Summary
Because of the vast reservoirs of water mass held in the great ice sheets of Antarctica and Greenland, determining the role that they play in global sea level has long been a major challenge of polar science. But until recent advances in measurement technology, the error in ice sheet mass budgets was greater than any net contribution to sea level change, and it was uncertain whether the ice sheets were causing sea level to rise or to fall. The Fourth Assessment report of the Intergovernmental Panel on Climate Change (IPCC AR4, 2007) was able, for the first time, to conclude that, taken together, the ice sheets in Greenland and Antarctica have very likely (>90% probability) been contributing to sea level rise over the period 1993 to 2003 at an average rate estimated at 0.4 mm of sea level per year. Since the cut-off date for work assessed by the IPCC AR4, a number of further studies of the mass budget of Greenland and Antarctica have been made using satellite altimetry, satellite gravity measurements and estimates of mass influx and discharge from a variety of techniques. Overall, these studies reinforce the conclusion that the ice sheets are contributing to present sea level rise, narrow the uncertainties in that sea level contribution, and strongly suggest that the rate of loss from the ice sheets may be increasing. Flow speeds of several of Greenland and Antarctica's outlet glaciers, which drain ice from the interior to the ocean, have increased. This dynamical change is often linked to the reduction in floating ice tongues or ice shelves and can explain much of the net ice loss from the ice sheets. Current models do not adequately treat the dynamic response of ice sheets to climate change, and this is the largest uncertainty in projecting future sea level rise.