IPSL-CM-Elmer/Ice: a new coupled ice sheet – climate model

The contribution of ice sheets to future sea level rise remains highly uncertain, and complex positive feedback mechanisms can lead to accelerating melt in a warming climate. Yet, few climate models explicitly represent ice flow of Greenland and Antarctica, or their interactions with the rest of the climate system.

Here we present the coupling of the Elmer/Ice ice sheet model with the IPSL-CM7 climate model. Two-way coupling with the atmospheric and oceanic components of IPSL-CM7 (LMDZ and NEMO, respectively) occurs every simulated year. On the atmospheric side, the surface mass balance from LMDZ is used to force the ice sheet model. In this coupling step, a positive degree day scheme is used to re-calculate surface melt and runoff for Greenland to yield more realistic results. The elevation of the LMDZ domain’s bottom surface is in turn updated to account for the new ice sheet geometry provided by Elmer/Ice. On the ocean side, sub-ice shelf melting is explicitly represented where NEMO's resolution allows it and is extrapolated near the grounding line and under small ice shelves, where the cavity geometry is not resolved by the ocean model. NEMO’s computational domain is updated yearly to account for new icy or wet cells.

We then present the results of two 100-year simulations, which were conducted to test the robustness of the coupling and the behaviour of the model in a warming climate. The first  simulation has a constant pre-industrial atmospheric CO₂ concentration, whereas in the second one  the CO₂ concentration increases by 1% every year. We describe some interesting features that emerge due to increasing CO₂ concentrations, such as the transition from cold to warm water on the continental shelf of the Amundsen Sea, and a retreat of the grounding line in this region.

While still in its early stage of development, this work is an important milestone in the addition of interactive ice sheets within the IPSL-CM7 climate model. Future developments include interactive ice fronts, which are currently fixed in the model, and the possibility of uncovering solid ground as ice sheets retreat.