increasing Lake Area. Our Findings Provide Comprehensive Inferences And Confidence For Lake Level Monitoring In The Third Pole In The Early Period Of The Swot Satellite.

Abstract: The Arctic Rivers Contribute More Than One-Third Of The Total Freshwater Streamflow Into The Arctic Ocean And Play An Essential Role In The Heat And Mass Circulation Of The Arctic Atmosphere/Ocean System. As The Arctic Is Warming Faster Than The Global Average, The Streamflow From Arctic Basins Increases. This Study Analyzed The Streamflow Of The Three Largest Siberian Rivers: The Lena, Yenisei, And Ob', At Multiple Temporal Scales. Results Show That The Annual Streamflow Of Each River Basin Exhibits Statistically Significant Increasing Trends, While The Seasonal Streamflow Of Sub-Basins Generally Decreases In The Summer But Increases In The Winter. Both Autocorrelation And Long-Term Persistency Are Often Found In The Streamflow Time Series, Which Indicates Significant Changes In The Large-Scale Climatological Environment. Therefore, Wavelet Coherence Between The Streamflow And Large-Scale Climate Patterns, Including The El Nino-Southern Oscillation (Enso), North Pacific Pattern (Np), Arctic Oscillation (Ao), And The Pacific/North America Pattern (Pna), Have Been Conducted. Np And Enso Are Found To Have Positive Relationships With The Precipitation And The Ratio Of Potential Evapotranspiration Over The Precipitation. Ao And The Pna Are Found To Have Positive Relationships With The Streamflow Of The Ob' And Yenisei Rivers At Decadal And Multidecadal Scales. This Study Demonstrates That The Existence Of Nonstationarities Within The Siberian Streamflow As The Combined Impact Of Climate Change Alters The Hydroclimatological And Terrestrial Environment Of Siberia. These Findings Provide New Insights Into The Mechanisms Underlying The Hydrologic Changes To Warming Trends And Oscillations Of Climate Patterns, Which Contribute To Our Understanding And The Prediction Of Streamflow Of These Northern Rivers.

Abstract: Rapid urbanization trends and urban lifestyles challenge urban populations to recognize ecosystems' contributions to their well-being, and urban planners to integrate nature at the core of urban development. This study assesses the relationships between ecosystems and people in the rapidly expanding Barranquilla Metropolitan Area (BMA) and extracts lessons for its planning as a BiodiverCity. Using 22 interviews and 400 face-to-face surveys we evaluated: 1) the perception of positive and negative contributions of specific types of ecosystems to human well-being (HWB); 2) the importance and vulnerability of multiple ecosystem services (ES) and disservices (EdS); and 3) the relationships between ES, EdS and relational values (RV), and the influence of socioeconomic factors in providing HWB, using a Structural Equation Model (SEM). Open-ended answers in the survey showed that rural and certain natural ecosystems, such as wetlands, mangroves and tropical dry forest were the least valued ecosystems and included some EdS. In contrast, urban and peri-urban ecosystems, namely the river, beaches, crops, urban green, and backyards, were the most valued. Overall, regulating ES were perceived as critical, as well as important and vulnerable. The results of the SEM model indicate that HWB is not only explained by socioeconomic factors such as income and education, but also by ES. We argue that the necessary sustainable socio-economic development of the BMA should be coupled with an urban planning that integrates ES and their contributions to HWB.

Abstract: Mineral dust is an important aerosol in the atmosphere and is known to reduce snow albedo upon deposition. Model predictions of dust deposition events in snow covered mountain regions are challenging due to the complexity of aerosol-cloud interactions and the specifics of mountain meteorological systems. We use a case study of dust deposition between 30 March and 5 April 2018 to the French alpine snowpack to study the processes that control dust deposition to the seasonal snowpack. To understand processes controlling dust transport and deposition to snow, we use a combination of in situ observations at Col du Lautaret in the French Alps, satellite remote sensing, the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis global atmospheric composition, and the regional model WRF-Chem. Specifically, we investigate the role of increased model spatial resolution within WRF-Chem in capturing mountain meteorology, precipitation, and predicted dust deposition. Regional model results are also compared to the reanalysis global CAMS products including aerosols in the atmosphere and predicted dust deposition fluxes. We conclude that predicted mountain meteorology (e.g., precipitation) is better with increased model resolution (3 x 3 km resolution WRF-Chem domain). This improved meteorology has significant impacts on predicted dry and wet dust deposition to the alpine snowpack. Dry deposition is important in the western part of the French Alps at low altitudes, while wet deposition dominates over the complex higher altitude mountain terrain.

Abstract: Arctic vegetation cover has been increasing over the last 40 years, which has been attributed mostly to increases in temperature. Yet, the temporal dimension of this greening remains overlooked as it is often viewed as a monotonic trend. Here, using 11 year long rolling windows on 30 m resolution Landsat data, we examined the temporal variations in greening in north-eastern Canada and its dependence on summer warming. We found two significant and distinct waves of greening, centred around 1996 and 2011, and observed in all land cover types (from boreal forest to arctic tundra). The first wave was more intense and correlated with increasing summer temperature while no such relation was found for the weaker second wave. More specifically, the greening lasted longer at higher elevation during the first wave which translates to a prolonged correlation between greening and summer warming compared to low-altitude vegetation. Our work explored a forsaken complexity of high latitude greening trends and associated drivers and has raised new questions that warrant further research highlighting the importance to include temporal dimension to greening analyses in conjunction with common spatial gradients.

Abstract: Glaciers and ice caps are experiencing strong mass losses worldwide, challenging water availability, hydropower generation, and ecosystems. Here, we perform the first-ever glacier evolution projections based on deep learning by modelling the 21st century glacier evolution in the French Alps. By the end of the century, we predict a glacier volume loss between 75 and 88%. Deep learning captures a nonlinear response of glaciers to air temperature and precipitation, improving the representation of extreme mass balance rates compared to linear statistical and temperature-index models. Our results confirm an over-sensitivity of temperature-index models, often used by large-scale studies, to future warming. We argue that such models can be suitable for steep mountain glaciers. However, glacier projections under low-emission scenarios and the behaviour of flatter glaciers and ice caps are likely to be biased by mass balance models with linear sensitivities, introducing long-term biases in sea-level rise and water resources projections. Deep learning unveils a nonlinear sensitivity of glacier mass changes to future climate warming, with important implications for water resources and sea-level rise coming from glaciers and particularly ice caps.

Abstract: The recent field measurements of katabatic winds on steep alpine slopes provide a unique database for theoretical analysis of the mean flow development and the determination of mixing properties. The theory is based on the depth-integrated momentum and heat equations, and demonstrates an increase in mean velocity U with downstream distance x according to x(n) (n <= 1/2). An equation for the mean wind velocity is established, expressing its dependency on the buoyancy flux, related to the heat flux to the ground, entrainment and bottom friction. No ambient stratification, and ambient wind and constant ground surface temperature, lead to U similar to x(1/2), while constant heat flux to the ground leads to U similar to x(1/3) and requires that the reduced gravity decreases as x(-1/3). Stable ambient stratification N causes, in addition to small-amplitude mean flow oscillations, a decrease in reduced gravity with x, in which case the assumption of constant surface heat flux along x is only an approximation. The turbulent fluxes are a function of gradient Richardson number Ri with the ratio of turbulent diffusivity to viscosity K-h/K-m changing from nearly 1.4 to approximately 0.5 at Ri approximate to 0.5. A new mixing efficiency is introduced that includes turbulence kinetic energy production or consumption by along-slope turbulent buoyancy flux. It increases with Ri up to 0.25 at Ri approximate to 0.5 and then remains nearly constant. The measurements allowed us to determine the bottom drag coefficients and interfacial entrainment, with the ground surface heat flux being determined from the mean buoyancy flux.

Abstract: Considerable expansion of shrubs across the Arctic tundra has been observed in recent decades. These shrubs are thought to have a warming effect on permafrost by increasing snowpack thermal insulation, thereby limiting winter cooling and accelerating thaw. Here, we use ground temperature observations and heat transfer simulations to show that low shrubs can actually cool the ground in winter by providing a thermal bridge through the snowpack. Observations from unmanipulated herb tundra and shrub tundra sites on Bylot Island in the Canadian high Arctic reveal a 1.21 degrees C cooling effect between November and February. This is despite a snowpack that is twice as insulating in shrubs. The thermal bridging effect is reversed in spring when shrub branches absorb solar radiation and transfer heat to the ground. The overall thermal effect is likely to depend on snow and shrub characteristics and terrain aspect. The inclusion of these thermal bridging processes into climate models may have an important impact on projected greenhouse gas emissions by permafrost. Arctic shrubs cool permafrost in winter by acting as a thermal bridge through the snowpack, according to ground temperature observations and heat transfer simulations.

Abstract: The Earth system model EC-Earth3 for contributions to CMIP6 is documented here, with its flexible coupling framework, major model configurations, a methodology for ensuring the simulations are comparable across different high-performance computing (HPC) systems, and with the physical performance of base configurations over the historical period. The variety of possible configurations and sub-models reflects the broad interests in the EC-Earth community. EC-Earth3 key performance metrics demonstrate physical behavior and biases well within the frame known from recent CMIP models. With improved physical and dynamic features, new Earth system model (ESM) components, community tools, and largely improved physical performance compared to the CMIP5 version, EC-Earth3 represents a clear step forward for the only European community ESM. We demonstrate here that EC-Earth3 is suited for a range of tasks in CMIP6 and beyond.

Abstract: Floods Are One Of The Most Affective Climate-Related Disasters, And Climate Change Has Altered Their Intensity And Frequency Worldwide. This Study Examined Long-Term Changes In Flood Characteristics (Including Magnitude, Frequency, And Timing) In 30 Alpine Headwaters Of The Large Endorheic Tarim River Basin, Central Asia. The Contributions Of Climatic Factors To Flood (Magnitude And Timing) Changes Were Investigated Using Numerical Experiments In Combination With The Random Forest Approach. The Following Results Were Obtained: (1) Annual Maximum Flood Peaks Increased At Most Stations (89% Stations) During 1961-2015 With Increased Flood Frequency. Earlier Flood Peaks Were Observed In Spring With A Rate Of 1.38 Day Per Decade; For Other Seasons, Changes In The Occurrence Time Of Flood Peaks Showed Strong Spatial Variability. (2) Precipitation Was The Dominant Factor For The Increased Flood Magnitude In Most Catchments Of The Southern Slope Of The Tianshan Mountains, And Temperature Played A Greater Role In The Northern Kunlun Mountains. (3) For Flood Timing Changes, Melt Level Height And Precipitation Were The Most Influential Factors In The Alpine Catchments In The Tarim River Basin. The Results Provide Information On The Spatiotemporal Variations Of Floods And Their Driving Factors In This Alpine Basin Under Climate Change.

Abstract: This study highlights the occurrence of atmospheric rivers (ARs) over northwest Africa towards Europe, which were accompanied by intense episodes of Saharan dust transport all the way to Scandinavia, in the winter season. Using a combination of observational and reanalysis data, we investigate two extreme dusty AR events in February 2021 and assess their impact on snow melt in the Alps. The warm, moist, and dusty air mass (spatially-averaged 2-meter temperature and water vapour mixing ratio anomalies of up to 8 K and 3 g kg(-1), and aerosol optical depths and dust loadings of up to 0.85 and 11 g m (2), respectively) led to a 50% and 40% decrease in snow depth and surface albedo, respectively, in less than one month during the winter season. ARs over northwest Africa show increasing trends over the past 4 decades, with 78% of AR events associated with severe dust episodes over Europe.

Abstract: Protected Areas (Pas) Are The Most Effective Tools To Protect Biodiversity And Ecosystem Services. They Have Proven To Be Effective In Stopping Extensive Land Use Conversion In Well-Conserved Terrestrial Ecosystems. However, Land Cover Changes Around Pas Threaten Biodiversity And Ecosystem Services Within Their Limits And Reduce Ecological Connectivity. In This Study, We Analysed The Urban Sprawls On The Boundaries Of 159 Pas (National, Regional, And Natural Parks) In Spain, Using 2.5 And 5 Km Non-Protected Buffer Zones From 1990 To 2018. We Clustered Pas Based On Biophysical And Socio-Economic Characteristics And Modelled Urban Sprawl In Different Buffers And Periods. Hierarchical Clustering Revealed Three Groups Of Pas: (A) Proximate Urban Parks, (B) Mountainous Parks, And (C) Parks In The Madrid Autonomous Region. We Found That Urbanisation In The Surroundings Of Pas In Spain Has Nearly Doubled Since 1990. General Linear Models Explained A Significant Proportion Of The Urbanisation Trends Observed, With The Number Of Municipalities In The Boundary Of The Pa, The Distance To A Main Road, And The Distance To A Big City Acting As The Most Important Drivers Of Urban Sprawl. Our Results Also Show That Some Pas Exert Significant Effects On Urbanisation Trends In Their Surroundings Through The Park-View Effect. Finally, We Highlight Three Coexisting Phenomena That Might Explain The Observed Urban Sprawl Processes: (A) Pas Attracting Urbanisation In Their Surroundings Due To The Park-View Effect, (B) Pas As A Deterrent For Urban Sprawl Within Their Limits, And (C) Pas Occupying Residual Areas Among Previously Urbanised Lands.

