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).

earlier related report
Finland warns of Arctic climate challenges
Helsinki, Finland (UPI) Apr 28, 2010 - Climate change is affecting the Arctic in more ways than can be termed positive and there is need for urgent action to meet the challenge, Finnish Minister for Foreign Trade and Development Paavo Vayrynen warned delegates attending the sixth Arctic Shipping Summit in Helsinki.

"Climate change, with effects on ice, snow, water and permafrost in the Arctic, appears to be reaching a tipping point," he warned, adding the neighboring countries need to act fast in response.

"For a long time the Arctic has meant sailing into unknown seas," he said. This, he indicated, was changing with more frequent consultation and literally with melting ice due to the climate change.

A better understanding of the global mechanisms in the Arctic environment was urgently needed to adapt to change and minimize risks, he said.

"We are faced with new and increasingly diverse challenges," said Vayrynen. "On one hand, we are benefiting from a wider access to natural resources as well as opening of new sea routes.

"But, on the other hand, our attention has to be attached to sustainable development, human health, environmental protection and the preservation of the livelihoods of indigenous peoples and Arctic communities," he said.

The International Polar Foundation has warned that changes in the Arctic will affect not only ecosystems and 4 million indigenous people but the rest of the world in profound ways.

GreenFacts, the Belgian non-profit organization, said on its Web site, "Arctic climate is now warming rapidly and much larger changes are projected." The average annual temperatures are projected to rise by 5 to 13 degrees Fahrenheit, with the greatest relative warming recorded in the winter months.

Precipitation is projected to increase by roughly 20 percent and the area of Arctic land covered by snow is expected to decrease by 10 to 20 percent, GreenFacts said.

The Arctic region includes sections of Canada, Finland, Greenland, Iceland, Norway, Russia, Sweden and the United States.

The region's economy depends largely on natural resources, from oil, gas and metal ores to fish, reindeer and birds and, more recently, tourism.

Vayrynen called for "a comprehensive approach" that ensured sustainable use of the natural resources and clean technologies.

"The capacity to respond to emergency situations in the Arctic should be improved by exchange of information, training and experience, technical development and support and the coordination of response," he said.

"Due to low population and infrastructure density, emergency response resources are thinly spread over a large area. This makes search and rescue operations difficult to stage and manage," he said.

He also called for a comprehensive European Union policy on the Arctic to send "clear signals that the EU recognizes the importance of the Arctic."

He said Arctic nations would need to innovate to respond to climate change and the opportunities and challenges it presents.

"The changes in the Arctic are rapid -- the climate change is twice as fast as elsewhere in the world," he said.

As a corollary to that change, he said, "new oil and gas reserves are expected to be revealed soon." The fish stock is already moving north, opening new fishing grounds, he said.

"The first trans-Arctic shipping lanes -- particularly the Northeastern Passage -- could be a reality in just a few years. Preparing ourselves for the future challenges demands international cooperation, communication and most of all the desire to fully -- and responsibly -- exploit the potential the Arctic offers," Vayrynen said.

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