April 22, 2008 -- Last summer Arctic sea ice hit a record low -- by a long shot -- leaving scientists to puzzle out why. Now researchers are closer to an answer with new findings showing that an especially sunny year is partly to blame.
Thinner ice is now more vulnerable to feedback cycles caused by increased sunshine, researchers believe.
"The 2007 summer decrease in ice extent was extraordinary," said Don Perovich of the Army Corps of Engineers Cold Regions Research and Engineering Laboratory in Hanover, N.H., who was not part of the new study.
The record minimum set last year dipped 22 percent below the previous minimum set in 2005, and more than 43 percent less than in 1979 when measurements started.
"The question now is what caused the decrease," he said. "Like any good mystery there is a large list of suspects." These include warmer air temperatures, changes in weather patterns, ice driven out of the Arctic by winds, warm ocean currents, increased sunshine, or a combination.
To examine one suspect -- increased sunshine -- Jennifer Kay and colleagues from the National Center for Atmospheric Research in Boulder, Colo. and Colorado State University in Fort Collins used data from two members of NASA's string of atmosphere-monitoring satellites called the "A-train."
The two satellites use radar and lidar -- a similar technique to radar that uses laser light instead of radio waves -- to generate a picture of the pattern of clouds throughout the region over time, including how the density of the clouds varies with height. This allowed the researchers to calculate how much solar radiation hit the Arctic in 2007 compared with the year before.
They found that cloud cover was 16 percent less in 2007 than 2006, which was an average year. More sunshine melted more ice and heated the exposed water more than usual.
"You were left with this dark water which can absorb a lot more sunlight, whereas before you had a bright surface that just would have reflected it," Kay said. The warmer waters can then melt more ice from below or along the edges. "The lack of clouds helped facilitate these classic feedbacks having to do with the brightness of sea ice," she added.
The teams findings were published today in Geophysical Research Letters.
The weather patterns that created the especially sunny weather are probably just part of the natural variability in Arctic weather, Kay said, but other years have seen even lower cloud cover without the same effect.
"While the weather conditions in the Arctic were unusual, they weren't unprecedented," Kay said. "We've seen them before and they haven't had the same impact. So something's different up there. It's likely that sea ice is thinner, so it's more vulnerable to this variability."
How much of a role sunshine had compared with other factors is still unresolved, said Julienne Stroeve of the National Snow and Ice Data Center in Boulder, Colo., but "the bottom line is that the ice has become very thin," she said.
Earlier this month, the center released an update on the state of the Arctic sea ice after it reached its annual maximum in March.
"This winter the ice is even thinner than last, so if we were to have a similar pattern as last summer, we would lose even more ice than we did in 2007," Stroeve said. "But even if we were to have conditions from other previous years, it's still likely that we could see dramatic ice loss again this summer."
Related Links:
Larry O'Hanlon's blog: Earth Impacts
National Snow and Ice Data Center