Sea ice loss major cause of Arctic warming: study

Marlowe Hood Yahoo News 28 Apr 10;

PARIS (AFP) – Melting sea ice has dramatically accelerated warming in the Arctic, where temperatures have risen faster in recent decades than the global average, according to a study released Wednesday.

The study, published in the journal Nature, also suggests that current forecasts underestimate the degree to which the polar region could heat up in the future.

"It was previously thought that loss of sea ice could cause further warming. Now we have confirmation this is already happening," said James Screen, a researcher at the University of Melbourne and co-author of the study.

While itself a consequence of climate change, the shrinking Arctic ice cap has contributed to a "positive feedback loop" in which global warming and loss of ice reinforce each other on a regional scale.

"The sea ice acts like a shiny lid floating on top of the Arctic Ocean, reflecting most of the incoming sunlight back into space," Screen explained by email.

But when the ice melts, more heat is absorbed by the darker water, which in turn heats the atmosphere above it.

"What we found is this feedback system has warmed the atmosphere at a faster rate than it would otherwise," he said.

From 1989 to 2008, global temperatures climbed on average by 0.5 Celsius (0.8 Fahrenheit), whereas the Arctic has warmed by 2.1 C (3.4 F) -- the most rapid increase of any place on the planet.

Up to now, scientists have sharply disagreed on the main causes of this discrepancy.

Using the most recent observational data from the European Centre for Medium-Range Weather Forecasting, Screen and co-author Ian Simmonds uncovered a nearly perfect season-by-season match during the 20-year period analysed between surface warming trends and reductions in sea ice cover.

The findings show that the main driver of so-called "polar amplification" -- warming in excess of the global average -- is shrinking ice cover, and not increased cloudiness or changes in ocean and atmospheric circulation, as others have argued.

Models used by the UN's top scientific authority, the Intergovernmental Panel on Climate Change (IPCC), seriously underestimated the recent loss of Arctic sea ice, Screen pointed out.

"They may also underestimate future sea ice loss and warming, but only time will tell for sure," he added.

At the end of northern hemisphere summer 2007, the Arctic ice cap shrank to the smallest size on record, 40 percent below the average 7.23 million square kilometers (2.8 million square miles) observed in 1979-2000, according to the US National Snow and Ice Data Center (NSIDC).

The sea ice pack thawed to its second smallest size in 2008, followed by the third smallest in 2009.

NASA satellite data has also shown that Arctic sea ice has thinned considerably.

During the period 2004-2008, the ice diminished in thickness by some 2.2 feet (67 centimeters).