New estimates for rise in sea levels due to ice sheet mass loss under future climate change

An international consortium of researchers under the aegis of CMIP6 has calculated new estimates for the melting of Earth’s ice sheets due to greenhouse gas emissions and its impact on sea levels, showing that the ice sheets could together contribute more than 40 cm by the end of 2100.

Flying over the vast ice sheet of Greenland, on board an Iceland Air flight from Reykjavik, Iceland, to Seattle, USA. Notice the small “lakes” and rivers of melt water (CC BY 2.0, Bernt Rostad/Flickr)

One of the many effects of global warming is the increase of sea levels due to the melting and retreat of the ice sheets in the Arctic and the Antarctic. As the sea level rises, large areas of densely populated coastal land will become uninhabitable. It is vital that we understand the impact climate change interventions could have on the rate of melting and, consequently, changes in sea level.

The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), consisting of more than 60 scientists from 36 institutions across the world – including Dr. Christopher Chambers and Professor Ralf Greve from the Institute of Low Temperature Science at Hokkaido University – has used the latest generation of models to estimate the impact of global warming on ice sheets. Their results were published in a special issue of the journal The Cryosphere on September 17.

The ISMIP6 team projected the changes of the Greenland and Antarctic Ice Sheets (GrIS/AIS) between 2015 and 2100 under global warming conditions predicted by climate modelling. The advantage of ISMIP6 was that it used fourteen different models to estimate the changes in the ice sheet under two greenhouse gas (GHG) emissions pathways: the pessimistic scenario, where there is no change in current trends, leading to a rapid increase in GHG emissions; and the optimistic scenario, where comprehensive steps are taken to reduce GHG emissions. Using different models to estimate the changes was vital to this endeavour, as they have different baselines and assumptions.

Ralf Greve and Christopher Chambers modelled the behavior of ice sheets using the SICOPOLIS model. The first version was released in 1995 by Greve, and since then it has been continuously developed and used for a large number of studies. The SICOPOLIS model uses data from 1990 as a baseline for the experiments. As the model has a 25-year history with an uninterrupted development and publication record, it brought a unique perspective to ISMIP6.

ISMIP6 mean projections for rise in sea levels through 2100 due to the melting of the Greenland Ice Sheet. The red-shaded area is the projection for the pessimistic scenario, while the blue-shaded is the projection for the optimistic scenario (Heiko Goelzer, et al., The Cryosphere, September 17, 2020).

The rate of ice sheet change was modelled under different forcings provided by climate models: future changes of precipitation, surface melting over the ice sheets, surface and ocean temperatures. The goal was to estimate how much the mass loss of the ice sheets would contribute to the rise in average sea levels beyond what has already been put in motion. The study found that, by 2100, the GrIS would raise sea levels by 4-14 cm under the pessimistic scenario, but only 1.5-5 cm under the optimistic scenario. For the AIS, the results point to a greater range of possibilities, from ice sheet change that decreases sea level by 7.8 cm to increasing it by 30 cm under the pessimistic scenario, and an increase of 0-3 cm under the optimistic scenario.

“Mass change from the AIS is notoriously difficult to predict: Increasing ocean temperatures erode the bottom of large floating ice shelves, causing loss; while the AIS can also gain mass by increased snowfall due to warmer air temperatures. However, we are constantly improving our understanding of the ice sheets and their interaction with the Earth’s climate system. Modelling intercomparison studies like ISMIP6 are an effective tool to provide society with the necessary information, including uncertainties, for rational decisions”, said Ralf Greve.

This effort took over six years of collaboration, and the findings of ISMIP6 will help inform the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC), scheduled for release in 2022. ISMIP6 is part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). CMIP6 currently has a total of 23 endorsed Model Intercomparison Projects (MIPs), and has been invaluable in assessing our understanding of climate and climate change.

Christopher Chambers and Ralf Greve, the collaborators from Hokkaido University, in front of the ice crystal mosaic in the Institute of Low Temperature Science (Photo: Ralf Greve).

Original Papers:

Heiko Goelzer, et al. The future sea-level contribution of the Greenland ice sheet: a multi-model ensemble study of ISMIP6. The Cryosphere. September 17, 2020.

DOI: 10.5194/tc-14-3033-2020

Hélène Seroussi, et al. ISMIP6 Antarctica: a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century. The Cryosphere. September 17, 2020.

DOI: 10.5194/tc-14-3071-2020

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