Abstract: Nepal is highly vulnerable to climate change with increased fire occurrences and fire burned areas in recent years; therefore, we accessed the climatic drivers for its variability using fire burned areas product of Moderate Resolution Imaging Spectroradiometer (MODIS) from 2001 and 2020. The peak fire burned areas were observed in the spring season (similar to 91%) from March to May, especially higher in the lowlands of the western and central parts. At the inter-annual timescale, low precipitation, humidity, soil moisture, and high temperature supported the existence of spring fire. Combining these factors induces drought conditions, enhancing evapotranspiration from vegetation and providing more combustible fuels. Furthermore, the El Nino phase in the central-eastern Pacific Ocean is related to the weakened westerly moisture transport and moisture divergence that creates dry and warm conditions leading to increased fire activities. Thus, this study could be helpful for preparedness, management, and policy-making to limit the multi-dimensional losses in the ecosystem and society due to fire.

Abstract: Glaciers showed a similar evolution in Greenland, Europe, the US and the tropical Andes during the Holocene. The authors propose the Atlantic Meridional Ocean Overturning Circulation as a key driver of this trend. Based on new and published cosmic-ray exposure chronologies, we show that glacier extent in the tropical Andes and the north Atlantic regions (TANAR) varied in-phase on millennial timescales during the Holocene, distinct from other regions. Glaciers experienced an early Holocene maximum extent, followed by a strong mid-Holocene retreat and a re-advance in the late Holocene. We further explore the potential forcing of TANAR glacier variations using transient climate simulations. Since the Atlantic Meridional Overturning Circulation (AMOC) evolution is poorly represented in these transient simulations, we develop a semi-empirical model to estimate the “AMOC-corrected” temperature and precipitation footprint at regional scales. We show that variations in the AMOC strength during the Holocene are consistent with the observed glacier changes. Our findings highlight the need to better constrain past AMOC behavior, as it may be an important driver of TANAR glacier variations during the Holocene, superimposed on other forcing mechanisms.

Abstract: Glaciers Are Key Components Of The Mountain Water Towers Of Asia And Are Vital For Downstream Domestic, Agricultural, And Industrial Uses. The Glacier Mass Loss Rate Over The Southeastern Tibetan Plateau Is Among The Highest In Asia And Has Accelerated In Recent Decades. This Acceleration Has Been Attributed To Increased Warming, But The Mechanisms Behind These Glaciers' High Sensitivity To Warming Remain Unclear, While The Influence Of Changes In Precipitation Over The Past Decades Is Poorly Quantified. Here, We Reconstruct Glacier Mass Changes And Catchment Runoff Since 1975 At A Benchmark Glacier, Parlung No. 4, To Shed Light On The Drivers Of Recent Mass Losses For The Monsoonal, Spring-Accumulation Glaciers Of The Tibetan Plateau. Our Modeling Demonstrates How A Temperature Increase (Mean Of 0.39 Degrees C Center Dot Dec(-1) Since 1990) Has Accelerated Mass Loss Rates By Altering Both The Ablation And Accumulation Regimes In A Complexmanner. The Majority Of The Post-2000 Mass Loss Occurred During Themonsoon Months, Caused By Simultaneous Decreases In The Solid Precipitation Ratio (From 0.70 To 0.56) And Precipitation Amount (-10%), Leading To Reduced Monsoon Accumulation (-26%). Higher Solid Precipitation In Spring (+18%) During The Last Two Decades Was Increasingly Important In Mitigating Glacier Mass Loss By Providing Mass To The Glacier And Protecting It From Melting In The Early Monsoon. With Bare Ice Exposed To Warmer Temperatures For Longer Periods, Icemelt And Catchment Discharge Have Unsustainably Intensified Since The Start Of The 21St Century, Raising Concerns For Long-Term Water Supply And Hazard Occurrence In The Region.

Abstract: A multi-experiment ensemble is performed using the WRF (Weather Research and Forecasting) model at high spatial resolution (1 km) over the Antisana glacier region (Ecuador), during the year 2005. Our goal is to identify the best model configurations to simulate atmospheric processes at diurnal and seasonal scales. The model is able to reproduce the complex zonal gradient of precipitation between the wet Amazon and the drier inter-Andean region. The main precipitation biases are (i) an overestimation in the afternoon (up to 6 mm/day) in the Antisana region related to local surface circulation patterns and (ii) a nighttime overestimation (up to 20 mm/day) in the Andes-Amazon transition zone associated with the regional circulation. Changing the microphysics scheme and/or the cumulus scheme primarily affect nighttime processes, while changing the topography forcing and activating slope radiation and shading options mostly affects afternoon processes. An adequate choice of the model configuration allows a correct representation of the diurnal cycle of precipitation, and in particular: (i) the mid-level easterly regional flow, (ii) the local moisture transport along and across the valleys, and (iii) the orographic mountain waves on the Antisana summit. For this specific area and year, the best configuration retained defined as “dSRTM_LRad” shows nighttime (daytime) precipitation biases smaller than 2 mm/day (3 mm/day); it is based on non-smoothed SRTM digital elevation model (dSRTM), Lin Purdue microphysics (L), and slope and shading radiation options (Rad). This 1-km resolution configuration requires the activation of the cumulus scheme, that improves the regional nighttime convection induced by the easterly regional flow on the Amazon-Andes transition region. It allows also a realistic strengthening of the daytime upward moisture transport. This study demonstrates that in the Antisana region, 1 km is a resolution still too coarse to deactivate cumulus schemes for a correct representation of cloud convection.

Abstract: Ice Aprons (Ias) Are Part Of The Critical Components Of The Alpine Cryosphere. As A Result Of The Changing Climate Over The Past Few Decades, Deglaciation Has Resulted In A Surface Decrease Of Ias, Which Has Not Yet Been Documented, Except For A Few Specific Examples. In This Study, We Quantify The Effects Of Climate Change On Ias Since The Mid-20Th Century In The Mont Blanc Massif (Western European Alps). We Then Evaluate The Role Of Meteorological Parameters And The Local Topography In The Behaviour Of Ias. We Precisely Mapped The Surface Areas Of 200 Ias Using High-Resolution Aerial And Satellite Photographs From 1952, 2001, 2012 And 2019. From The Latter Inventory, The Surface Area Of The Present Individual Ias Ranges From 0.001 To 0.04 Km(2). Ias Have Lost Their Surface Area Over The Past 70 Years, With An Alarming Increase Since The Early 2000S. The Total Area, From 7.93 Km(2) In 1952, Was Reduced To 5.91 Km(2) In 2001 (-25.5 %) Before Collapsing To 4.21 Km(2) In 2019 (-47 % Since 1952). We Performed A Regression Analysis Using Temperature And Precipitation Proxies To Better Understand The Effects Of Meteorological Parameters On Ia Surface Area Variations. We Found A Strong Correlation Between Both Proxies And The Relative Area Loss Of Ias, Indicating The Significant Influence Of The Changing Climate On The Evolution Of Ias. We Also Evaluated The Role Of The Local Topographic Factors In The Ia Area Loss. At A Regional Scale, Factors Like Direct Solar Radiation And Elevation Influence The Behaviour Of Ias, While Others Like Curvature, Slope And Size Of The Ias Seem To Be Rather Important On A Local Scale.

Abstract: Over The Past Two Decades, Ice Loss From The Greenland Ice Sheet (Gris) Has Increased Owing To Enhanced Surface Melting And Ice Discharge To The Ocean(1-5). Whether Continuing Increased Ice Loss Will Accelerate Further, And By How Much, Remains Contentious(6-9). A Main Contributor To Future Ice Loss Is The Northeast Greenland Ice Stream (Negis), Greenland'S Largest Basin And A Prominent Feature Of Fast-Flowing Ice That Reaches The Interior Of The Gris(10-)(12). Owing To Its Topographic Setting, This Sector Is Vulnerable To Rapid Retreat, Leading To Unstable Conditions Similar To Those In The Marine-Based Setting Of Ice Streams In Antarctica(13-20). Here We Show That Extensive Speed-Up And Thinning Triggered By Frontal Changes In 2012 Have Already Propagated More Than 200 Km Inland. We Use Unique Global Navigation Satellite System (Gnss) Observations, Combined With Surface Elevation Changes And Surface Speeds Obtained From Satellite Data, To Select The Correct Basal Conditions To Be Used In Ice Flow Numerical Models, Which We Then Use For Future Simulations. Our Model Results Indicate That This Marine-Based Sector Alone Will Contribute 13.5-15.5 Mm Sea-Level Rise By 2100 (Equivalent To The Contribution Of The Entire Ice Sheet Over The Past 50 Years) And Will Cause Precipitous Changes In The Coming Century. This Study Shows That Measurements Of Subtle Changes In The Ice Speed And Elevation Inland Help To Constrain Numerical Models Of The Future Mass Balance And Higher-End Projections Show Better Agreement With Observations.

Abstract: In The Northern Hemisphere, -1500 Glaciers, Accounting For 28% Of Glacierized Area Outside The Greenland Ice Sheet, Terminate In The Ocean. Glacier Mass Loss At Their Ice-Ocean Interface, Known As Frontal Ablation, Has Not Yet Been Comprehensively Quantified. Here, We Estimate Decadal Frontal Ablation From Measurements Of Ice Discharge And Terminus Position Change From 2000 To 2020. We Bias-Correct And Cross-Validate Estimates And Uncertainties Using Independent Sources. Frontal Ablation Of Marine-Terminating Glaciers Contributed An Average Of 44.47 +/- 6.23 Gt A(-1) Of Ice To The Ocean From 2000 To 2010, And 51.98 +/- 4.62 Gt A(-1) From 2010 To 2020. Ice Discharge From 2000 To 2020 Was Equivalent To 2.10 +/- 0.22 Mm Of Sea-Level Rise And Comprised Approximately 79% Of Frontal Ablation, With The Remainder From Terminus Retreat. Near-Coastal Areas Most Impacted Include Austfonna, Svalbard, And Central Severnaya Zemlya, The Russian Arctic, And A Few Alaskan Fjords.

Abstract: Active rock glaciers are some of the most frequent cryospheric landforms in midlatitude high-elevation mountain ranges. Their activity strongly influences the hydrology and geomorphology of alpine environments over short (years to decades) and long (centuries to millennia) timescales. Being conspicuous expressions of mountain permafrost and important water reserves in the form of ground ice, rock glaciers are seen as increasingly important actors in the geomorphological and hydrological evolution of mountain systems, especially in the context of current climate change. Over geological timescales, rock glaciers both reflect paleoclimate conditions and transport rock boulders produced by headwall erosion, and they therefore participate in shaping high mountain slopes. However, the dynamics of rock glaciers and their evolution over different timescales remain under-constrained. In this study, we adopt a multi-method approach, including field observations, remote sensing, and geochronology, to investigate the rock glacier system of the Vallon de la Route (Combeynot Massif, western French Alps). Remotely sensed images and correlation techniques are used to document the displacement field of the rock glacier over timescales ranging from days to decades. Additionally, to estimate displacement over periods from centuries to millennia, we employ terrestrial cosmogenic nuclide (quartz Be-10) surface-exposure dating on rock boulder surfaces located along the central flow line of the rock glacier, targeting different longitudinal positions from the headwall to the rock glacier terminus. The remote sensing analysis demonstrates that between 1960 and 2018 the two lower units of the rock glacier were motionless, the transitional unit presented an integrated surface velocity of 0.03 +/- 0.02 m a(-1), and the two upper active units above 2600 m a.s.l. showed a velocity between 0.14 +/- 0.08 and 0.15 +/- 0.05 m a(-1). Our results show Be-10 surface-exposure ages ranging from 13.10 +/- 0.51 to 1.88 +/- 0.14 ka. The spatial distribution of dated rock glacier boulders reveals a first-order inverse correlation between Be-10 surface-exposure age and elevation and a positive correlation with horizontal distance to the headwall. These observations support the hypothesis of rock boulders falling from the headwall and remaining on the glacier surface as they are transported down valley, and they may therefore be used to estimate rock glacier surface velocity over geological timescales. Our results also suggest that the rock glacier is characterized by two major phases of activity. The first phase, starting around 12 ka, displays a Be-10 age gradient with a rock glacier surface velocity of about 0.45 m a(-1), following a quiescent period between ca. 6.2 and 3.4 ka before the emplacement of the present-day upper two active units. Climatic conditions have favored an integrated rock glacier motion of around 0.18 m a(-1) between 3.4 ka and present day. These results allow us to quantify back-wearing rates of the headwall of between 1.0 and 2.5 mm a(-1), higher than catchment-integrated denudation rates estimated over millennial timescales. This suggests that the rock glacier system promotes the maintenance of high rock wall erosion by acting as debris conveyor and allowing freshly exposed bedrock surfaces to be affected by erosion processes.

