Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeof the glacier losses are from the southern halfof Greenland, especially the southeast sector,east-central, and west-central. In the northwest,losses were already significant in the early1990s and did not increase in recent decades.In the southwest, losses are low but slightlyincreasing. In the north, losses are very low,but also slightly increasing in the northwestand northeast.Comparison of 2005 ICESat data with 1998–89airborne laser surveys shows losses during theinterim of 80 ± 25 Gt a –1 (Thomas et al., 2006),and this is probably an underestimate becauseof sparse coverage of regions where otherinvestigations show large losses.The pattern of thickening/thinning overGreenland, derived from laser-altimeter data,is shown in Figure 2.4, with the various massbalanceestimates summarized in Figure 2.5. Itis clear that the SRALT-derived estimate differswidely from the others, each of which is basedon totally different methods, suggesting thatthe SRALT interpretations underestimate totalice loss for reasons discussed in Section 3.1.1.Figure 2.4. Rates of elevation change (dS/dt) forGreenland derived from comparisons at more than16,000 locations where ICESat data from Oct/Novand May/June 2004 overlay ATM surveys in 1998/9,averaged over 50-km grid squares. Locations of rapidlythinning outlet glaciers at Jakobshavn (J), Kangerdlugssuaq(K), Helheim (H), and along the southeastcoast (SE) are shown, together with plots showingtheir estimated mass balance (10 6 Gt a –1 ) versus time(Rignot and Kanagaratnam, 2006).47