Multistability of the Greenland ice sheet and the effects of an adaptive mass balance formulation

The effect of a warmer climate on the Greenland ice sheet as well as its ability to regrow from a reduced geometry is important knowledge when studying future climate. Here we use output from a general circulation model to construct adaptive temperature and precipitation patterns to force an ice flow model off-line taking into consideration that the patterns change in a non-uniform way (both spatially and temporally) as the geometry of the ice sheet evolves and as climate changes. In a series of experiments we investigate the retreat from the present day configuration, build-up from ice free conditions of the ice sheet during a warmer-than-present climate and how the ice sheet moves between states. The adaptive temperature and accumulation patterns as well as two different constant-pattern formulations are applied and all experiments are run to steady state. All results fall into four different groups of geometry regardless of the applied accumulation pattern and initial state. We find that the ice sheet is able to survive and build up at higher temperatures using the more realistic adaptive patterns compared to the classic constant patterns. In contrast, decay occurs at considerably higher temperatures than build-up when the other formulations are used. When studying the motion between states it is clear that the initial state is crucial for the result. The ice sheet is thus multistable at least for certain temperature forcings, and this implies that the ice sheet not does not necessarily return to its initial configuration after a temperature excursion.