18.104.22.168 Glaciers, Ice Sheets and Ice Shelves
During the 20th century, glaciers generally lost mass with the strongest retreats in the 1930s and 1940s and after 1990 (Section 4.5). The widespread shrinkage appears to imply widespread warming as the probable cause (Oerlemans, 2005), although in the tropics changes in atmospheric moisture might be contributing (Section 4.5.3). Over the last half century, both global mean winter accumulation and summer melting have increased steadily (Ohmura, 2004; Dyurgerov and Meier, 2005; Greene, 2005), and at least in the NH, winter accumulation and summer melting correlate positively with hemispheric air temperature (Greene, 2005); the negative correlation of net balance with temperature indicates the primary role of temperature in forcing the respective glacier fluctuations.
There have been a few studies for glaciers in specific regions examining likely causes of trends. Mass balances for glaciers in western North America are strongly correlated with global mean winter (October–April) temperatures and the decline in glacier mass balance has paralleled the increase in temperature since 1968 (Meier et al., 2003). Reichert et al. (2002a) forced a glacier mass balance model for the Nigardsbreen and Rhône glaciers with downscaled data from an AOGCM control simulation and conclude that the rate of glacier advance during the ‘Little Ice Age’ could be explained by internal climate variability for both glaciers, but that the recent retreat cannot, implying that the recent retreat of both glaciers is probably due to externally forced climate change. As well, the thinning and acceleration of some polar glaciers (e.g., Thomas et al., 2004) appear to be the result of ice sheet calving driven by oceanic and atmospheric warming (Section 22.214.171.124).
Taken together, the ice sheets of Greenland and Antarctica are shrinking. Slight thickening in inland Greenland is more than compensated for by thinning near the coast (Section 126.96.36.199). Warming is expected to increase low-altitude melting and high-altitude precipitation in Greenland; altimetry data suggest that the former effect is dominant. However, because some portions of ice sheets respond only slowly to climate changes, past forcing may be influencing ongoing changes, complicating attribution of recent trends (Section 188.8.131.52).