On the role of melt lakes on the speed of ice-sheet deglaciation
Written by Aurélien Quiquet no comments
Under current and forecasted climate change, it is expected that the current large ice-sheets (Greenland and Antarctica) will partially melt. One question is whether current ice sheet models include the necessary processes to dynamically simulate ice-sheet retreat. To better understand the capability of current ice-sheet models to simulate the dynamical behavior of ice-sheet retreat in periods of large climate warming, one period of interest is the last deglaciation (21 ka B.P. - 7 ka B.P.). When ice-sheet are retreating, the melt water is forming large lakes that are formed in the area where the surface is depressed by the ice-sheets. We show that these lakes can lead to large-scale ice sheet collapse and rapid sea-level rise.
During the last deglaciation, these so-called proglacial lakes were numerous in the Northern Hemisphere and could reach considerable sizes. For example, Lake Agassiz-Ojibway, which bordered the southern margin of the North American ice sheet, was about 1.5 million km², an area equivalent to the extent of Italy, France, Switzerland, Germany, Belgium and the Netherlands combined. The importance that these lakes may have had on climate change is widely recognised, but their role in ice sheet dynamics has never been quantified until now.
In this study we used a set of numerical simulations covering the period of the last deglaciation to investigate the role of proglacial lakes on the dynamics of the North American ice sheet. We were able to show that lakes can lead to large-scale mechanical instabilities that can account for half of the mass loss of the ice sheet when it retreats in the present Hudson Bay region.
This mechanism is explained by the fact that the grounding line, i.e. the transition from grounded to floating ice, is unstable when the bedrock slopes backwards (upstream is deeper than downstream). The sinking of the bedrock under the North American ice sheet during the ice age (isostasy) produces exactly this configuration and it is in these isostatic depressions that the large proglacial lakes appear.
These results allow us to revisit the mechanisms at work during the last deglaciation. While surface melting linked to climate warming was the main mechanism put forward to explain the retreat of the North American ice sheet, we have been able to show that purely mechanical aspects, i.e. independent of climate, can also play a crucial role.
These results were made possible by the recent improvements in the representation of the mechanical instabilities that can occur at the grounding line. These instabilities could affect a large part of Antarctica in the future.
Figure. Ice flow velocities of the North American ice sheet, plotted on the 3D topography for two periods separated by 1000 years. The instability is characterised by a rapid retreat of the grounding line and an acceleration of the ice flow velocities.
A. Quiquet, C. Dumas, D. Paillard, G. Ramstein, C. Ritz, and D.M. Roche (2021): Deglacial Ice Sheet Instabilities Induced by Proglacial Lakes, Geophysical Research Letters, 48, e2020GL092141, doi:10.1029/2020GL092141.