Thomas Homer-dixon: Arctic melt is just part of a vicious climate circle

Thomas Homer-dixon: Arctic melt is just part of a vicious climate circle

The vast expanse of ice floating on the surface of the Arctic Ocean always recedes in the summer, reaching its lowest point sometime in September. Every winter it expands again, as the long Arctic night descends and temperatures plummet. Each summer over the past six years, global warming has trimmed this ice's total area a little more, and each winter the ice's recovery has been a little less robust. These trends alarmed climate scientists, but most thought that sea ice wouldn't disappear completely in the Arctic summer before 2040 at the earliest.

But this past summer sent scientists scrambling to redo their estimates. Week by week, the National Snow and Ice Data Center in Boulder, Colo., reported the trend: from 2.23 million square miles of ice remaining on Aug. 8 to 1.6 million square miles on Sept. 16, an astonishing drop from the previous low of 2.05 million square miles, reached in 2005.