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 new study titled The central role of diminishing sea ice in recent Arctic temperature amplification.

While itself a consequence of climate change, the shrinking Arctic ice cap is contributing to a positive feedback loop in which global warming and loss of ice reinforce each other. White sea ice reflects most of the incoming sunlight back into space. But when the ice melts, more heat is absorbed by the darker water, which in turn heats the atmosphere above it.

An APF story about the study quotes study co-author James Screen:

It was previously thought that loss of sea ice could cause further warming. Now we have confirmation this is already happening.

The findings show that the main driver of  “polar amplification” – warming in excess of the global average – is shrinking ice cover, rather than increased cloudiness or changes in ocean and atmospheric circulation.  The study’s findings also suggest that current forecasts underestimate the degree to which the polar region could heat up in the future.

A new report from the U.S. Environmental Protection Agency titled Climate Change Indicators in the U.S. states the extent of Arctic sea ice in 2009 was 24% below the 1979 to 2000 historical average. The area covered by ice is typically smallest in September, after the summer melting season. September 2007 had the least ice of any year on record, followed by 2008 and 2009.

The National Snow and Ice Data Center reports Arctic sea ice running only a bit below the 1979-2000 average – so far.

But the ice is thin, and it’s still early.

Here’s the abstract of the Screen  & Ian Simmonds study, published this week in the journal Nature:

The rise in Arctic near-surface air temperatures has been almost twice as large as the global average in recent decades — a feature known as ‘Arctic amplification’. Increased concentrations of atmospheric greenhouse gases have driven Arctic and global average warming; however, the underlying causes of Arctic amplification remain uncertain. The roles of reductions in snow and sea ice cover and changes in atmospheric and oceanic circulation cloud cover and water vapour are still matters of debate. A better understanding of the processes responsible for the recent amplified warming is essential for assessing the likelihood, and impacts, of future rapid Arctic warming and sea ice loss. Here we show that the Arctic warming is strongest at the surface during most of the year and is primarily consistent with reductions in sea ice cover. Changes in cloud cover, in contrast, have not contributed strongly to recent warming. Increases in atmospheric water vapour content, partly in response to reduced sea ice cover, may have enhanced warming in the lower part of the atmosphere during summer and early autumn. We conclude that diminishing sea ice has had a leading role in recent Arctic temperature amplification. The findings reinforce suggestions that strong positive ice–temperature feedbacks have emerged in the Arctic increasing the chances of further rapid warming and sea ice loss, and will probably affect polar ecosystems, ice-sheet mass balance and human activities in the Arctic.