With more dark, open water in the summer, less of the sun's heat is reflected back into space. So the entire Earth is absorbing more heat than expected, according to a study published 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 8 per cent darker between 1979 and 2011, Eisenman found, measuring how much sunlight is reflected back into space.
"Basically, it means more warming," Eisenman said in an interview.