Darker days during Arctic summer

From Acapulco, Mexico, at a meeting of the American Geophysical Union

Satellite observations of Earth indicate that Arctic regions reflected less sunlight into space in the summer of 2006 than in other recent years. That change could contribute to the warming of Earth’s climate.

Sensors on board NASA’s Terra satellite have been observing Earth’s surface since May 2000, says Roger Davies, a climate physicist at the University of Auckland in New Zealand. By analyzing those data in 36-day sets, Davies has identified seasonal trends in the planet’s albedo, the percentage of sunlight that it reflects into space.

Between 2000 and 2005, the albedo during any given season remained about the same, says Davies. In May and June of 2006, however, albedo fell significantly in comparison with the same months in previous years. In particular, regions north of 55°N—approximately the latitude of Copenhagen and the southern part of the Alaskan Panhandle—had darkened enough to decrease Earth’s average albedo from 31 percent to 30 percent.

One small mystery: Sea ice coverage during the Arctic summer has dramatically decreased throughout the past decade, but changes in albedo didn’t show up until last year. It’s possible, Davies says, that the effect of replacing large areas of white sea ice with dark, open water was masked by clouds before 2006—an issue that he is now researching.