Limit global warming to 1.5°C and halve the land ice contribution to sea level this century

We can halve the contribution of land ice to sea level rise this century, if we increase the ambition of our emissions commitments sufficiently to meet the goals of the Paris Agreement

The latest Special Report on Ocean and Cryosphere Change in a Changing Climate (SROCC) prepared by the Intergovernmental Panel on Climate Change (IPCC) in 2019 projects a global average surface atmospheric warming in 2100 of between 1°C and more than 4°C relative to the period 1986-2005 depending on the evolution of all human activities, and an increase in global and average ocean surface temperature of between just under 1°C and 3°C. These temperature change projections would lead to increased melting of mountain glaciers and polar ice caps at the surface, as well as an acceleration in the rate of movement of outlet glaciers from the polar ice caps, increasing iceberg calving, and increased melting beneath glaciers terminating in the ocean. Together, these processes would result in a loss of continental ice mass, which would contribute to the rise in mean sea level. The study led by Tamsin Edwards of King’s College London published in the journal Nature, including the collaboration of more than 80 researchers and 58 research institutes from many countries, including several researchers from the IGE and LSCE, estimated that limiting global warming to 1. 5°C above pre-industrial values (period 1850-1900), thus complying with the December 2015 Paris agreements, would halve the estimate, based on current greenhouse gas emission reduction commitments (Nationally Determined Contributions, NDCs), of the contribution of continental ice to mean sea level rise by 2100, from 25 to 13 cm SLE*.

The strength and originality of this study is for the first time to take into account polar ice caps and mountain glaciers in a coherent uncertainty estimation framework, using a statistical emulator. Indeed, the explicit (and not statistical) simulation of all the more than 210,000 mountain glaciers of the planet over long time periods has only been possible for a few years, and the first international scientific exercise of comparison of global physical glacier models (GlacierMIP) dates only from last year. This new study led by Tamsin Edwards explores the probability distributions of the contribution of continental ice to mean sea level from different glacier models but also from different polar cap models. In addition, these results use the new socio-economic scenarios (SSPs) of climate change created by the IPCC as well as the latest temperature change projections. The inclusion of glaciers in the statistical emulator showed that they will probably (median response) be responsible for half of the sea level rise from continental ice over the 21st century.

Finally, this study has confirmed the existing uncertainties regarding the contribution of Antarctica to the rise in mean sea level for the end of the 21st century. The response of Antarctica to anthropogenic greenhouse gas emission scenarios is uncertain due to the competition between increased snowfall under a warmer climate and increased ice mass loss via melting under floating ice shelves and associated ice flow acceleration. An important result of this statistical emulation is also the identification of a much higher risk of sea level rise associated with Antarctica. Indeed, under plausible but pessimistic physical assumptions (e.g. collapse of some ice shelves and significant basal melting under some outlet glaciers), still for a climate change scenario based on current national commitments (NDCs), the loss of Antarctic mass could be 5 times larger than the value currently considered most likely and thus increase the median contribution of continental ice to sea level to 42 cm SLE. Moreover, there is a significant risk (95 percentile) that sea level rise could exceed 50 cm, even in an optimistic scenario of keeping the temperature below 1.5°C of warming ! This risk could jeopardize flood mitigation and adaptation strategies along the coast.

In conclusion, even considering a single scenario of changes in greenhouse gas emissions (NDCs here), the range of uncertainty is still very large : the median estimate of global mean sea level rise in 2100 is 25 cm and the most pessimistic estimate (i.e., with plausible but pessimistic physical assumptions for the polar caps) is 42 cm. Thus, adaptation strategies should allow for a higher uncertainty factor until "climate" policies and Antarctic response to climate change are better constrained and known.

* SLE stands for sea-level equivalent, i.e. whether the freshwater input to the ocean is evenly distributed over the ocean (unit is meters).

Reference : Edwards,T. et al, Projected land ice contributions to 21 century sea level rise. Nature (2021).

Scientific contacts : Nicolas Champollion (nicolas.champollion univ-grenoble-alpes.fr) and Nicolas Jourdain (Nicolas.jourdain univ-grenoble-alpes.fr)

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