Projections of global mean sea level rise calculated with a 2D energy-balance climate model and dynamic ice sheet models

Projections of changes in surface air temperature and global mean sea level over the next century are presented for all IS92 radiative forcing scenarios. A zonal mean energy-balance climate model is used to estimate temperature changes and thermal expansion, precipitation-dependent sensitivity values are used to estimate the sea-level contribution of glaciers and small ice caps and dynamic ice-sheet models coupled to surface mass balance models are employed with regard to the Greenland and Antarctic ice sheets. A few of the sea-level projections have been included in the IPCC96-report for comparison with the revised IPCC96 projections. Here it is demonstrated that the observed inter-model differences are similar for all IS92 radiative forcing scenarios: the projections of global surface air temperature change resemble the revised IPCC96 projections, but the projections of global sea-level rise are 30%—50% smaller than the revised IPCC96 projections. In this paper, the reasons for the inter-model differences in sea-level results are considered. The largest inter-model differences in individual sea-level contributions are found for thermal expansion and for the Antarctic ice-sheet. Sensitivity experiments are presented that show the importance of different assumptions about the temperature forcing of the glacier and ice-sheet models and about weakening of the ocean circulation. Furthermore, uncertainties in thermal expansion caused by uncertainties in ocean heat mixing are considered. It is concluded that the inter-model differences in sea-level projections are caused by the use of essentially different models in this paper and in the revised IPCC96 projections.