Are the glaciers going to run out of steam, slow down and return to normal? Are the changes going to propagate far inland and do so rapidly? We do not have an answer to these questions. Yet we know that the changes taking place now in Greenland are unlikely to have had any precedent this century. It was warmer in Greenland during the 1930s and1940s, and glaciers were losing mass rapidly at that time, but Jakobshavn Isbrae, Helheim Gletscher and Kangerdlugssuaq Gletscher did not undergo a major speed up, ice shelf collapse and several kilometer retreat as a result of warming. The warming of the 1930s was rapid but of finite duration. The warming that we are experiencing since the 1980s is longer and will not end next year.
Analogs to Greenland glaciers exist. They correspond to the warm tidewater glaciers of Alaska. These glaciers reach states of equilibrium almost independent of climate, but climate change can trigger them out of equilibrium, at which point their behavior is more guided by the glacier geometry, in particular, how deep below sea level they are grounded, than climate. As they retreat into deeper waters, they produce larger icebergs, more rapidly, and continue to retreat. In Greenland, glaciers ending on a shoal in deep water and grounded well below sea level upstream, will retreat abruptly and rapidly for a long period of time into the interior before they may eventually stabilize on higher grounds. Jakobshavn Isbrae, for instance, is grounded below sea level over several hundred kilometers into the interior. It has maintained high flow speeds, and the effects of its speed up are now felt more than 100 km inland, with no sign of reversal. In contrast, Rinks Glacier, a few hundred kilometers north, is grounded well above sea level, and has been stable since the 1960s.
Until numerical models catch up with the real world - as they probably will - we are limited to observations to figure out what may come next. Because the ice sheet changes are rapid, we will learn fast. It is incomparably easier to study the dynamics of a changing system than those of a static system. Major progress will take place in the coming years from the coupling of satellite observations with numerical models, especially now that this problem has drawn urgent interest beyond the world of glaciology.
A recent study (Rahmstorf, 2007) proposed an alternative prediction of sea level rise in the future, without requiring sophisticated numerical models of ice sheet flow, which is illuminative of the current uncertainties in predicting the future of ice sheets. A simple regression between global temperature and sea level rise suggests that sea level will rise 1 m (about 3 ft) by 2100 (Rahmstorf, 2007), or twice more than asserted by the IPCC. At the other extreme, during the Eemian interglacial (127,000 years ago), the Greenland Ice Sheet was smaller than at present, not completely melted, but enough to have raised sea level by 3 m (about 10 ft) (Overpeck et al, 2006). Depending on how fast global temperature rises, sea level may or may not be driven up by 1-3 m (about 3-10 ft) by 2100. So, there is no reason to panic at this point, but also no basis for providing reassurance.
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