| Summary: | Understanding the sensitivity of ice sheets to climate forcing is crucial for predicting future behavior of ice sheets. Multiple studies have analyzed the inception of the Quaternary ice sheets using time-dependent global climate models. The results have indicated that although carbon dioxide levels, summer insolation, and sea level are important, ice sheet growth is most dependent on the distribution and quantity of precipitation. I test this conclusion by analyzing how precipitation and temperature effect the growth of an ice sheet using two different sets of climate data. I use the Parallel Ice Sheet Model (PISM), a coupled ice sheet and ice shelf model, to simulate the mass balance and dynamics of the Fennoscandian Ice Sheet (FIS) at the time of its inception. One climate scenario is constructed using modern temperature and precipitation records scaled to represent conditions at the last glacial maximum (LGM) with a -7ᵒC temperature offset and 50% decrease in precipitation. I compare this scenario with the LGM modeled climate from the PMIP3 palaeo-GCM. When forced with the scaled modern climate, PISM first initiates glaciation the southern Scandinavian mountains and an ice sheet expands north and south over 1000 years. The thickest ice remains in the south near the present day Jotunheimen ice cap. In contrast, the evolution of the ice sheet using the PMIP3 climate features unrealistically large ice thicknesses along the southern coast of Norway and Sweden, extending across the Norwegian Channel into Denmark. Glaciers do not propagate along the Scandinavian mountains to the north despite the presence of very cold temperatures. The difference between these two models of the inception of the FIS is largely controlled by differences in the spatial patterns of precipitation between the two climate data sets. A sensitivity test examining the influence of incremental changes in precipitation and temperature ranging from the present modern condition to the LGM scaling of modern patterns shows that while temperature limits growth of the ice sheet to some extent, it does not change the general pattern of ice accumulation. Precipitation has a much larger influence on ice sheet evolution. I conclude that the inception of the actual FIS is likely to have been sensitive to patterns of precipitation and assert that future work on modeling ice sheets must consider the potential impact of small changes in precipitation on ice sheet mass balance and movement.
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