Glacier behavior unpredictable

Greenland's ice giants fop climate scientists

Calving zone of the Helheim Glacier in the southeastern part of Gr nland. © NASA / JPL
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The glacier's response to global warming is far more complex than previously thought. As researchers now report in "Science", two of Greenland's largest ice streams have defrosted extremely quickly between 2004 and 2005, but have now normalized their melting rates. Possibly such "pulses" of accelerated melting, but also sudden collapse, are typical of the climate consequences in the polar regions.

Just a year ago, researchers in the journal Science warned that the speed at which Greenland's glaciers were melting nearly doubled between 2000 and 2005. This led to the assumption that heating could trigger a gradual acceleration of defrosting in the future as well. But now, new data seems to have disproved this assumption: scientists from the universities of Washington and Colorado examined the glaciers Kangerdlugssuaq and Helheim in southeastern Greenland. The fronts of both glaciers lead directly into the sea and together they are responsible for 35 percent of the runoff of the whole of Greenland.

Pulsating back and forth

The measurements showed that Kangerdlugssuaq had the largest loss of ice in 2005, with more than 80 percent increase in a single year recorded here. In 2006, however, defrosting speeds dropped again by 25 percent. The researchers also observed something similar at Helheim: Here, too, the discharge speed of ice peaked between 2004 and 2005, before sinking again slightly.

"Although the shrinkage rates of these two glaciers have stabilized again, we do not know if they will remain stable, regrow, or collapse in the near future, " explains Ian Howat, a glaciologist at the National Snow and Ice Data Center of the University of Colorado, "Our primary knowledge is that the behavior of these glaciers changes from year to year, so we can not expect to predict future behavior due to current changes."

In the opinion of the researcher, future warming may lead to faster pulses of advance and increased melting rather than a slow, even retreat. Responsible for the now observed at the two Greenland glaciers change is, according to the scientists, during the rapid defrosting changing shape of the ice giants: they were longer and thinner and the ice front, which initially swam on the water, lowered again with increasing loss of ice and slowed down the ice flow. But to what extent this process could also be used on other glaciers is still completely unclear. display

Too little knowledge for accurate forecasts

This uncertainty in the prediction of glacier behavior was also taken into account by the scientists of the IPCC Panel, who wrote in their climate report published on 2 February 2007 that climate change causes an increased susceptibility of glaciers, but that the understanding of the associated processes is extremely limited and therefore there is no consensus on the extent of the consequences. "I think the IPCC authors made a responsible decision here by calling the accelerated melt rates alarming, but at the same time emphasizing that we do not know enough yet to make accurate predictions, " commented Ian Joughin, a glaciologist from the University of Washington and co-author of the current science study.

The current investigations - including their own - are also short and incomplete snapshots of glacier development. "Events in just two short years on these glaciers - that's not the classical way of thinking of glaciologists, " explains Howat. We usually think of ice sheets in the order of centuries or even millions of years.

(University of Washington, 13.02.2007 - NPO)