We will investigate the stability of the Greenland ice sheet under anthropogenic warming and its potential contribution to sea level rise over coming centuries and millennia. The paleoclimate record highlights the susceptibility of ice sheets and sea level to increased global temperatures, even for global warming much less severe than that predicted for future climate. The critical role of climate feedbacks in regulating ice sheets over long timescales highlights the need to use coupled icesheet/climate models in assessments of past and future sea level rise. However, coupled climate models are only beginning to include dynamic ice sheets, coupling mechanisms between ice sheets and climate, and the spatial resolution needed to properly simulate the coupled ice-sheet/climate system.

Here, we propose to use a state-of-the-art version of the Community Earth System Model (CESM) coupled to the Community Ice Sheet Model (CISM) to assess the processes and feedbacks that control the response of the Greenland ice sheet to past and projected future warm climate states. We will focus on the coupled long-timescale evolution of the climate and the Greenland ice sheet for three time periods of warmer temperatures: the mid-Pliocene, the Last Interglacial, and future projections to year 3000. Each period has different forcings, but all have significant (though uncertain) contributions of the Greenland ice sheet to sea level rise.

Past warm climate states are ideal proving grounds for models that are to be used for sea level projections. Thus, from a policy perspective, realistic simulation of previous warm periods lends confidence to assessments of future changes. The climate and Greenland ice sheet simulations of the mid-Pliocene and the Last Interglacial periods will be benchmarked against paleo observations to provide a robust validation of model performance in warm past climate states. To this end, our CESMCISM simulations of past warm states will be used as critical confirmation of the coupled model, prior to assessing future ice-sheet/climate evolution and sea level rise. Recent pioneering developments in CESM and CISM have enabled a complex, physicallybased, and conservative two-way coupling between ice sheets and the atmosphere and land surface, allowing the new CESM-CISM model to be used for the first time to simulate fully coupled climate/ice-sheet interactions during these periods. In addition, increases in computing power and parallel optimization of codes have made possible millennial and longer simulations. The model developments and simulations we propose are aligned with DOE’s ESM and RGCM long-timescale climate modeling element and the DOE Earth and Environmental System Modeling program objective to "develop and analyze high fidelity community models representing Earth and climate system variability and change."