Abstract: The Monitoring Of Glaciers In Switzerland Has A Long Tradition, Yet Glacier Changes During The 20Th Century Are Only Known Through Sparse Observations. Here, We Estimate A Halving Of Swiss Glacier Volumes Between 1931 And 2016 By Mapping Historical Glacier Elevation Changes At High Resolution. Our Analysis Relies On A Terrestrial Image Archive Known As Terra, Which Covers About 86 % Of The Swiss Glacierised Area With 21 703 Images Acquired During The Period 1916-1947 (With A Median Date Of 1931). We Developed A Semi-Automated Workflow To Generate Digital Elevation Models (Dems) From These Images, Resulting In A 45 % Total Glacier Coverage. Using The Geodetic Method, We Estimate A Swisswide Glacier Mass Balance Of -0.52 +/- 0.09 M W.E. A(-1) Between 1931 And 2016. This Equates To A 51.5 +/- 8.0 % Loss In Glacier Volume. We Find That Low-Elevation, High-Debriscover, And Gently Sloping Glacier Termini Are Conducive To Particularly High Mass Losses. In Addition To These Glacier-Specific, Quasi-Centennial Elevation Changes, We Present A New Inventory Of Glacier Outlines With Known Timestamps And Complete Attributes From Around 1931. The Fragmented Spatial Coverage And Temporal Heterogeneity Of The Terra Archive Are The Largest Sources Of Uncertainty In Our Glacier-Specific Estimates, Reaching Up To 0.50 M W.E. A(-1). We Suggest That The High-Resolution Mapping Of Historical Surface Elevations Could Also Unlock Great Potential For Research Fields Other Than Glaciology.

Abstract: This article presents a comprehensive hydrometeorological dataset collected over the past two decades throughout the Cordillera Blanca, Peru. The data-recording sites, located in the upper portion of the Rio Santa valley, also known as the Callejon de Huaylas, span an elevation range of 3738-4750ma.s.l. As many historical hydrological stations measuring daily discharge across the region became defunct after their installation in the 1950s, there was a need for new stations to be installed and an opportunity to increase the temporal resolution of the streamflow observations. Through inter-institutional collaboration, the hydrometeorological network described in this paper was deployed with the goal of evaluating how progressive glacier mass loss was impacting stream hydrology, and understanding better the local manifestation of climate change over diurnal to seasonal and interannual time scales. The four automatic weather stations supply detailed meteorological observations and are situated in a variety of mountain landscapes, with one on a high-mountain pass, another next to a glacial lake, and two in glacially carved valleys. Four additional temperature and relative humidity loggers complement the weather stations within the Llanganuco valley by providing these data across an elevation gradient. The six streamflow gauges are located in tributaries to the Rio Santa and collect high-temporal-resolution runoff data. The datasets presented here are available freely from https://doi.org/10.4211/hs.35a670e6c5824ff89b3b74fe45ca90e0 (Mateo et al., 2021). Combined, the hydrological and meteorological data collected throughout the Cordillera Blanca enable detailed research of atmospheric and hydrological processes in tropical high-mountain terrain.

Abstract: Dense seismic array monitoring combined with advanced processing can help retrieve and locate a variety of seismic sources with unprecedented resolution and spatial coverage. We present a methodology that goes beyond classical localization algorithms through gathering various types of sources (impulsive or continuous) using a single scheme based on a gradient-descent optimization and evaluating different levels of phase coherence. We apply our methodology on an Alpine glacier and demonstrate that we can retrieve the dynamics of active crevasses with a metric resolution using sources associated with high phase coherence; the presence of diffracting materials (e.g., rocks) trapped in transverse crevasses using sources with moderate phase coherence; and the two-dimensional time evolution of the subglacial hydrology system using sources with low phase coherence. Our study highlights the strength of using an appropriate and systematic seismological approach to image a wide range of subsurface structures and phenomena in settings with complex wavefields.

Abstract: Dense seismic array monitoring has recently allowed the detailed investigation of sources of ground vibrations and their spatiotemporal dynamics. In a context where traditional monitoring techniques for fluvial processes often do not provide data with adequate temporal and spatial resolution, the use of dense arrays could allow the identification and tracking of different sources of river-induced seismic ground vibrations (e.g. turbulence and bedload transport). Here, we study the potential of dense seismic array monitoring by investigating a high-flow event that occurred in summer 2019 along a 600-m-long braided reach of the Severaisse River (French Alps). We use a network of 80 seismometers deployed on both river banks, and we supplement the seismic observations with flow gauging measurements and time-lapse imagery. During this event, we observe impulsive signals that are coherently detected over the array, and which we interpret as being associated with the bedload transport of clusters of coarse grains (blocks). Through phase-delay analysis we are able to locate these seismic events on the bend apex of an active branch of the reach. These results demonstrate the capability of such a method to locate bedload activity at high spatiotemporal resolution, providing crucial information for geomorphological investigations and natural risk management.

Abstract: Surface Melting On The Antarctic Ice Sheet Has Been Monitored By Satellite Microwave Radiometry For Over 40 Years. Despite This Long Perspective, Our Understanding Of The Microwave Emission From Wet Snow Is Still Limited, Preventing The Full Exploitation Of These Observations To Study Supraglacial Hydrology. Using The Snow Microwave Radiative Transfer (Smrt) Model, This Study Investigates The Sensitivity Of Microwave Brightness Temperature To Snow Liquid Water Content At Frequencies From 1.4 To 37 Ghz. We First Determine The Snowpack Properties For Eight Selected Coastal Sites By Retrieving Profiles Of Density, Grain Size And Ice Layers From Microwave Observations When The Snowpack Is Dry During Wintertime. Second, A Series Of Brightness Temperature Simulations Is Run With Added Water. The Results Show That (I) A Small Quantity Of Liquid Water (P-Center Dot 0.5 Kgm(-2)) Can Be Detected, But The Actual Quantity Cannot Be Retrieved Out Of The Full Range Of Possible Water Quantities; (Ii) The Detection Of A Buried Wet Layer Is Possible Up To A Maximum Depth Of 1 To 6 M Depending On The Frequency (6-37 Ghz) And On The Snow Properties (Grain Size, Density) At Each Site; (Iii) Surface Ponds And Water-Saturated Areas May Prevent Melt Detection, But The Current Coverage Of These Waterbodies In The Large Satellite Field Of View Is Presently Too Small In Antarctica To Have Noticeable Effects; And (Iv) At 1.4 Ghz, While The Simulations Are Less Reliable, We Found A Weaker Sensitivity To Liquid Water And The Maximal Depth Of Detection Is Relatively Shallow (< 10 M) Compared To The Typical Radiation Penetration Depth In Dry Firn (Asymptotic To 1000 M) At This Low Frequency. These Numerical Results Pave The Way For The Development Of Improved Multi Frequency Algorithms To Detect Melt Intensity And The Depth Of Liquid Water Below The Surface In The Antarctic Snowpack.

Abstract: Black Carbon And Dust Deposition Advanced The End Of The Snow Season By 17 Days On Average Over The Last 40 Years In The French Alps And The Pyrenees. The Warming-Induced Snow Cover Decline Was Partly Offset By Decreases In Black Carbon Deposition Observed Since The 1980S. By Darkening The Snow Surface, Mineral Dust And Black Carbon (Bc) Deposition Enhances Snowmelt And Triggers Numerous Feedbacks. Assessments Of Their Long-Term Impact At The Regional Scale Are Still Largely Missing Despite The Environmental And Socio-Economic Implications Of Snow Cover Changes. Here We Show, Using Numerical Simulations, That Dust And Bc Deposition Advanced Snowmelt By 17 +/- 6 Days On Average In The French Alps And The Pyrenees Over The 1979-2018 Period. Bc And Dust Also Advanced By 10-15 Days The Peak Melt Water Runoff, A Substantial Effect On The Timing Of Water Resources Availability. We Also Demonstrate That The Decrease In Bc Deposition Since The 1980S Moderates The Impact Of Current Warming On Snow Cover Decline. Hence, Accounting For Changes In Light-Absorbing Particles Deposition Is Required To Improve The Accuracy Of Snow Cover Reanalyses And Climate Projections, That Are Crucial For Better Understanding The Past And Future Evolution Of Mountain Social-Ecological Systems.

Abstract: The Cordillera Blanca (central Andes of Peru) represents the largest concentration of tropical glaciers in the world. The atmospheric processes related to precipitations are still scarcely studied in this region. The main objective of this study is to understand the atmospheric processes of interaction between local and regional scales controlling the diurnal cycle of precipitation over the Santa River basin located between the Cordillera Blanca and the Cordillera Negra. The rainy season (December-March) of 2012-2013 is chosen to perform simulations with the WRF (Weather Research and Forecasting) model, with two domains at 6 km (WRF-6 km) and 2 km (WRF-2 km) horizontal resolutions, forced by ERA5. WRF-2 km precipitation shows a clear improvement over WRF-6 km in terms of the daily mean and diurnal cycle, compared to in situ observations. WRF-2 km shows that the moisture from the Pacific Ocean is a key process modulating the diurnal cycle of precipitation over the Santa River basin in interaction with moisture fluxes from the Amazon basin. In particular, a channeling thermally orographic flow is described as controlling the afternoon precipitation along the Santa valley. In addition, in the highest parts of the Santa River basin (in both cordilleras) and the southern part, maximum precipitation occurs earlier than the lowest parts and the bottom of the valley in the central part of the basin, associated with the intensification of the channeling flow by upslope cross-valley winds during mid-afternoon and its decrease during late afternoon/early night.

Abstract: Mountains Are An Essential Component Of The Global Life-Support System. They Are Characterized By A Rugged, Heterog-Enous Landscape With Rapidly Changing Environmental Conditions Providing Myriad Ecological Niches Over Relativelysmall Spatial Scales. Although Montane Species Are Well Adapted To Life At Extremes, They Are Highly Vulnerable To Human Derived Ecosystem Threats. Here We Build On The Manifesto'World Scientists' Warning To Humanity',Issuedbythe Alliance Of World Scientists, To Outline The Major Threats To Mountain Ecosystems. We Highlight Climate Change Asthe Greatest Threat To Mountain Ecosystems, Which Are More Impacted Than Their Lowland Counterparts. We Further Dis-Cuss The Cascade Of“Knock-On”Effects Of Climate Change Such As Increased Uv Radiation, Altered Hydrological Cycles,And Altered Pollution Profiles; Highlighting The Biological And Socio-Economic Consequences. Finally, We Presenthow Intensified Use Of Mountains Leads To Overexploitation And Abstraction Of Water, Driving Changes In Carbonstock, Reducing Biodiversity, And Impacting Ecosystem Functioning. These Perturbations Can Provide Opportunitiesfor Invasive Species, Parasites And Pathogens To Colonize These Fragile Habitats, Driving Further Changes And Losses Ofmicro- And Macro-Biodiversity, As Well Further Impacting Ecosystem Services. Ultimately, Imbalances In The Normalfunctioning Of Mountain Ecosystems Will Lead To Changes In Vital Biological, Biochemical, And Chemical Processes, Crit-Ically Reducing Ecosystem Health With Widespread Repercussions For Animal And Human Wellbeing. Developing Tools Inspecies/Habitat Conservation And Future Restoration Is Therefore Essential If We Are To Effectively Mitigate Against The Declining Health Of Mountains

Abstract: This Study Finds The Relationship Between Increases In Precipitable Water Vapor (Pwv), And Intense Rainfall Events In Four Different Climatological Regions Of South America'S Equatorial Northwest: The Coast, Andes Valley, High Mountains, And Amazon. First, The Pwv Was Derived From Tropospheric Zenith Delay Measured By Global Navigation Satellite System (Gnss) Instrumentation Located Near Meteorological Stations Within The Regions Of Interest Using Hourly Data From The Year 2014. A Harmonic Analysis Approach Through Continuous Wavelet Cross-Spectrum And Coherence, As Well As Discrete Wavelets, Was Used To Determine A Measure Of The Lags Found Between Pwv And Specific Heavy Rain Events And Then Compared With Satellite Ir Images And Meteorological Anomalies. The Link Between Pwv Peaks And Rainfall Was The Most Evident On The Coast, And Less Discernible In The Other Stations Possibly Due To Local Dynamic Factors. The Results Showed A Lag Of 11 H Between The Preceding Pwv Increase And An Intense Rainfall Event. This Was Apparent In All Of The Stations, Except In Amazon Where It Was 6 H, With The Highest Precision At The Coast And With The Largest Dispersion In The High Mountains. The Interpretation Of This Lag For Each Region Is Also Discussed.

