Darker Arctic turns up the heat
The Arctic isn't nearly as bright and white as it used to be because of more ice melting in the ocean, and that's becoming a global problem, a new study has found.
With more dark, open water in the summer, less of the sun's heat is reflected back into space. So the Earth is absorbing more heat than expected, according to the study published on Monday in The Proceedings of the National Academy of Sciences.
That extra absorbed energy is so big that it measures about one-quarter of the entire heat-trapping effect of carbon dioxide, said the study's lead author, Ian Eisenman, a climate scientist at the Scripps Institution of Oceanography in California.
The Arctic grew darker by 8 percent between 1979 and 2011, Eisenman found, measuring how much sunlight is reflected back into space.
"Basically, it means more warming," he said in an interview.
The North Pole region comprises an ocean that is mostly crusted on the surface with ice that shrinks in the summer and reforms in the autumn. At its peak melt in September, the ice has shrunk on average by nearly 90,650 square km a year since 1979.
Snow-covered ice reflects several times more heat than dark, open ocean, which replaces the ice when it melts, Eisenman said.
As more summer sunlight falls onto the ocean, the water becomes warmer and it takes longer for the ice to form again in the autumn, Jason Box of the Geological Survey of Denmark and Greenland said in an e-mail. He was not part of the study.
While earlier studies used computer models, Eisenman said his is the first to use satellite measurements to gauge sunlight reflection and to take into account cloud cover. The results show the darkening is as much as two to three times greater than previous estimates, he said.
Associated Press
(China Daily 02/21/2014 page10)
