This composite satellite image shows the Arctic in May 2009. Photo: NSIDC

The annual melting of Arctic sea ice is trending toward another record-low.While it’s still too early to say whether the 2009 melt will exceed the record 2007 melt — the annual low-point isn’t reached until September — the trend line for 2009 for the first time has dipped below 2007, according to the latest data from the National Snow and Ice Data Center.

The melting of Arctic sea ice is one of the clearest signals of global warming, and a leading indicator of what is to come. The melting is also an example — one of many — of a positive feedback loop that scientists expect will accelerate global warming: As sea ice melts, the darker water that is exposed absorbs more of the sun’s energy, which leads to warmer waters and more melting ice.

The Arctic is referred to as the “Earth’s air conditioner,” moderating climate worldwide.

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