Arctic Melt Opens Northwest Passage

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But this is the first time the Northwest Passage has been "fully navigable" since ESA began measuring the Arctic sea ice with satellites.

Commercial shipping may be a while off yet. The passage is seasonal, is likely to be unstable enough to endanger commercial vessels, and still lacks supporting ports along the way.

"Extreme" Ice Loss

According to ESA, 2007 represents the lowest amount of Arctic sea ice on record.

The area of sea ice cover has dropped to around 1.2 million square miles (3 million square kilometers). This is about 386,000 square miles (a million square kilometers) less than the previous record set in 2005.

(Related: "Arctic Ice at All-Time Low" [August 20, 2007].)

Leif Toudal Pedersen of the Danish National Space Center said the sea ice has declined by about 38,600 square miles (100,000 square kilometers) a year for the last decade, making this year's decline "extreme."

"The strong reduction in just one year certainly raises flags that the ice [in summer] may disappear much sooner than expected and that we urgently need to understand better the processes involved," he said in a media statement.

According to NSIDC's Serreze, scientists are exploring several theories that may explain the mismatch between observations and climate models.

He said the models may start with sea ice that is too thick, they may lack a key dynamic in ocean circulation patterns, or they may underestimate how much the melting ice is allowing nearby water to warm and thus melt even more ice.

"It's all got us a bit puzzled," he said.

Tipping Point?

The recording-breaking Arctic sea ice melt this summer suggests that the region is either near or has reached a so-called tipping point in the climate system, Serreze noted.

A tipping point is when the climate system reaches a threshold where gradual changes suddenly kick in to high gear, he explained.

"Are we there now? We don't know, but my guess is we've got to be darn close if we haven't hit one already," he said.

Serreze cautioned, however, against drawing conclusions from just this year alone. Natural variability in the climate system, for example, could have caused the record low.

"But it looks like the overall trend is toward declining sea ice cover," he said, "and 2007 is just setting that exclamation point."

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