Melting sea ice has accelerated warming in the Arctic, which in recent decades has warmed twice as quickly as the global average, according to a new study.

"The findings reinforce suggestions that strong positive ice-temperature feedbacks have emerged in the Arctic, increasing the changes of further rapid warming and sea ice loss," concludes the research published yesterday by the journal Nature.

The work by James Screen and Ian Simmonds at Australia's University of Melbourne echoes earlier studies that identified the same basic feedback loop.

The feedback loop begins with warmer Arctic springs and summers, which cause more sea ice to melt each summer.

That widespread melting leaves huge swaths of dark ocean water that absorbs more heat from the sun than the white, reflective sea ice it replaces. And that heat appears to be cycling back into the atmosphere each fall, when the amount of sunlight dips and sea ice re-forms.

The Australian researchers say their work suggests that other proposed causes for the large Arctic warming -- such as changes in air and ocean circulation patterns, cloud cover and water vapor -- are dwarfed by the effect of the decline in sea ice.

But the scientists do note that the increases in the amount of water vapor in the air in summer and early fall "may have enhanced" warming during those seasons. The increase in the amount of water vapor could be related to the decline in sea ice, they say.

Water vapor and clouds may play a role
The Arctic's summer ice cover hit a record low in 2007, when it dipped about 40 percent below the average ice cover recorded since 1979, when scientists began monitoring the region with satellites.

Although the ice cover has increased over the past few years, the Arctic's sea ice is now much thinner than it was just a few years ago, making it more vulnerable to future warming.

Julienne Stroeve of the National Snow and Ice Data Center said the new study reaches the same basic conclusion she and her colleagues did in work published in 2007, although it relies on a new data set.

"The only thing that they showed that we didn't look at is changes in cloud cover and atmospheric moisture trends," she said.

But Jennifer Francis, a Rutgers University researcher who uses satellite data to study Arctic climate change, said she had questions about the research.

"This study highlights the very sensitive nature of sea ice and snow to slight changes in the surface energy balance," she said.

But Francis also said she believes that data the researchers used to test their conclusions "shows serious problems" with identifying clouds present over Arctic snow and ice. That's important, she said, because cloud cover influences when in spring sea ice begins melting.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500