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"NASA satellite data reveals how this year's minimum sea ice extent, reached on Sept. 9 as depicted here, declined to a level far smaller than the 30-year average (in yellow) and opened up Northwest Passage shipping lanes (in red)." (Credit: NASA Goddard's Scientific Visualization Studio)

Arctic sea ice consists of frozen layers of seawater. Each winter the ice in the Arctic Ocean increases significantly and then in the summer some of the newer ice melts.

In the early 2000s more and more of the ice melted away, opening vast areas of the Arctic Ocean. In fact, the Northwest Passage, searched for by explorers since the 15th century, was nearly open to navigation at the time.

The year 2007 marked the lowest extent of the sea ice until 2012 when it reached the lowest extent in recorded history. Even multi-year sea ice has declined significantly. Multi-year ice is thicker and older. It survives from year to year as opposed to annual ice that forms in the winter and melts in the summer. According to NASA, multi-year ice has declined by 42 percent.

NASA climate models have forecasted a decline in Arctic sea ice. Some scientists have suggested that the Arctic Ocean will be completely free of sea ice in the summer by 2100, but the actual sea ice retreat has been faster than predicted. One result of this decline is that more of the Earth surface is darker in color, thus absorbing more of the sun’s energy instead of reflecting the energy back into space as lighter snow and ice does.

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