Abstract
Abstract. The early 21st century retreat of Jakobshavn Isbræ into its overdeepened bedrock trough was accompanied by acceleration to unprecedented ice stream speeds. Such dramatic changes suggested the possibility of substantial mass loss over the rest of this century. Here we use a three-dimensional ice sheet model with parameterizations to represent the effects of ice mélange buttressing, crevasse-depth-based calving and submarine melting to adequately reproduce its recent evolution. We are the first study on Jakobshavn Isbræ that solves for three-dimensional ice flow coupled with representations of hydro-fracturing-induced calving and mélange buttressing. Additionally, the model can accurately replicate interannual variations in grounding line and terminus position, including seasonal fluctuations that emerged after arriving at the overdeepened basin and the disappearance of its floating ice shelf. Our simulated ice viscosity variability due to shear margin evolution is particularly important in reproducing the large observed interannual changes in terminus velocity. We use this model to project Jakobshavn's evolution over this century, forced by ocean temperatures from seven Earth system models and surface runoff derived from RACMO, all under the IPCC RCP4.5 climate scenario. In our simulations, Jakobshavn's grounding line continues to retreat ∼18.5 km by the end of this century, leading to a total mass loss of ∼2068 Gt (5.7 mm sea level rise equivalent). Despite the relative success of the model in simulating the recent behavior of the glacier, the model does not simulate winter calving events that have become relatively more important.
Highlights
Jakobshavn Isbræ (Fig. 1) is Greenland’s largest and fastest outlet glacier, with transient speeds of up to 17 km a−1 (Joughin et al, 2014)
The model can accurately replicate interannual variations in grounding line and terminus position, including seasonal fluctuations that emerged after arriving at the overdeepened basin and the disappearance of its floating ice shelf
Using the HadGEM2-ES forcing, which is the same model used to force RACMO with α and γ set to their best estimates (Fig. 4) gives 2044 Gt. We suggest that this may be the lower reasonable bound of mass loss since the HadGEMES ocean temperatures rise notably slower than the ensemble mean (Fig. 6)
Summary
Jakobshavn Isbræ (Fig. 1) is Greenland’s largest and fastest outlet glacier, with transient speeds of up to 17 km a−1 (Joughin et al, 2014). Since 1997, measurements indicate that the water entering Ilulissat Fjord, where Jakobshavn Isbræ terminates, is about 1.1 ◦C warmer than it was during 1987–1991 (Holland et al, 2008) This rise in water temperature coincided with the onset of dramatic thinning, speedup and retreat of Jakobshavn Isbræ. By 2003 its velocity near the grounding line had reached ∼ 12.6 km a−1, more than double that of 1992, and the ice shelf in the fjord had disintegrated (Joughin et al, 2004). By 2012 the seasonal velocity fluctuations 4 km upstream from the calving front were nearly ±8 km a−1 and the grounding line of Jakobshavn Isbræ had reached the bottom of a subglacial bedrock trough after years of downslope migration (Joughin et al, 2014)
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