TC The Cryosphere TC 1994-0424 Copernicus GmbH Göttingen, Germany 10.5194/tc-6-589-2012 An assessment of key model parametric uncertainties in projections of Greenland Ice Sheet behavior Applegate P. J. 1 5 Kirchner N. 1 Stone E. J. 2 Keller K. 3 Greve R. 4 Physical Geography and Quaternary Geology/Bert Bolin Centre for Climate Research, Stockholm University, Sweden BRIDGE, School of Geographical Sciences, University of Bristol, Bristol, UK Department of Geosciences/Earth and Environmental Sciences Institute, Pennsylvania State University, University Park, Pennsylvania, USA Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan now at: Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA 30 05 2012 6 3 589 606 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.the-cryosphere.net/6/589/2012/tc-6-589-2012.html The full text article is available as a PDF file from http://www.the-cryosphere.net/6/589/2012/tc-6-589-2012.pdf

Lack of knowledge about the values of ice sheet model input parameters introduces substantial uncertainty into projections of Greenland Ice Sheet contributions to future sea level rise. Computer models of ice sheet behavior provide one of several means of estimating future sea level rise due to mass loss from ice sheets. Such models have many input parameters whose values are not well known. Recent studies have investigated the effects of these parameters on model output, but the range of potential future sea level increases due to model parametric uncertainty has not been characterized. Here, we demonstrate that this range is large, using a 100-member perturbed-physics ensemble with the SICOPOLIS ice sheet model. Each model run is spun up over 125 000 yr using geological forcings and subsequently driven into the future using an asymptotically increasing air temperature anomaly curve. All modeled ice sheets lose mass after 2005 AD. Parameters controlling surface melt dominate the model response to temperature change. After culling the ensemble to include only members that give reasonable ice volumes in 2005 AD, the range of projected sea level rise values in 2100 AD is ~40 % or more of the median. Data on past ice sheet behavior can help reduce this uncertainty, but none of our ensemble members produces a reasonable ice volume change during the mid-Holocene, relative to the present. This problem suggests that the model's exponential relation between temperature and precipitation does not hold during the Holocene, or that the central-Greenland temperature forcing curve used to drive the model is not representative of conditions around the ice margin at this time (among other possibilities). Our simulations also lack certain observed physical processes that may tend to enhance the real ice sheet's response. Regardless, this work has implications for other studies that use ice sheet models to project or hindcast the behavior of the Greenland Ice Sheet.

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