Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 11996 19983.00E+07Total Melt AreaApril - October20072007elted (km 2 )Area M2.50E+072.00E+07071987199119951998200220051.50E+071.00E+071983199219965.00E+061978 1983 1988 1993 1998 2003 2008YearKonrad Steffen and Russell Huff, CIRES, University of Colorado at BoulderFigure 1.3. The graph shows the total melt area 1979 to 2007 for the Greenland ice sheet derivedfrom passive microwave satellite data. Error bars represent the 95% confidence interval. The mapinserts display the area of melt for 1996, 1998, and the record year 2007 (from K. Steffen, CIRES,University of Colorado).Because sites of global deep water formationoccur immediately adjacent to the Greenlandand Antarctic ice sheets, any significant increasein freshwater fluxes from these ice sheetsmay induce changes in ocean heat transportand thus climate. This topic is addressed inChapter 4 of this report.SummaryThe Greenland and Antarctic Ice Sheets arelosing mass, likely at an accelerating rate. Muchof the loss from Greenland is by increasedsummer melting as temperatures rise, but anincreasing proportion of the combined massloss is caused by increasing ice discharge fromthe ice-sheet margins, indicating that dynamicalresponses to warming may play a much greaterrole in the future mass balance of ice sheetsthan previously considered. The interactionof warm waters with the periphery of the icesheets is very likely one of the most significantmechanisms to trigger an abrupt rise in globalsea level. The potentially sensitive regions forrapid changes in ice volume are thus likely thoseice masses grounded below sea level such asthe West Antarctic Ice Sheet or large glaciersin Greenland like the Jakobshavn Isbræ withan over-deepened channel (channel below sealevel, see Chapter 2, Fig. 2.10) reaching far inland.Ice-sheet models currently do not includethe physical processes that may be governingthese dynamical responses, so quantitativeassessment of their possible contribution to sealevel rise is not yet possible. If these processesprove to be significant to the long-term massbalance of the ice sheets, however, current sealevel projections based on present-generationnumerical models will likely need to be revisedsubstantially upwards.5. Abrupt Change in LandHydrologyMuch of the research on the climate responseto increased GHG concentrations, and mostof the public’s understanding of that work,has been concerned with global warming.Accompanying this projected globally uniformincrease in temperature, however, are spatially16