The skin of sea ice that covers the Arctic Ocean has reached its maximum extent for 2013, the National Snow and Ice Data Center announced Monday, and the annual melt season has begun. As of March 15, ice covered 5.84 million square miles of ocean, the sixth-lowest since satellite observations began in the 1970’s, and 283,000 square miles lower than the 1979-2000 average. Reflecting the influence of global warming, the 10 lowest sea ice maximums have all occurred over the past 10 years.
Last summer’s ice minimum, moreover, was the lowest on record, with 2007 coming in a distant second. Taken together, it’s one more sign that the planet is warming under the influence of heat-trapping greenhouse gases.
The Arctic is warming especially quickly, however, thanks to a sort of vicious cycle that operates between ice, ocean and sunlight. When the sea is covered with bright, reflective ice, incoming sunlight bounces back into space. When the darker water underneath is exposed, some of the Sun’s energy is absorbed, heating the seawater. That warms the air in turn, increasing the melting and exposing even more dark seawater to the incoming sunlight, and so on.
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