By the end of the annual summer melting season Sept. 17, Arctic ice covered 1.94 million square miles – about the same span last year, but far below the 2.4-million-square-mile average set between 1981 and 2010.

It has receded by about two-thirds since NASA began monitoring the Arctic ice by satellite in 1978.

[JACK LEW: Global Warming Costing U.S.]

“Arctic sea ice basically acts as a big air conditioner for the planet,” Nathan Kurtz, a research scientist at NASA’s Goddard Space Flight Center, said in a video accompanying the announcement. “When the sun shines down on it, most of the energy is reflected away, so it helps keep the planet cool.”

During the past 10 years, however, as climate change has pushed the global average temperature ever higher, the Arctic has actually warmed at twice or three times the rate, thinning the ice sheet and making it less resilient to the summer melting season.