Abstract: The Decadal Variability In Summer Precipitation Over The Central Andes (10 Degrees-30 Degrees S) Is Investigated From 1921 To 2010 Using Low-Pass Filtered Time Series Of The Central And Eastern El Nino-Southern Oscillation (Enso) Pacific (C And E) Indices, The South Pacific Convergence Zone (Spcz) Index, The Atlantic Sst Indices, Atlantic Multidecadal Oscillation (Amo) Index, North Atlantic Oscillation (Nao) Index, And Era-20C Reanalysis. Additionally, An Empirical-Statistical Downscaling (Esd) Model Was Built. A Rotated Empirical Orthogonal Function (Reof) Analysis Shows That The First Leading Mode Of Precipitation (Rpc1) Represents 38.2% Of The Total Decadal Variance. Rpc2, Rcp3, And Rpc4 Represent 18.8, 12.8, And 9.7% Of The Total Decadal Variance, Respectively. Furthermore, Rpc1 Features Highest Loadings Over Most Of The Region. Rpc2 Features A Dipole Of Highest Loadings Over The Southernmost Bolivian Altiplano And Lowest Loadings Over The Northwestern Argentinian Andes. Conversely, Rpc3 Presents Highest Loadings Over The Eastern-Central Bolivian Altiplano And Northwestern Argentinian Andes. Rpc4 Features Highest Loadings Over The Southern Bolivian Andes. Rpc1 And Rpc3 Wet Summers Are Associated With Moisture Transport From The Amazon Basin, But Rpc1 Features The Strengthening Upper-Level Bolivian High-Nordeste Low System Over South America. Conversely, Rpc2 And Rpc4 Wet Summers Are Associated With Local Processes Induced By Southward Displacement Of The South Atlantic Convergence Zone And Warm Sea Surface Temperature (Sst) Anomalies Over The Indian Ocean, Respectively. According To The Esd Model, The Decadal Variability In The Central And Eastern Pacific (Cp And Ep) And Atlantic Ocean Reproduces The Decadal Component Of The Djf Precipitation Over Most Of The Central Andes.

Abstract: The Southern Hemisphere cryosphere has recently shown regionally-contrasted responses to climate change, in particular to the positive phases of the Southern Annular Mode. However, the understanding of the impacts of this mode on ice-shelf basal melt at a circum-Antarctic scale is still limited. Here, we performed idealized experiments with a pan-Antarctic regional ice-shelf cavity-resolving ocean-sea-ice model for different phases of the Southern Annular Mode. We show that positive phases lead to increased upwelling and subsurface ocean temperature and salinity close to ice shelves, while the opposite occurs for negative phases. A one-standard-deviation increase of the Southern Annular Mode leads to a net basal mass loss of 40 Gt yr(-1), with strong regional contrasts: increased ice-shelf basal melt in the Bellingshausen and Western Pacific sectors and the opposite response in the Amundsen sector. Estimates of 1000-1200 and 2090-2100 ice-shelf basal melt changes due to the Southern Annular Mode are -86.6 Gt yr(-1) and 55.0 to 164.9 Gt yr(-1), respectively, compared to the present. Positive phases of the Southern Annular Mode lead to enhanced basal melt overall in the Antarctic ice shelves, with strong losses in the Bellingshausen and Western Pacific sectors and gains in the Amundsen Sea, according to ice-ocean model experiments.

Abstract: The hydromechanical processes by which basal water controls sliding at the glacier bed are poorly known, despite glacier basal motion being responsible for a large part of ice flux in temperate alpine glaciers. Previous studies suggest that sliding strongly relates to the quantity of water being stored at the ice-bedrock interface. However, this water storage is difficult to quantify accurately on the basis of surface-motion observations, given that uplift can also be affected by changes in vertical-strain rates and sliding velocity change. Here, we use a comprehensive data set of in situ measurements performed over 2 years on the Argentiere Glacier in the French Alps to investigate the relationships between horizontal and vertical velocities, basal sliding, subglacial runoff and bed separation. We observe strikingly large uplifts varying spatially between 0.20 and 0.90 m over the winter/spring seasons between January and June and with a consistent spatial pattern from 1 year to another. We show, based on observations and three dimensional ice-flow modeling, that these large uplifts cannot be explained solely by changes in strain rates or in sliding up an inclined bed. Our results reveal that more than 80% of the observed uplift is related to enhanced bed separation through cavitation, allowing us to estimate the volume occupied by water-filled subglacial cavities. Our interpretation of uplift being mainly caused by increased cavitation is also consistent with an associated increase in the observed surface horizontal velocity. These findings provide important observational constraints for testing subglacial hydrological models.

Abstract: Owing To Drifting Snow Processes, Snow Accumulation And Surface Density In Polar Environments Are Variable In Space And Time. We Present New Field Data Of Manual Measurements, Repeat Terrestrial Laser Scanning And Snow Micro-Penetrometry From Dronning Maud Land, Antarctica, Showing The Density Of New Snow Accumulations. We Combine These Data With Published Drifting Snow Mass Flux Observations, To Evaluate The Performance Of The 1-D, Detailed, Physics-Based Snow Cover Model Snowpack In Representing Drifting Snow And Surface Density. For Two Sites In East Antarctica With Multiple Years Of Data, We Found A Coefficient Of Determination For The Simulated Drifting Snow Of R(2) = 0.42 And R(2) = 0.50, Respectively. The Field Observations Show The Existence Of Low-Density Snow Accumulations During Low Wind Conditions. Successive High Wind Speed Events Generally Erode These Low-Density Layers While Producing Spatially Variable Erosion/Deposition Patterns With Typical Length Scales Of A Few Metres. We Found That A Model Setup That Is Able To Represent Low-Density Snow Accumulating During Low Wind Speed Conditions, As Well As Subsequent Snow Erosion And Redeposition At Higher Densities During Drifting Snow Events Was Mostly Able To Describe The Observed Temporal Variability Of Surface Density In The Field.

Abstract: Rapid warming is a major threat for the alpine biodiversity but, at the same time, accelerated glacial retreat constitutes an opportunity for taxa and communities to escape range contraction or extinction. We explored the first steps of plant primary succession after accelerated glacial retreat under the assumption that the first few years are critical for the success of plant establishment. To this end, we examined plant succession along a very short post-glacial chronosequence in the tropical Andes of Ecuador (2-13 years after glacial retreat). We recorded the location of all plant individuals within an area of 4,200 m(2) divided into plots of 1 m(2). This sampling made it possible to measure the responses of the microenvironment, plant diversity and plants traits to time since the glacial retreat. It also made it possible to produce species-area curves and to estimate positive interactions between species. Decreases in soil temperature, soil moisture, and soil macronutrients revealed increasing abiotic stress for plants between two and 13 years after glacial retreat. This increasing stress seemingly explained the lack of positive correlation between plant diversity and time since the glacial retreat. It might explain the decreasing performance of plants at both the population (lower plant height) and the community levels (lower species richness and lower accumulation of species per area). Meanwhile, infrequent spatial associations among plants indicated a facilitation deficit and animal-dispersed plants were almost absent. Although the presence of 21 species on such a small sampled area seven years after glacial retreat could look like a colonization success in the first place, the increasing abiotic stress may partly erase this success, reducing species richness to 13 species after 13 years and increasing the frequency of patches without vegetation. This fine-grain distribution study sheds new light on nature's responses to the effects of climate change in cold biomes, suggesting that faster glacial retreat would not necessarily result in accelerated plant colonization. Results are exploratory and require site replications for generalization.

Abstract: In the 2019/2020 austral summer, the surface melt duration and extent on the northern George VI Ice Shelf (GVIIS) was exceptional compared to the 31 previous summers of distinctly lower melt. This finding is based on analysis of near-continuous 41-year satellite microwave radiometer and scatterometer data, which are sensitive to meltwater on the ice shelf surface and in the near-surface snow. Using optical satellite imagery from Landsat 8 (2013 to 2020) and Sentinel-2 (2017 to 2020), record volumes of surface meltwater ponding were also observed on the northern GVIIS in 2019/2020, with 23% of the surface area covered by 0.62 km(3) of ponded meltwater on 19 January. These exceptional melt and surface ponding conditions in 2019/2020 were driven by sustained air temperatures >= 0 degrees C for anomalously long periods (55 to 90 h) from late November onwards, which limited meltwater refreezing. The sustained warm periods were likely driven by warm, low-speed (<= 7.5 m s(-1)) northwesterly and northeasterly winds and not by foehn wind conditions, which were only present for 9 h total in the 2019/2020 melt season. Increased surface ponding on ice shelves may threaten their stability through increased potential for hydrofracture initiation; a risk that may increase due to firn air content depletion in response to near-surface melting.

Abstract: The warming-induced expansion of shrubs in the Arctic is transforming snowpacks into a mixture of snow, impurities and buried branches. Because snow is a translucent medium into which light penetrates up to tens of centimetres, buried branches may alter the snowpack radiation budget with important consequences for the snow thermal regime and microstructure. To characterize the influence of buried branches on radiative transfer in snow, irradiance profiles were measured in snowpacks with and without shrubs near Umiujaq in the Canadian Low Arctic (56.5 degrees N, 76.5 degrees W) in November and December 2015. Using the irradiance profiles measured in shrub-free snowpacks in combination with a Monte Carlo radiative transfer model revealed that the dominant impurity type was black carbon (BC) in variable concentrations up to 185 ng g(-1). This allowed the separation of the radiative effects of impurities and buried branches. Irradiance profiles measured in snowpacks with shrubs showed that the impact of buried branches was local (i.e. a few centimetres around branches) and only observable in layers where branches were also visible in snowpit photographs. The local-effect hypothesis was further supported by observations of localized melting and depth hoar pockets that formed in the vicinity of branches. Buried branches therefore affect snowpack properties, with possible impacts on Arctic flora and fauna and on the thermal regime of permafrost. Lastly, the unexpectedly high BC concentrations in snow are likely caused by nearby open-air waste burning, suggesting that cleaner waste management plans are required for northern community and ecosystem protection.

Abstract: Debris-covered glaciers constitute a large part of the world's cryosphere. However, little is known about their long-term response to multi-millennial climate variability, in particular in the Southern Hemisphere. Here, we provide first insights into the response of a debris-covered glacier to multi-millennial climate variability in the sub-Antarctic Kerguelen Archipelago, which can be compared to that of recently investigated debris-free glaciers. We focus on the Gentil Glacier and present 13 new Cl-36 cosmic-ray exposure ages from moraine boulders. The Gentil Glacier experienced at least two glacial advances: the first one during the Late Glacial (19.0-11.6 ka) at similar to 14.3 ka and the second one during the Late Holocene at similar to 2.6 ka. Both debris-covered and debris-free glaciers advanced broadly synchronously during the Late Glacial, most probably during the Antarctic Cold Reversal event (14.5-12.9 ka). This suggests that both glacier types at Kerguelen were sensitive to abrupt temperature changes recorded in Antarctic ice cores, associated with increased moisture. However, during the Late Holocene, the advance at similar to 2.6 ka was not observed in other glaciers and seems to be an original feature of the debris-covered Gentil Glacier, related to either distinct dynamics or to distinct sensitivity to precipitation changes.

Abstract: Atmospheric boundary layer (ABL) dynamics over glaciers mediate the response of glacier mass balance to large-scale climate forcing. Despite this, very few ABL observations are available over mountain glaciers in complex terrain. An intensive field campaign was conducted in June 2015 at the Athabasca Glacier outlet of Columbia Icefield in the Canadian Rockies. Observations of wind and temperature profiles with novel kite and radio-acoustic sounding systems showed a well-defined mesoscale circulation developed between the glacier and snow-free valley in fair weather. The typical vertical ABL structure above the glacier differed from that expected for “glacier winds”; strong daytime down-glacier winds extended through the lowest 200 m with no up-valley return flow aloft. This structure suggests external forcing at mesoscale scales or greater and is provisionally termed an “icefield breeze.” A wind speed maximum near the surface, characteristic of a “glacier wind,” was only observed during night-time and one afternoon. Lapse rates of air temperature down the glacier centerline show the interaction of down-glacier cooling driven by sensible heat loss into the ice, entrainment and periodic disruption and warming. Down-glacier cooling was weaker in “icefield breeze” conditions, while in “glacier wind” conditions, stronger down-glacier cooling enabled large increases in near-surface temperature on the lower glacier during periods of surface boundary layer (SBL) disruption. These results raise several questions, including the impact of Columbia Icefield on the ABL and melt of Athabasca Glacier. Future work should use these observations as a testbed for modeling spatio-temporal variations in the ABL and SBL within complex glaciated terrain.

