The Intergovernmental Panel on Climate Change (IPCC), the international panel of scientists that weighs in every five years or so on the state of the climate, predicted earlier this year that seas would rise between 18 and 59 centimeters this century. While the higher figure is quite worrisome in low-lying coastal areas, the numbers are still small enough–and the prospect far enough in the future–to seem manageable for most of the world. But the IPCC, in explaining its numbers, added that it could provide neither a “best estimate” nor an “upper bound” for how much higher sea levels might rise if the ice sheets disintegrate. (See “Sea-Level Riddle.”)
Indeed, the IPCC’s sea-level estimates are based on math that takes into account only a few well-understood processes. One is the expansion of seawater as it warms. Another is the melting of mountain glaciers in temperate zones–places like the Alps, Andes, and Himalayas. Third is the melting of the ice sheets’ surfaces and the glaciers’ seaward migration under the pull of gravity, though this may be partly balanced by increased snowfall that occurs because warmer air holds more moisture. The problem is that other processes may actually prove far more consequential. Warmer and higher oceans undermine the glaciers that flank Greenland and Antarctica, yanking them away from their seabed moorings. With those bulwarks weakened, inland glaciers slide much faster toward the sea. In a complementary process, water gushes down through fractures and holes in the Greenland ice sheet, making it easier for its glaciers to slide. As the glaciers reach lower (and therefore warmer) elevations, the melting and sliding will further accelerate.
And that’s exactly what appears to be happening. Recent observations have shown that the movement of major glaciers in both Greenland and the West Antarctic Ice Sheet is in fact accelerating. To pick just two examples, by 2005 a vast glacier in Greenland, the Jakobshavn, was slipping seaward twice as fast as it had in 1996, and another, the Kangerdlugssuaq, was slipping three times as fast as it had in 2000. “The current dynamical changes that we are seeing on the ice sheet are not captured in any climate model,” says Prasad Gogineni, director of the Center for Remote Sensing of Ice Sheets at the University of Kansas, which is a participant in the Danish-led effort. “That seems to indicate a huge uncertainty.” Today’s climate models, he says, simply can’t be relied on to predict what will happen to the great ice sheets.
Greenland’s melting ice sheet is now contributing more than half a millimeter per year to sea-level rise, according to a study coauthored by Eric Rignot, a senior research scientist at NASA’s Jet Propulsion Laboratory who has collaborated with Gogineni and others at the University of Kansas. That seemingly small figure is noteworthy because it’s more than twice the upper limit of Greenland’s contribution as estimated by the IPCC in its report earlier this year. “The [existing models] don’t assume any change in velocity in the glaciers, except on very long time scales,” Rignot says. “What we are seeing today is that those glaciers do speed up in a significant fashion in response to climate warming.”
The melting of the ice sheets is really just getting started, says Rignot. The current surge in the velocity of the glaciers has accompanied the 0.74 ºC of warming the planet has seen over the past 100 years. The IPCC is predicting that global temperatures will rise far more in the next 100 years–1.8 ºC to 4 ºC, depending on future emissions of greenhouse gases. The resulting loss of ice will dwarf any increases in snowfall, Rignot says. But Rignot disagrees with the conclusions drawn by the IPCC: he believes oceans will rise more than a full meter before the end of the century, nearly twice the upper bound of the IPCC’s predictions. “We have to acknowledge that we don’t have reliable ways to predict what ice sheets will do, but that they will certainly react much more strongly to climate warming in the future,” he says. “There is no reason to alarm people that the end of the world is coming. But there is no reason to reassure them, either, that there is nothing to worry about with the ice sheets.”