Reducing the uncertainty in the contribution of Greenland to sea-level rise in the 20th and 21st centuries

Three methods to calculate summer snow- and ice melt are combined with a simplified climate model to estimate past, present and future values of accumulation and ablation on the Greenland ice sheet. This allows the reliability of the computationally efficient temperature-based parameterizations of melting to be compared to that of a more complicated physical model of the snow cover which calculates explicitly the formation of meltwater, refreezing and runoff. Six runs are subject to the same observed climatic forcing over the 20th century, with different model parameters chosen. The range of change in sea level which accompanies these six runs is < 1 cm. Because of a near-perfect cancellation between increases in accumulation and runoff, for a reference climate scenario similar to the Intergovernmental Panel on Climate Change's IS92a, the Greenland ice sheet is not expected to contribute significantly to changes in the ocean level over the 21st century. The uncertainty in these predictions is estimated by repeating the calculation for a range of climate-change scenarios.