Abstract: We present projections of West Antarctic surface mass balance (SMB) and surface melt to 2080-2100 under the RCP8.5 scenario and based on a regional model at 10 km resolution. Our projections are built by adding a CMIP5 (Coupled Model Intercomparison Project Phase 5) multi-model-mean seasonal climate-change anomaly to the present-day model boundary conditions. Using an anomaly has the advantage to reduce CMIP5 model biases, and a perfect-model test reveals that our approach captures most characteristics of future changes despite a 16 %-17 % underestimation of projected SMB and melt rates. SMB over the grounded ice sheet in the sector between Getz and Abbot increases from 336 Gt yr(-1) in 1989-2009 to 455 Gt yr(-1) in 2080-2100, which would reduce the global sea level changing rate by 0.33 mm yr(-1). Snowfall indeed increases by 7.4 % degrees C-1 to 8.9 % degrees C-1 of near-surface warming due to increasing saturation water vapour pressure in warmer conditions, reduced sea-ice concentrations, and more marine air intrusion. Ice-shelf surface melt rates increase by an order of magnitude in the 21st century mostly due to higher downward radiation from increased humidity and to reduced albedo in the presence of melting. There is a net production of surface liquid water over eastern ice shelves (Abbot, Cosgrove, and Pine Island) but not over western ice shelves (Thwaites, Crosson, Dotson, and Getz). This is explained by the evolution of the melt-to-snowfall ratio: below a threshold of 0.60 to 0.85 in our simulations, firn air is not entirely depleted by melt water, while entire depletion and net production of surface liquid water occur for higher ratios. This suggests that western ice shelves might remain unaffected by hydrofracturing for more than a century under RCP8.5, while eastern ice shelves have a high potential for hydrofracturing before the end of this century.

Abstract: Optical sensors are increasingly sought to estimate the amount of chlorophyll a (chla) in freshwater bodies. Most, whether empirical or semi-empirical, are data-oriented. Two main limitations are often encountered in the development of such models. The availability of data needed for model calibration, validation, and testing and the locality of the model developed-the majority need a re-parameterization from lake to lake. An Unmanned aerial vehicle (UAV) data-based model for chla estimation is developed in this work and tested on Sentinel-2 imagery without any re-parametrization. The Ensemble-based system (EBS) algorithm was used to train the model. The leave-one-out cross validation technique was applied to evaluate the EBS, at a local scale, where results were satisfactory (R-2 = Nash = 0.94 and RMSE = 5.6 μg chla L-1). A blind database (collected over 89 lakes) was used to challenge the EBS' Sentine-2-derived chla estimates at a regional scale. Results were relatively less good, yet satisfactory (R-2 = 0.85, RMSE= 2.4 μg chla L-1, and Nash = 0.79). However, the EBS has shown some failure to correctly retrieve chla concentration in highly turbid waterbodies. This particularity nonetheless does not affect EBS performance, since turbid waters can easily be pre-recognized and masked before the chl_a modeling.

Abstract: The frequency and spatial distributions of precipitation extremes (PEs) and deep convective clouds (DCC) across the Amazon region were assessed using satellite-derived data. For PEs, CHIRPS dataset for the period 1981-2018 were used to calculate a set of absolute, threshold, duration, and percentile-based threshold indices defined by the Expert Team on Climate Change Detection and Indices. DCC occurrence was assessed based on the Advanced Microwave Sounding Unit data for the period 2002-2018. In northern Amazon (north of 5 degrees S) PEs and DCC are more frequent (>= 60% frequency) during February-June. Averaged trends over these months have shown increase in daily rainfall above 20 mm of near 3 days over the 1981-2018 period, and an increase of 2 consecutive wet days (P >= 1 mm) in the same period. South of 5.S prevalence of PEs and DCC is largely observed during November-March (>= 60% frequency), whereas the longest persistence of dry days is observed during June-August. Though all PE trends point to an intensification of rainfall in November-March, only consecutive dry days in winter (JJA) and spring (SON) show significant trends, pointing to an increase of 7 days over the 38-yr winters. Rainfall extremes over the entire Amazon region were found to be moderate to strongly correlated with the mean vertically integrated moisture divergence, and in southern Amazon also to upper level divergence and upward vertical velocity. Increased frequency of DCC were found over the whole basin (similar to 18% yr(-1)), in contrast to decreased convective overshooting (up to similar to 15.4% yr(-1)).

Abstract: The thickness of a supraglacial layer is critical to the magnitude and time frame of glacier melt. Field-based, short pulse, ground-penetrating radar (GPR) has successfully measured debris thickness during a glacier's melt season, when there is a strong return from the ice-debris interface, but profiling with GPR in the absence of a highly reflective ice interface has not been explored. We investigated the performance of 960 MHz signals over 2 km of transects on Changri Nup Glacier, Nepal, during the post-monsoon. We also performed laboratory experiments to interpret the field data and investigate electromagnetic wave propagation into dry rocky debris. Laboratory tests confirmed wave penetration into the glacier ice and suggest that the ice-debris interface return was missing in field data because of a weak dielectric contrast between solid ice and porous dry debris. We developed a new method to estimate debris thicknesses by applying a statistical approach to volumetric backscatter, and our backscatter-based calculated thickness retrievals gave reasonable agreement with debris depths measured manually in the field (10-40 cm). We conclude that, when melt season profiling is not an option, a remote system near 1 GHz could allow dry debris thickness to be estimated based on volumetric backscatter.

Abstract: Nature's contributions to people (NCP) do not flow automatically from ecosystems to society, but they result from a co-production process of interactions between societal and ecological systems. In this study, we used the collective capabilities approach to address the social dimensions of co-production of the material NCP of cheese. These are the benefits collective structures retrieve from social-ecological interactions that individuals could not have achieved on their own and which frequently exceed pure instrumental values. Collective structures mobilise different types of social capitals in order to generate these collective capabilities. Here, we specifically investigated linkages between collective capabilities and their contributions to common perceptions and local identities. We conducted 44 semi-structured interviews with two distinct different actors' groups in a French Alpine agricultural system surrounding the production of the quality labelled Beaufort cow cheese. We analysed the interviews qualitatively and conducted quantitative analyses as well as content and sentiment analysis to identify the different levels and types of collective investment mobilised by actors to generate collective capabilities. We found that collective capabilities involved in NCP co-production contributed to common perceptions and to specific dimensions of local identities. These can be viewed as the results of relational value construction. Further, the analysis suggests that collective capability relies on dense social interactions between actors that contribute to a good quality of life in itself. This study advances previous attempts to further investigate the role of intra-societal relations for NCP co-production.

Abstract: Missing data is a critical pitfall in the investigation of remotely sensed displacement measurement because it prevents from a full understanding of the physical phenomenon under observation. In the sight of reconstructing incomplete displacement data, this letter presents a data-driven spatiotemporal gap-filling method, which is an extension of the expectation-maximization-empirical orthogonal function (EM-EOF) method. The presented method decomposes an augmented spatiotemporal covariance of a displacement time series into EOF modes and then selects the optimal set of EOF modes to reconstruct the time series. This selection is based on the cross-validation root-mean-square error and a confidence index associated with each eigenvalue. The estimated missing values are then iteratively updated until convergence. Results on displacement time series derived from cross correlation of Sentinel-2 optical images over Fox Glacier in New-Zealand's Alps show that the reconstruction accuracy is improved compared with the EM-EOF method. The proposed extension can tackle challenging cases, i.e., short time series with heterogeneous displacement behaviors corrupted by a large amount of missing data and noise.

Abstract: Glaciers distinct from the Greenland and Antarctic ice sheets are shrinking rapidly, altering regional hydrology(1), raising global sea level(2) and elevating natural hazards(3). Yet, owing to the scarcity of constrained mass loss observations, glacier evolution during the satellite era is known only partially, as a geographic and temporal patchwork(4,5). Here we reveal the accelerated, albeit contrasting, patterns of glacier mass loss during the early twenty-first century. Using largely untapped satellite archives, we chart surface elevation changes at a high spatiotemporal resolution over all of Earth's glaciers. We extensively validate our estimates against independent, high-precision measurements and present a globally complete and consistent estimate of glacier mass change. We show that during 2000-2019, glaciers lost a mass of 267 +/- 16 gigatonnes per year, equivalent to 21 +/- 3 per cent of the observed sea-level rise(6). We identify a mass loss acceleration of 48 +/- 16 gigatonnes per year per decade, explaining 6 to 19 per cent of the observed acceleration of sea-level rise. Particularly, thinning rates of glaciers outside ice sheet peripheries doubled over the past two decades. Glaciers currently lose more mass, and at similar or larger acceleration rates, than the Greenland or Antarctic ice sheets taken separately(7-9). By uncovering the patterns of mass change in many regions, we find contrasting glacier fluctuations that agree with the decadal variability in precipitation and temperature. These include a North Atlantic anomaly of decelerated mass loss, a strongly accelerated loss from northwestern American glaciers, and the apparent end of the Karakoram anomaly of mass gain(10). We anticipate our highly resolved estimates to advance the understanding of drivers that govern the distribution of glacier change, and to extend our capabilities of predicting these changes at all scales. Predictions robustly benchmarked against observations are critically needed to design adaptive policies for the local- and regional-scale management of water resources and cryospheric risks, as well as for the global-scale mitigation of sea-level rise. Analysis of satellite stereo imagery uncovers two decades of mass change for all of Earth's glaciers, revealing accelerated glacier shrinkage and regionally contrasting changes consistent with decadal climate variability.

Abstract: Climate change over High Mountain Asia (HMA, including the Tibetan Plateau) is investigated over the period 1979-2014 and in future projections following the four Shared Socioeconomic Pathways: SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5. The skill of 26 Coupled Model Intercomparison Project phase 6 (CMIP6) models is estimated for near-surface air temperature, snow cover extent and total precipitation, and 10 of them are used to describe their projections until 2100. Similarly to previous CMIP models, this new generation of general circulation models (GCMs) shows a mean cold bias over this area reaching -1.9 [ -8.2 to 2.9] degrees C (90 % confidence interval) in comparison with the Climate Research Unit (CRU) observational dataset, associated with a snow cover mean overestimation of 12 % [ -13 % to 43 %], corresponding to a relative bias of 52 % [ -53 % to 183 %] in comparison with the NOAA Climate Data Record (CDR) satellite dataset. The temperature and snow cover model biases are more pronounced in winter. Simulated precipitation rates are overestimated by 1.5 [0.3 to 2 9] mm d(-1), corresponding to a relative bias of 143 % [31 % to 281 %], but this might be an apparent bias caused by the undercatch of solid precipitation in the APHRODI1E (Asian Precipitation-Highly-Resolved Observational Data Integration Towards Evaluation of Water Resources) observational reference. For most models, the cold surface bias is associated with an overestimation of snow cover extent, but this relationship does not hold for all models, suggesting that the processes of the origin of the biases can differ from one model to another. A significant correlation between snow cover bias and surface elevation is found, and to a lesser extent between temperature bias and surface elevation, highlighting the model weaknesses at high elevation. The models with the best performance for temperature are not necessarily the most skillful for the other variables, and there is no clear relationship between model resolution and model skill. This highlights the need for a better understanding of the physical processes driving the climate in this complex topographic area, as well as for further parameterization developments adapted to such areas. A dependency of the simulated past trends on the model biases is found for some variables and seasons; however, some highly biased models fall within the range of observed trends, suggesting that model bias is not a robust criterion to discard models in trend analysis. The HMA median warming simulated over 2081-2100 with respect to 1995-2014 ranges from 1.9 [1.2 to 2.7] degrees C for SSP1-2.6 to 6.5 [4.9 to 9.0] degrees C for SSP5-8.5. This general warming is associated with a relative median snow cover extent decrease from -9.4 % [ – 16.4 % to -5.0 %] to -32.2 % [ -49.1 % to -25.0 %] and a relative median precipitation increase from 8.5 % [4.8 % to 18.2 %] to 24.9 % [14.4 % to 48.1 %] by the end of the century in these respective scenarios. The warming is 11 % higher over HMA than over the other Northern Hemisphere continental surfaces, excluding the Arctic area. Seasonal temperature, snow cover and precipitation changes over HMA show a linear relationship with the global surface air temperature (GSAT), except for summer snow cover which shows a slower decrease at strong levels of GSAT.

