April 22, 2008
The shrinking expanse of Arctic sea ice is increasingly vulnerable to summer sunshine. Unusually sunny weather contributed to last summer's record loss of Arctic ice, while similar weather conditions in past summers did not appear to have comparable impacts, new research concludes.
"The relative importance of solar radiation in the summer is changing," says Jennifer Kay of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, who is lead author of the study. "The amount of sunshine reaching the Arctic is increasingly influential, as there is less ice to reflect it back into space," she says.
The findings by Kay and colleagues at NCAR and Colorado State University (CSU) in Fort Collins indicate that the presence or absence of clouds now has greater implications for sea ice loss.
"A single unusually clear summer can now have a dramatic impact," Kay says.
A report on the new results will be published April 22, 2008 in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).
Last summer's loss of Arctic sea ice set a modern-day record, with the ice extent shrinking in September to a minimum of about 4.1 million square kilometers (1.6 million square miles). That was 43 percent less ice coverage than in 1979, when accurate satellite observations began.
The study draws on observations from new NASA satellite radar and lidar instruments. Lidar devices make measurements using lasers. Looking at the first 2 years of satellite data from those sensors, Kay and her colleagues found that total 2007 summertime cloud cover was 16 percent less than the year before, largely because of a strong high-pressure system centered north of Alaska that kept skies clear.
Over a 3-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 2.4° C (4.3° F). 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's 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. The scientists found that the 2007 weather and cloud pattern was unusual but not unprecedented. Five other years — 1968, 1971, 1976, 1977, and 1991 — appeared to have lower summertime cloud cover than 2007, but without the same impact on sea ice.
"In a warmer world, the thinner sea ice is becoming increasingly sensitive to year-to-year variations in weather and cloud patterns," Kay says.
The research suggests that warmth from the Sun will increasingly affect Arctic climate in the summer. As the ice shrinks, incoming sunshine triggers a feedback mechanism: 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.
The authors note that, in addition to solar radiation, other factors such as changes in wind patterns and, possibly, shifts in ocean circulation patterns also influence sea ice loss. In particular, strong winds along regions of sea ice retreat were important to last year's loss of ice. The relative importance of these factors, and the precise extent to which global climate change is driving them, are not yet known.