Abstract
In order to understand ice sheet response to climate change, it is critical to examine errors associated with ice flow model boundary conditions and forcing. It is also important to understand how these errors propagate through numerical ice sheet models and contribute to uncertainty in model output. Using established uncertainty quantification methods within the Ice Sheet System Model (ISSM), we investigate the sensitivity of ice flow within the Northeast Greenland Ice Stream (NEGIS) to key fields, including ice viscosity and basal drag, and compare them with model sensitivity to climate forcing. In addition, we examine how errors in model input manifest as mass flux uncertainties during a forward simulation of the NEGIS from 1989 to 2010. Overall, we find that mass flux is most uncertain in the main outlets, Nioghalvfjerdsbrae and Zachariae Isstrom, and that mass flux is most sensitive to basal drag, though errors associated with basal drag are poorly constrained and difficult to quantify. Given our knowledge of errors associated with the thermal properties of ice, we estimate that in the ablation area, the effects of cryohydrologic warming contribute over 4 times more mass flux uncertainty that do errors in geothermal heat flux. We find that NEGIS total ice discharge is associated with a 0.7 Gt/yr (2.6%) uncertainty due to errors in geothermal heat flux and a 3.3 Gt/yr (11.6%) uncertainty due to the added effects of cryohydrologic warming. In comparison, errors in surface mass balance contribute 4.5 Gt/yr to NEGIS total discharge uncertainty.
Highlights
The Greenland Ice Sheet is amongst the largest contributors to sea level rise [Gardner et al, 2013; Intergovernmental Panel on Climate Change, 2013], and observations indicate that its rate of ice loss has accelerated by more than 25 Gt/yr2 during the last decade [Wouters et al, 2013; Enderlin et al, 2014]
We simulate Northeast Greenland Ice Stream (NEGIS) ice flow from 1989 to 2010 and use uncertainty quantification tools to investigate how errors in boundary conditions compare to the influence of climate forcing on the ice flow in Northeast Greenland
This estimate is less than the 5.7 Gt/yr (19.8%) uncertainty associated with a 20% error in the basal drag coefficient and less than the 3.3 Gt/yr (11.6%) uncertainty associated with additional changes in the ice viscosity parameter due to surface meltwater subglacial drainage and refreeze
Summary
The Greenland Ice Sheet is amongst the largest contributors to sea level rise [Gardner et al, 2013; Intergovernmental Panel on Climate Change, 2013], and observations indicate that its rate of ice loss has accelerated by more than 25 Gt/yr during the last decade [Wouters et al, 2013; Enderlin et al, 2014]. Ice sheet models vary in their treatment of key processes (i.e., assumptions during initialization, implementation of model forcing, choice of flow equations and sliding laws, etc.). Such differences lead to a significant spread in model behavior and ice flow response to climate forcing [Nowicki et al, 2013]. Projections of Greenland’s sensitivity to climate change vary dramatically and are largely uncertain
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