Abstract: The lower parts of two glaciers in the Am range on the western Tibetan Plateau (TP) collapsed on 17 July and 21 September 2016, respectively, causing fatal damage to local people and their livestock. The giant ice avalanches, with a total volume of 150 x 10(6) m(3), had almost melted by September 2019 (about 30 % of the second ice avalanche remained). The impact of these extreme disasters on downstream lakes has not been investigated yet. Based on in situ observation, bathymetry survey and satellite data, we explore the impact of the ice avalanches on the two downstream lakes (i.e., Am Co and Memar Co) in terms of lake morphology, water level and water temperature in the subsequent 4 years (2016-2019). After the first glacier collapse, the ice avalanche slid into Aru Co along with a large amount of debris, which generated great impact waves in Aru Co and significantly modified the lake's shoreline and underwater topography. An ice volume of at least 7.1 x 10(6) m(3 )was discharged into Aru Co, spread over the lake surface and considerably lowered its surface temperature by 2-4 degrees C in the first 2 weeks after the first glacier collapse. Due to the large amount of meltwater input, Memar Co exhibited more rapid expansion after the glacier collapses (2016-2019) than before (2003-2014), in particular during the warm season. The melting of ice avalanches was found to contribute to about 23 % of the increase in lake storage between 2016 and 2019. Our results indicate that the Am glacier collapses had both short-term and long-term impacts on the downstream lakes and provide a baseline in understanding the future lake response to glacier melting on the TP under a warming climate.

Abstract: Twenty-seven models participated in the Earth System Model-Snow Model Intercomparison Project (ESM-SnowMIP), the most data-rich MIP dedicated to snow modeling. Our findings do not support the hypothesis advanced by previous snow MIPs: evaluating models against more variables and providing evaluation datasets extended temporally and spatially does not facilitate identification of key new processes requiring improvement to model snow mass and energy budgets, even at point scales. In fact, the same modeling issues identified by previous snow MIPs arose: albedo is a major source of uncertainty, surface exchange parameterizations are problematic, and individual model performance is inconsistent. This lack of progress is attributed partly to the large number of human errors that led to anomalous model behavior and to numerous resubmissions. It is unclear how widespread such errors are in our field and others; dedicated time and resources will be needed to tackle this issue to prevent highly sophisticated models and their research outputs from being vulnerable because of avoidable human mistakes. The design of and the data available to successive snow MIPs were also questioned. Evaluation of models against bulk snow properties was found to be sufficient for some but inappropriate for more complex snow models whose skills at simulating internal snow properties remained untested. Discussions between the authors of this paper on the purpose of MIPs revealed varied, and sometimes contradictory, motivations behind their participation. These findings started a collaborative effort to adapt future snow MIPs to respond to the diverse needs of the community.

Abstract: This study is based on the analysis of detailed measurements of firn dielectric properties performed in Antarctica through coring down to 106 m. Dielectric measurements in the frequency band (0.4-2.5 GHz) have been carried out using an open-resonator probe. Density was also measured for the same samples. The experimental results confirmed the well-known dependence of the real part of permittivity epsilon' on depth and density, showing an increase of epsilon' with density. The imaginary part also increases with depth with a rather complex dependence on frequency, probably due to the presence of salts or impurities. The analysis of the experimental data was performed by implementing 3D and 2D full wave numerical models, to simulate a mixture of firn crystals at prescribed densities, corresponding to the measured densities on the ice cores. The numerical analysis of the ensemble of inclusions showed that the usual symmetric formulae used for modeling ice dielectric properties agree with the average results of the simulation, but they are not able to explain the spreading of the measured data at given density. A dielectric model was then developed allowing for quantification of the dependence of dielectric properties on density, by combining two models: one consisting in firn crystals into an air host, the other assuming the presence of air inclusions into a homogeneous firn host. The weighted equation is based on the volume fraction. A simple geometric shape (ellipsoidal) is assumed for both ice crystals and air inclusions. This kind of shape is reasonable for the purpose of the dielectric study. The result is a mixture, smoothly changing from firn particles in air (low density) to air bubbles in an ice matrix (high density). A statistical analysis has been accomplished to investigate the dependence of the dielectric properties on the geometrical arrangement of the inclusions. For that purpose, a large number of simulations with different arrangements (micro-states) giving rise to the same average density (macro-states) has been carried out. The permittivity change due to micro-state variability appears to be at least two-three times the model variation due to density alone, and comparable to the measured variability at a given depth, suggesting that firn structure has a significant effect on the dielectric properties.

Abstract: Spatio-temporal variability of the winter surface mass balance is a major uncertainty in the modelling of annual surface mass balance. Moreover, its measurement at high spatio-temporal resolution (sub-200 m) is very useful to force, calibrate or validate models. This study presents the results of year-round field campaigns to study the evolution of the surface mass balance in a 2 km(2) portion of the accumulation zone of the Mer de Glace (France). It is based on repeated LiDAR acquisitions, submergence-velocity measurements and meteorological records. The two methods used to quantify submergence velocities show good agreement. They present a linear temporal evolution without significant seasonal changes but display significant spatial variability. We conclude that a dense network of submergence velocity measurements is required to reduce the uncertainties when computing winter and annual surface mass balance from digital elevation model differencing. Finally, a hight spatio-temporal variability of the winter surface mass balance is highlighted (e.g., a std dev. of 0.92 m in April) even though the topography is homogeneous (std dev. of 25 m). Attempts to relate this variability to different morpho-topographic variables and wind-related indexes show the need for studies conducted at the snowfall event scale to obtain a better understanding of the variability in mass balance at the glacier scale.

Abstract: Rock debris covers similar to 30% of glacier ablation areas in the Central Himalaya and modifies the impact of atmospheric conditions on mass balance. The thermal properties of supraglacial debris are diurnally variable but remain poorly constrained for monsoon-influenced glaciers over the timescale of the ablation season. We measured vertical debris profile temperatures at 12 sites on four glaciers in the Everest region with debris thickness ranging from 0.08 to 2.8 m. Typically, the length of the ice ablation season beneath supraglacial debris was 160 days (15 May to 22 October)-a month longer than the monsoon season. Debris temperature gradients were approximately linear (r(2) > 0.83), measured as -40 degrees C m(-1) where debris was up to 0.1 m thick, -20 degrees C m(-1) for debris 0.1-0.5 m thick, and -4 degrees C m(-1) for debris greater than 0.5 m thick. Our results demonstrate that the influence of supraglacial debris on the temperature of the underlying ice surface, and therefore melt, is stable at a seasonal timescale and can be estimated from near-surface temperature. These results have the potential to greatly improve the representation of ablation in calculations of debris-covered glacier mass balance and projections of their response to climate change.

Abstract: Study region: The upper part of the Guayllabamba and Napo basins (78.2 degrees W, 0.3 degrees S; 18,500 km(2)) in the equatorial Andes, which are vulnerable to stress on the ecosystem services. Study focus: This paper analyses the diurnal cycle of precipitation over a transect from the Andes to the Amazon. The diurnal cycle is estimated as the diurnal distribution of precipitation for 2014-2019 using records from 80 stations. Cluster analysis performed on the diurnal cycle estimates depicts the spatial association between the diurnal and seasonal cycles of precipitation. New hydrological insights: A northwest-southeast spatial variation in the diurnal and seasonal cycles is identified with four groups of stations. In the western part, the seasonal cycles of Groups 1 and 2 are bimodal with precipitation maxima in the March-April and October-November seasons and a short drier season in July-August. In the eastern part, Group 3 also presents bimodality, but a weaker seasonal cycle. Conversely, Group 4 is unimodal with a peak in June. Distinct diurnal cycles are observed in both drier and wetter seasons of Groups 1-3; no marked diurnal cycle is observed in Group 4. Groups 3 and 4 are the most spatially heterogeneous, with an exceptional horizontal variation of 330 mm/yr/km. The analysis of these variations provides insight into the atmospheric dynamics driving precipitation in this zone, and may help to better optimize the water supply system.

Abstract: The Choco low-level jet is among the main regional circulation mechanisms related to the advection of water vapor from the eastern Pacific to northwestern South America. Variations in the intensity of position of the jet core are identified as determinant for regional moisture transport and associated rainfall. This paper analyzes the annual cycle of intensity and latitudinal location of this jet according to different reanalysis and observational datasets. Moreover, we compare possible changes in the Choco jet occurred during past climates, like the little ice age (LIA), with those associated with future scenarios of greenhouse gas concentrations (RCP8.5), using simulations from the Paleoclimate Modelling Intercomparison Project Phase 3 (PMIP3) and the Coupled Model Intercomparison Project Phase 5 (CMIP5). Our results suggest that according to reanalysis/observational data, as well as the CMIP5 models with the best representation of the Choco jet in present climate, there is a positive correlation between the jet intensity and its latitudinal location, and such relationship is associated with the sea level pressure (SLP) difference between the eastern tropical Pacific and the northwestern South American landmass. Hence, stronger (weaker) SLP differences favor a stronger (weaker) intensity and a northward (southward) location of the Choco jet. PMIP3 simulations suggest a stronger and northward Choco jet during LIA due to a stronger SLP difference in comparison to present climate. However, under the RCP8.5 scenario, there is not robust agreement among CMIP5 models although the best models suggest a southward jet at the end of the 21st century. This suggests that the mechanisms influencing the Choco jet may play different roles during past natural climate changes with respect to anthropogenically-forced climate changes.

Abstract: In climate models, the snow albedo scheme generally calculates only a narrowband or broad-band albedo, which leads to significant uncertainties. Here, we present the Versatile ALbedo calculation metHod based on spectrALLy fixed radiative vAriables (VALHALLA version 1.0) to optimize spectral snow albedo calculation. For this optimization, the energy absorbed by the snowpack is calculated by the spectral albedo model Two-streAm Radiative TransfEr in Snow (TARIES) and the spectral irradiance model Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART). This calculation takes into account the spectral characteristics of the incident radiation and the optical properties of the snow based on an analytical approximation of the radiative transfer of snow. For this method, 30 wavelengths, called tie points (TPs), and 16 reference irradiance profiles are calculated to incorporate the absorbed energy and the reference irradiance. The absorbed energy is then interpolated for each wavelength between two TPs with adequate kernel functions derived from radiative transfer theory for snow and the atmosphere. We show that the accuracy of the absorbed energy calculation primarily depends on the adaptation of the irradiance of the reference profile to that of the simulation (absolute difference < 1 W m(-2) for broadband absorbed energy and absolute difference < 0.005 for broadband albedo). In addition to the performance in terms of accuracy and calculation time, the method is adaptable to any atmospheric input (broadband, narrowband) and is easily adaptable for integration into a radiative scheme of a global or regional climate model.

Abstract: Mass balance assessments of the East Antarctic ice sheet (EAIS) are highly sensitive to changes in firn thickness, causing substantial disagreement in estimates of its contribution to sea-level. To better constrain the uncertainty in recent firn thickness changes, we develop an ensemble of 54 model scenarios of firn evolution between 1992 and 2017. Using statistical emulation of firn-densification models, we quantify the impact of firn compaction formulation, differing climatic forcing, and surface snow density on firn thickness evolution. At basin scales, the ensemble uncertainty in firn thickness change ranges between 0.2 and 1.0 cm yr(-1) (15%-300% relative uncertainty), with the choice of climate forcing having the largest influence on the spread. Our results show the regions of the ice sheet where unexplained discrepancies exist between observed elevation changes and an extensive set of modeled firn thickness changes estimates, marking an important step toward more accurately constraining ice sheet mass balance.

Abstract: The 2007-19 glaciological mass-balance series of Mera Glacier in the Everest Region, East Nepal, is reanalysed using the geodetic mass balance assessed by differencing two DEMs obtained from Pleiades stereo-images acquired in November 2012 and in October 2018. The glaciological glacier-wide annual mass balance of Mera Glacier has to be systematically decreased by 0.11 m w.e. a(-1) to match the geodetic mass balance. We attribute part of the positive bias of the glaciological mass balance to an over-estimation of the accumulation above 5520 m a.s.l., likely due to a measurement network unable to capture its spatial variability. Over the period 2007-19, Mera Glacier has lost mass at a rate of -0.41 +/- 0.20 m w.e. a(-1), in general agreement with regional averages for the central Himalaya. We observe a succession of negative mass-balance years since 2013.

