Sunny year sped Arctic ice melt

As Arctic sea ice shrinks, it becomes more vulnerable to sunny weather.

Summer sunshine is starting to play a more important role in the melting of Arctic sea ice, according to research published in Geophysical Research Letters (2008, 35, L08503) by scientists at the National Center for Atmospheric Research (NCAR).

The research group, led by Jennifer Kay of NCAR, examined data from a new set of NASA satellites called the "A-Train" that use radar and lidar technologies to see the location and thickness of clouds. The first 2 years of data revealed that in the summer of 2007, the Arctic had 16% less cloud cover than the year before because of a high-pressure system north of Alaska. The extra sunshine was intense enough to melt about a foot of surface ice.

Last summer's sea ice shrank to a record minimum of 1.6 million square miles, the lowest since satellite observations began in 1979. Although unusually sunny weather contributed to that record-breaking loss, the researchers say, similar conditions in past years did not appear to cause the same effect.

The findings suggest that sunny weather could accelerate further Arctic sea-ice loss. In a warmer world, Kay says, thinner ice is more sensitive to sunshine levels. As ice shrinks, darker water is exposed, and it absorbs the sun's radiation instead of reflecting it, creating a positive feedback loop.

The extent of Arctic sea ice has been declining at a rate of nearly 8% per decade since 1953. A previous study (Geophys. Res. Lett. 2007, 34, L09501), led by Julienne Stroeve of NCAR, attributed part of that trend to increases in human-made greenhouse gases.

The new paper highlights an emerging understanding of potential linkages between short-term natural variability in atmospheric conditions and longer-term changes in climate driven by greenhouse gases. When summertime ice is "vulnerably thin," the researchers write, natural year-to-year variations in summer atmospheric conditions, clouds, and sunshine become more important in determining the extent of sea-ice cover. —ERIKA ENGELHAUPT