Are sea level projections missing a key mechanism?

Models used to predict sea level rise in the context of global warming may be missing an essential process rooted at the base of the Antarctic ice cap. At least that’s what a study published in the famous journal NatureCommunications this September 14.

Sea level rise has two main determinants. On the one hand, the warming of the water which causes its dilation, and on the other the melting of the continental ice which are the glaciers and the polar caps of Greenland andAntarctic. It should be recalled in this respect that the uncertainty with respect to the future rise in sea level depends very largely on the behavior of the southern ice cap, the instabilities and other retroactive processes of which remain difficult to quantify.

Sea level rise: the potentially underestimated contribution of basal melt

However, a team of researchers from the University of Stanford (California, United States) has identified a process which is not currently taken into account by any predictive model or forecasting calculation but which would nevertheless be able to contribute significantly to the sea ​​level rise in the coming decades. It is basal melting in Antarcticathat is to say the one that takes place at the base of the cap, just above the bedrock (see the diagram below).

Schematic representation of the basal melting that takes place above the bedrock (brown). A frozen bed can be identified in the figure (frozen bed), a bed that is frozen but whose temperature is close to the melting point (thawable bed) and a bed in the thawing phase (thawed bed). Finally, the black arrows indicate the ice flow. Credits: Eliza J. Dawson & coll. 2022.

If the phenomenon is not new as such, it was thought that it only varied over long time scales. However, the results obtained by the scientists show that this is not the case and that with a rapid warming of the climate, basal melting can amplify in just a few decadesat least for certain sectors, and therefore contribute significantly to sea level rise in the relatively short term.

These latter ” represent about 11% of the total area of ​​the ice sheet and would be likely to undergo a mass loss of an order of magnitude greater than the dynamic loss observed over the last four decades “, reports the study, which corresponds to several millimeters per year. In question, mechanical changes such as the accelerated sliding of the ice on the bedrock, a source of friction and therefore of heating by friction.

A significant point of the study is that the areas most at risk are in East Antarctica, a region generally assumed to be stable. ” The whole community is really focused on Thwaites Glacier right now “says Dustin Schroeder, co-author of the study. ” But some of the regions that are the usual suspects of significant changes are not the most impacting areas in this study. “.

Mechanisms that remain to be clarified

A question that still remains unanswered is to know more precisely what accelerates the basal slip. According to the first results obtained using numerical simulations taking into account the temperature at the foot of the Antarctic ice cap, warming ocean waters would be one of the main factors because it weakens the ice shelves and consequently induces an acceleration of the flow of the glaciers located upstream.

Warmer ocean water does not necessarily reach these parts of East Antarctica as it does in parts of West Antarctica, but it is close, so there has development potential “, notes the researcher. ” When considering recent theoretical work showing that bed-level thermal processes can be easily activated, this makes short-term thawing of the ice sheet bed appear to be a much easier lever to activate than we might expect. pension “.

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