Abstract: The retreat and acceleration of Greenland glaciers since the mid-1990s have been attributed to the enhanced intrusion of warm Atlantic Waters (AW) into fjords, but this assertion has not been quantitatively tested on a Greenland-wide basis or included in models. Here, we investigate how AW influenced retreat at 226 marine-terminating glaciers using ocean modeling, remote sensing, and in situ observations. We identify 74 glaciers in deep fjords with AW controlling 49% of the mass loss that retreated when warming increased undercutting by 48%. Conversely, 27 glaciers calving on shallow ridges and 24 in cold, shallow waters retreated little, contributing 15% of the loss, while 10 glaciers retreated substantially following the collapse of several ice shelves. The retreat mechanisms remain undiagnosed at 87 glaciers without ocean and bathymetry data, which controlled 19% of the loss. Ice sheet projections that exclude ocean-induced undercutting may underestimate mass loss by at least a factor of 2.

Abstract: Mountainous regions are a hotspot for water scarcity and anthropogenic pressure on water resources. Substantial uncertainty surrounds projections of future climate and water availability. Furthermore, quantitative and distributed data on water demand are generally scarce, dispersed, and highly heterogeneous. This forms a major bottleneck to studying water resources issues and developing strategies to improve water resource management. Here we present a methodology to produce and evaluate high-resolution gridded maps of anthropogenic surface water demand with application to the Andean region. These data are disaggregated according to the major types of water demand: domestic users, irrigated area, and hydropower. This dataset was built by homogenizing, integrating, and interpolating data obtained from various national institutions in charge of water resource management as well as relevant global datasets. The maps can be used to research anthropogenic impacts on water resources, and to guide regional decision-making in regions such as the Andes.

Abstract: The biodiversity and productivity of the Amazon floodplain depend on nutrients and organic matter transported with suspended sediments. Nevertheless, there are still fundamental unknowns about how hydrological and rainfall variability influence sediment flux in the Amazon River. To address this gap, we analyzed 3069 sediment samples collected every 10 days during 1995-2014 at five gauging stations located in the main rivers. We have two distinct fractions of suspended sediments, fine (clay and silt) and coarse (sand), which followed contrasting seasonal and long-term patterns. By taking these dynamics into account, it was estimated, for first time, in the Amazon plain, that the suspended sediment flux separately measured approximately 60% fine and 40% coarse sediment. We find that the fine suspended sediments flux is linked to rainfall and higher coarse suspended sediment flux is related with discharge. Additionally this work presents the time lag between rainfall and discharge, which is related to the upstream area of the gauging. This result is an important contribution to knowledge of biological and geomorphological issues in Amazon basin.

Abstract: Glacier mass balance (MB) data are crucial to understanding and quantifying the regional effects of climate on glaciers and the high-mountain water cycle, yet observations cover only a small fraction of glaciers in the world. We present a dataset of annual glacier-wide mass balance of all the glaciers in the French Alps for the 1967-2015 period. This dataset has been reconstructed using deep learning (i.e. a deep artificial neural network) based on direct MB observations and remote -sensing annual estimates, meteorological reanalyses and topographical data from glacier inventories. The method's validity was assessed previously through an extensive cross -validation against a dataset of 32 glaciers, with an estimated average error (RMSE) of 0.55 mw.e. a(-1), an explained variance (r(2)) of 75 % and an average bias of -0.021 m w.e. a(-1.) We estimate an average regional area-weighted glacier-wide MB of -0.69 +/- 0.21 (1 sigma) mw.e. a(-1) for the 1967-2015 period with negative mass balances in the 1970s (-0.44 m w.e. a-1), moderately negative in the 1980s (-0.16 m w.e. a(-1)) and an increasing negative trend from the 1990s onwards, up to -1.26 m w.e. a(-1) in the 2010s. Following a topographical and regional analysis, we estimate that the massifs with the highest mass losses for the 1967-2015 period are the Chablais (-0.93 mw.e. a(-1)), Champsaur (-0.86 m w.e. a(-1)), and Haute-Maurienne and Ubaye ranges (-0.84 mw.e. a(-1) each), and the ones presenting the lowest mass losses are the Mont-Blanc (-0.68 mw.e. a1), Oisans and Haute-Tarentaise ranges (-0.75 m w.e. a(-1) each). This dataset available at https://doi.org/10.5281/zenodo.3925378 (Bolibar et al., 2020a) provides relevant and timely data for studies in the fields of glaciology, hydrology and ecology in the French Alps in need of regional or glacier-specific annual net glacier mass changes in glacierized catchments.

Abstract: Two decades into the 21st century there is growing evidence for global impacts of Antarctic and Southern Ocean climate change. Reliable estimates of how the Antarctic climate system would behave under a range of scenarios of future external climate forcing are thus a high priority. Output from new model simulations coordinated as part of the Coupled Model Intercomparison Project Phase 6 (CMIP6) provides an opportunity for a comprehensive analysis of the latest generation of state-of-the-art climate models following a wider range of experiment types and scenarios than previous CMIP phases. Here the main broad-scale 21st century Antarctic projections provided by the CMIP6 models are shown across four forcing scenarios: SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5. End-of-century Antarctic surface-air temperature change across these scenarios (relative to 1995-2014) is 1.3, 2.5, 3.7 and 4.8 degrees C. The corresponding proportional precipitation rate changes are 8, 16, 24 and 31%. In addition to these end-of-century changes, an assessment of scenario dependence of pathways of absolute and global-relative 21st century projections is conducted. Potential differences in regional response are of particular relevance to coastal Antarctica, where, for example, ecosystems and ice shelves are highly sensitive to the timing of crossing of key thresholds in both atmospheric and oceanic conditions. Overall, it is found that the projected changes over coastal Antarctica do not scale linearly with global forcing. We identify two factors that appear to contribute: (a) a stronger global-relative Southern Ocean warming in stabilisation (SSP2-4.5) and aggressive mitigation (SSP1-2.6) scenarios as the Southern Ocean continues to warm and (b) projected recovery of Southern Hemisphere stratospheric ozone and its effect on the mid-latitude westerlies. The major implication is that over coastal Antarctica, the surface warming by 2100 is stronger relative to the global mean surface warming for the low forcing compared to high forcing future scenarios.

Abstract: Permafrost is a ubiquitous phenomenon in the Arctic. Its future evolution is likely to control changes in northern high-latitude hydrology and biogeochemistry. Here we evaluate the permafrost dynamics in the global models participating in the Coupled Model Intercomparison Project (present generation – CMIP6; previous generation – CMIP5) along with the sensitivity of permafrost to climate change. Whilst the northern high-latitude air temperatures are relatively well simulated by the climate models, they do introduce a bias into any subsequent model estimate of permafrost. Therefore evaluation metrics are defined in relation to the air temperature. This paper shows that the climate, snow and permafrost physics of the CMIP6 multi-model ensemble is very similar to that of the CMIP5 multi-model ensemble. The main differences are that a small number of models have demonstrably better snow insulation in CMIP6 than in CMIP5 and a small number have a deeper soil profile. These changes lead to a small overall improvement in the representation of the permafrost extent. There is little improvement in the simulation of maximum summer thaw depth between CMIP5 and CMIP6. We suggest that more models should include a better-resolved and deeper soil profile as a first step towards addressing this. We use the annual mean thawed volume of the top 2 m of the soil defined from the model soil profiles for the permafrost region to quantify changes in permafrost dynamics. The CMIP6 models project that the annual mean frozen volume in the top 2 m of the soil could decrease by 10 %-40% degrees C-1 of global mean surface air temperature increase.

Abstract: This work is motivated by the identification of the land-atmosphere interactions as one of the key sources of uncertainty in climate change simulations. It documents new developments in related processes, namely, boundary layer/convection/clouds parameterizations and land surface parameterization in the Earth System Model of the Institut Pierre Simon Laplace (IPSL). Simulations forced by prescribed oceanic conditions are produced with different combinations of atmospheric and land surface parameterizations. They are used to explore the sensitivity to the atmospheric physics and/or soil physics of major biases in the near surface variables over continents, the energy and moisture coupling established at the soil/atmosphere interface in not too wet (energy limited) and not too dry (moisture limited) soil moisture regions also known as transition or “hot-spot” regions, the river runoff at the outlet of major rivers. The package implemented in the IPSL-Climate Model for the Phase 6 of the Coupled Models Intercomparison Project (CMIP6) allows us to reduce several biases in the surface albedo, the snow cover, and the continental surface air temperature in summer as well as in the temperature profile in the surface layer of the polar regions. The interactions between soil moisture and atmosphere in hotspot regions are in better agreement with the observations. Rainfall is also significantly improved in volume and seasonality in several major river basins leading to an overall improvement in river discharge. However, the lack of consideration of floodplains and human influences in the model, for example, dams and irrigation, impacts the realism of simulated discharge.

Abstract: In polar regions, where the boundary layer is often stably stratified, atmospheric models produce large biases depending on the boundary-layer parametrizations and the parametrization of the exchange of energy at the surface. This model intercomparison focuses on the very stable stratification encountered over the Antarctic Plateau in 2009. Here, we analyze results from 10 large-eddy-simulation (LES) codes for different spatial resolutions over 24 consecutive hours, and compare them with observations acquired at the Concordia Research Station during summer. This is a challenging exercise for such simulations since they need to reproduce both the 300-m-deep convective boundary layer and the very thin stable boundary layer characterized by a strong vertical temperature gradient (10 K difference over the lowest 20 m) when the sun is low over the horizon. A large variability in surface fluxes among the different models is highlighted. The LES models correctly reproduce the convective boundary layer in terms of mean profiles and turbulent characteristics but display more spread during stable conditions, which is largely reduced by increasing the horizontal and vertical resolutions in additional simulations focusing only on the stable period. This highlights the fact that very fine resolution is needed to represent such conditions. Complementary sensitivity studies are conducted regarding the roughness length, the subgrid-scale turbulence closure as well as the resolution and domain size. While we find little dependence on the surface-flux parametrization, the results indicate a pronounced sensitivity to both the roughness length and the turbulence closure.

Abstract: During the last decade, the number of available satellite observations has increased significantly, allowing for far more frequent measurements of the glacier speed. Appropriate methods of post-processing need to be developed to efficiently deal with the large volumes of data generated and relatively large intrinsic errors associated with the measurements. Here, we process and combine together measurements of ice velocity of Russell Gletscher in Greenland from three satellites-Sentinel-1, Sentinel-2, and Landsat-8, creating a multi-year velocity database with high temporal and spatial resolution. We then investigate post-processing methodologies with the aim of generating corrected, ordered, and simplified time series. We tested rolling mean and median, cubic spline regression, and linear non-parametric local regression (LOWESS) smoothing algorithms to reduce data noise, evaluated the results against ground-based GPS in one location, and compared the results between two locations with different characteristics. We found that LOWESS provides the best solution for noisy measurements that are unevenly distributed in time. Using this methodology with these sensors, we can robustly derive time series with temporal resolution of 2-3 weeks and improve the accuracy on the ice velocity to about 10 m/yr, or a factor of three compared to the initial measurements. The presented methodology could be applied to the entire Greenland ice sheet with an aim of reconstructing comprehensive sub-seasonal ice flow dynamics and mass balance.

Abstract: Andean glaciers are among the fastest shrinking and largest contributors to sea level rise on Earth. They also represent crucial water resources in many tropical and semi-arid mountain catchments. Yet the magnitude of the recent ice loss is still debated. Here we present Andean glacier mass changes (from 10 degrees N to 56 degrees S) between 2000 and 2018 using time series of digital elevation models derived from ASTER stereo images. The total mass change over this period was -22.9 +/- 5.9 Gt yr(-1) (-0.72 +/- 0.22 m w.e. yr(-1) (m w.e., metres of water equivalent)), with the most negative mass balances in the Patagonian Andes (-0.78 +/- 0.25 m w.e. yr(-1)) and the Tropical Andes (-0.42 +/- 0.24 m w.e. yr(-1)), compared to relatively moderate losses (-0.28 +/- 0.18 m w.e. yr(-1)) in the Dry Andes. Subperiod analysis (2000-2009 versus 2009-2018) revealed a steady mass loss in the tropics and south of 45 degrees S. Conversely, a shift from a slightly positive to a strongly negative mass balance was measured between 26 and 45 degrees S. In the latter region, the drastic glacier loss in recent years coincides with the extremely dry conditions since 2010 and partially helped to mitigate the negative hydrological impacts of this severe and sustained drought. These results provide a comprehensive, high-resolution and multidecadal data set of recent Andes-wide glacier mass changes that constitutes a relevant basis for the calibration and validation of hydrological and glaciological models intended to project future glacier changes and their hydrological impacts.

Abstract: The 30-year simulations of seasonal snow cover in 22 physically based models driven with bias-corrected meteorological reanalyses are examined at four sites with long records of snow observations. Annual snow cover durations differ widely between models, but interannual variations are strongly correlated because of the common driving data. No significant trends are observed in starting dates for seasonal snow cover, but there are significant trends towards snow cover ending earlier at two of the sites in observations and most of the models. A simplified model with just two parameters controlling solar radiation and sensible heat contributions to snowmelt spans the ranges of snow cover durations and trends. This model predicts that sites where snow persists beyond annual peaks in solar radiation and air temperature will experience rapid decreases in snow cover duration with warming as snow begins to melt earlier and at times of year with more energy available for melting.

