Arctic ice more vulnerable to sunny weather, new study shows

Last summer's loss of Arctic sea ice set a modern-day record, with the ice extent shrinking to a minimum of about 1.6 million square miles (4.1 million square kilometers) in September. That was 43 percent less ice coverage than in 1979, when accurate satellite observations began.

Looking at the first two years of data from radar and lidar on the A-Train satellites, Kay and her colleagues found that total summertime cloud cover in the Western Arctic was 16 percent less in 2007 than the year before. A strong high-pressure system centered north of Alaska kept skies relatively clear. Over a three-month period in the summer, the increased sunshine was strong enough to melt about a foot of surface ice. Over open water, it was sufficient to increase sea-surface temperatures by 4.3 degrees Fahrenheit (2.4 degrees Celsius). Warmer ocean waters can contribute to sea ice loss by melting the ice from the bottom, thereby thinning it and making it more susceptible to future melt.

"Satellite radar and lidar measurements allow us to observe Arctic clouds in a new way," says CSU scientist Tristan L'Ecuyer, a co-author of the study. "These new instruments not only provide a very precise view of where clouds exist but also tell us their height and thickness, which are key properties that determine the amount of sunlight clouds reflect back to space."

The research team also examined longer-term records of Arctic cloud and weather patterns, including a 62-year-long record of cloudiness from surface observations at Barrow, Alaska. They found that the 2007 weather and cloud pattern was unusual but not unprecedented. At Barrow, five other years--1968, 1971, 1976, 1977, and 1991--had less summertime cloud cover than 2007, but without the same impact on sea ice.

-----A summer feedback cycle-----

The research suggests that warmth from the Sun will increasingly affect Arctic sea ice loss in the summer. As the ice shrinks, incoming sunshine triggers a spiraling effect: the newly exposed dark ocean waters, much darker than the ice, absorb the Sun's radiation instead of reflecting it. This warms the water and melts more ice, which in turn leads to more absorption of radiation and still more warming.

"Our research indicates that the relative importance of solar radiation in the summer is changing," Kay says. "The sunshine reaching the Arctic is increasingly influential, as there is less ice to reflect it back into space. Dry, sunny conditions in a single summer can now act as a potent force to melt sea ice."