Arctic sea ice reached its minimum extent of the year in Sept. 23, according to the satellite-derived data from the National Snow and Ice Data Center. Since the beginning of the satellite-based sea ice record in 1979, there has been a relatively steady decline in sea-ice extent.

This year's minimum was 1.77 million square miles, which is an area slightly bigger than half the area of the lower 40 contiguous states. This figure is tied with 2008 and 2010 for the sixth-lowest minimum extents, and all 12 of the lowest extents have occurred in the last 12 years.

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The Arctic is warming at a much faster rate than most of Earth, likely due to the positive feedback loop provided by the melting trend. Ice is highly reflective of sunlight and so turns away most solar radiation.

Open water, however, absorbs much of the incoming radiation provided by sunlight. So as Arctic summers get warmer and more ice melts, the tendency of the region to melt more and warm more is likely to accelerate.