Abstract: Study region: The Amazonas River and its tributaries (Peru), where riparian farmers face hydrological events that put their lowland crops at high risk of production loss during the flood recession period. Study focus: This paper analyzes the hydro-meteorological mechanisms over the Andes-Amazon basins that produce “repiquetes”, which are sudden reversals of the river stage. They are defined and characterized for the period 1996-2018 by using river stage data from three hydrological gauging stations for the Amazonas, Maranon and Ucayali Rivers. Daily rainfall and low-level winds depict the large-scale atmospheric patterns associated with repiquetes. New hydrological insights: Among 73 significant repiquetes (reversal >= 20 cm) observed in the Amazonas River, 64 % were preceded by repiquetes only in the Maranon River, 5 % by repiquetes only in the Ucayali River, 21 % by repiquetes in both rivers and 10 % was only registered in the Amazonas River without upstream precursor. These results show that repiquetes in the Maranon River are the primary precursors of repiquetes in the Amazonas River. Most repiquetes are associated with abundant rainfall over the Peruvian and Ecuadorian Andes-Amazon transition region related to a remarkable change in the direction of the meridional wind, from north to south, and an easterly flow five to three days before the beginning of a repiquete in the Amazonas River.

Abstract: We present in this paper the results of the Russian Academy of Sciences North Atlantic Atmospheric Downscaling (RAS-NAAD) project, which provides a 40-yr 3D hindcast of the North Atlantic (10 degrees-80 degrees N) atmosphere at 14-km spatial resolution with 50 levels in the vertical direction (up to 50 hPa), performed with a regional setting of theWRF-ARW3.8.1model for the period 1979-2018 and forced by ERA-Interim as a lateral boundary condition. The dataset provides a variety of surface and free-atmosphere parameters at sigma model levels and meets many demands of meteorologists, climate scientists, and oceanographers working in both research and operational domains. Three-dimensional model output at 3-hourly time resolution is freely available to the users. Our evaluation demonstrates a realistic representation of most characteristics in both datasets and also identifies biases mostly in the ice-covered regions. High-resolution and nonhydrostatic model settings in NAAD resolve mesoscale dynamics first of all in the subpolar latitudes. NAAD also provides a new view of the North Atlantic extratropical cyclone activity with a much larger number of cyclones as compared with most reanalyses. It also effectively captures highly localized mechanisms of atmospheric moisture transports. Applications of NAAD to ocean circulation and wave modeling are demonstrated.

Abstract: Monomethylmercury (MMHg) concentrations in aquatic biota from Lake Titicaca are elevated although the mercury (Hg) contamination level of the lake is low. The contribution of sediments to the lake MMHg pool remained however unclear. In this work, seven cores representative of the contrasted sediments and aquatic ecotopes of Lake Titicaca were sliced and analyzed for Hg and redox-sensitive elements (Mn, Fe, N and S) speciation in pore-water (PW) and sediment to document early diagenetic processes responsible for MMHg production and accumulation in PW during organic matter (OM) oxidation. The highest MMHg concentrations (up to 12.2 ng L-1 and 90% of THg) were found in subsurface PWs of the carbonate-rich sediments which cover 75% of the small basin and 20% of the large one. In other sediment facies, the larger content of OM restricted MMHg production and accumulation in PW by sequestering Hg in the solid phase and potentially also by decreasing its bioavailability in the PW. Diagenetically reduced S and Fe played a dual role either favoring or restricting the availability of Hg for biomethylation. The calculation of theoretical diffusive fluxes suggests that Lake Titicaca bottom sediments are a net source of MMHg, accounting for more than one third of the daily MMHg accumulated in the water column of the Lago Menor. We suggest that in the context of rising anthropogenic pressure, the enhancement of eutrophication in high altitude Altiplano lakes may increase these MMHg effluxes into the water column and favor its accumulation in water and biota. (C) 2020 Elsevier B.V. All rights reserved.

Abstract: Bathymetry (seafloor depth), is a critical parameter providing the geospatial context for a multitude of marine scientific studies. Since 1997, the International Bathymetric Chart of the Arctic Ocean (IBCAO) has been the authoritative source of bathymetry for the Arctic Ocean. IBCAO has merged its efforts with the Nippon Foundation-GEBCO-Seabed 2030 Project, with the goal of mapping all of the oceans by 2030. Here we present the latest version (IBCAO Ver. 4.0), with more than twice the resolution (200 x 200m versus 500 x 500m) and with individual depth soundings constraining three times more area of the Arctic Ocean (similar to 19.8% versus 6.7%), than the previous IBCAO Ver. 3.0 released in 2012. Modern multibeam bathymetry comprises similar to 14.3% in Ver. 4.0 compared to similar to 5.4% in Ver. 3.0. Thus, the new IBCAO Ver. 4.0 has substantially more seafloor morphological information that offers new insights into a range of submarine features and processes; for example, the improved portrayal of Greenland fjords better serves predictive modelling of the fate of the Greenland Ice Sheet. Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12369314

Abstract: Iceberg calving is a major component of glacier mass ablation that is not well understood due to a lack of detailed temporal and spatial observations. Here, we measure glacier speed and surface elevation at 3 min intervals using a portable radar interferometer at Kangilernata Sermia, West Greenland, for a period of 2 weeks in July 2016. We detect a 20% diurnal variation in glacier speed in phase with tidal height propagating kilometers inland. We find no speedup from ice shedding off the calving face or the detachment of floating ice blocks but a 30% speedup within a few hundred meters of the ice front that persists for days when calving removes full-thickness grounded ice blocks. Within one ice thickness from the calving front, we detect maximum strain rates 2 to 3 times larger than observable from satellite data, which has implications for studying iceberg calving as a fracturing process.

Abstract: The monitoring of snow-covered surfaces on Earth is largely facilitated by the wealth of satellite data available, with increasing spatial resolution and temporal coverage over the last few years. Yet to date, retrievals of snow physical properties still remain complicated in mountainous areas, owing to the complex interactions of solar radiation with terrain features such as multiple scattering between slopes, exacerbated over bright surfaces. Existing physically based models of solar radiation across rough scenes are either too complex and resource-demanding for the implementation of systematic satellite image processing, not designed for highly reflective surfaces such as snow, or tied to a specific satellite sensor. This study proposes a new formulation, combining a forward model of solar radiation over rugged terrain with dedicated snow optics into a flexible multi-sensor tool that bridges a gap in the optical remote sensing of snow-covered surfaces in mountainous regions. The model presented here allows one to perform rapid calculations over large snow-covered areas. Good results are obtained even for extreme cases, such as steep shadowed slopes or, on the contrary, strongly illuminated sun-facing slopes. Simulations of Sentinel-3 OLCI (Ocean and Land Colour Instrument) scenes performed over a mountainous region in the French Alps allow us to reduce the bias by up to a factor of 6 in the visible wavelengths compared to methods that account for slope inclination only. Furthermore, the study underlines the contribution of the individual fluxes to the total top-of-atmosphere radiance, highlighting the importance of reflected radiation from surrounding slopes which, in midwinter after a recent snowfall (13 February 2018), accounts on average for 7% of the signal at 400 nm and 16% at 1020 nm (on 13 February 2018), as well as of coupled diffuse radiation scattered by the neighbourhood, which contributes to 18% at 400 nm and 4% at 1020 nm. Given the importance of these contributions, accounting for slopes and reflected radiation between terrain features is a requirement for improving the accuracy of satellite retrievals of snow properties over snow-covered rugged terrain. The forward formulation presented here is the first step towards this goal, paving the way for future retrievals.

Abstract: Manually collected snow data are often considered as ground truth for many applications such as climatological or hydrological studies. However, there are many sources of uncertainty that are not quantified in detail. For the determination of water equivalent of snow cover (SWE), different snow core samplers and scales are used, but they are all based on the same measurement principle. We conducted two field campaigns with 9 samplers commonly used in observational measurements and research in Europe and northern America to better quantify uncertainties when measuring depth, density and SWE with core samplers. During the first campaign, as a first approach to distinguish snow variability measured at the plot and at the point scale, repeated measurements were taken along two 20 m long snow pits. The results revealed a much higher variability of SWE at the plot scale (resulting from both natural variability and instrumental bias) compared to repeated measurements at the same spot (resulting mostly from error induced by observers or very small scale variability of snow depth). The exceptionally homogeneous snowpack found in the second campaign permitted to almost neglect the natural variability of the snowpack properties and focus on the separation between instrumental bias and error induced by observers. Reported uncertainties refer to a shallow, homogeneous tundra-taiga snowpack less than 1 m deep (loose, mostly recrystallised snow and no wind impact). Under such measurement conditions, the uncertainty in bulk snow density estimation is about 5% for an individual instrument and is close to 10% among different instruments. Results confirmed that instrumental bias exceeded both the natural variability and the error induced by observers, even in the case when observers were not familiar with a given snow core sampler.

Abstract: Glacier mass loss is recognized as a major contributor to current sea level rise. However, large uncertainties remain in projections of glacier mass loss on global and regional scales. We present an ensemble of 288 glacier mass and area change projections for the 21st century based on 11 glacier models using up to 10 general circulation models and four Representative Concentration Pathways (RCPs) as boundary conditions. We partition the total uncertainty into the individual contributions caused by glacier models, general circulation models, RCPs, and natural variability. We find that emission scenario uncertainty is growing throughout the 21st century and is the largest source of uncertainty by 2100. The relative importance of glacier model uncertainty decreases over time, but it is the greatest source of uncertainty until the middle of this century. The projection uncertainty associated with natural variability is small on the global scale but can be large on regional scales. The projected global mass loss by 2100 relative to 2015 (79 +/- 56 mm sea level equivalent for RCP2.6, 159 +/- 86 mm sea level equivalent for RCP8.5) is lower than, but well within, the uncertainty range of previous projections.

Abstract: Getz Ice Shelf, the largest producer of ice shelf meltwater in Antarctica, buttresses glaciers that hold enough ice to raise sea level by 22 cm. We present a new bathymetry of its sub-ice shelf cavity using a three-dimensional inversion of airborne gravity data constrained by multibeam bathymetry at sea and a reconstruction of the bedrock from mass conservation on land. The new bathymetry is deeper than previously estimated with differences exceeding 500 m in a number of regions. When incorporated into an ocean model, it yields a better description of the spatial distribution of ice shelf melt, specifically along glacier grounding lines. While the melt intensity is overestimated because of a positive bias in ocean thermal forcing, the study reveals the main pathways along which warm oceanic water enters the cavity and corroborates the observed rapid retreat of Berry Glacier along a deep channel with a retrograde bed slope.

Abstract: Water flowing below glaciers exerts a major control on glacier basal sliding. However, our knowledge of the physics of subglacial hydrology and its link with sliding is limited because of lacking observations. Here we use a 2-year-long dataset made of on-ice-measured seismic and in situ-measured glacier basal sliding speed on Glacier d'Argentiere (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels' hydraulic radius and hydraulic pressure gradient. At seasonal timescales we find that hydraulic radius and hydraulic pressure gradient respectively exhibit a 2- and 6-fold increase from spring to summer, followed by comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels' behaviour at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behaviour that is consistent with channels behaving out of equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which prove that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speed, which supports that the widespread in-efficient drainage system (cavities) is likely pressurized concomitantly with the channel system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speed) through an efficient hydraulic connection between the two systems. The present findings provide an essential basis for testing the physics represented in subglacial hydrology and glacier sliding models.

Abstract: Regional changes in dry-season water availability over recent decades can be attributed to human-induced climate change, according to analyses of global reconstructions. Human-induced climate change impacts the hydrological cycle and thus the availability of water resources. However, previous assessments of observed warming-induced changes in dryness have not excluded natural climate variability and show conflicting results due to uncertainties in our understanding of the response of evapotranspiration. Here we employ data-driven and land-surface models to produce observation-based global reconstructions of water availability from 1902 to 2014, a period during which our planet experienced a global warming of approximately 1 degrees C. Our analysis reveals a spatial pattern of changes in average water availability during the driest month of the year over the past three decades compared with the first half of the twentieth century, with some regions experiencing increased and some decreased water availability. The global pattern is consistent with climate model estimates that account for anthropogenic effects, and it is not expected from natural climate variability, supporting human-induced climate change as the cause. There is regional evidence of drier dry seasons predominantly in extratropical latitudes and including Europe, western North America, northern Asia, southern South America, Australia and eastern Africa. We also find that the intensification of the dry season is generally a consequence of increasing evapotranspiration rather than decreasing precipitation.