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Thursday, June 24, 2010

Andrew Freedman: Arctic sea ice melting unusually fast

Arctic sea ice melting unusually fast

* Hotter & more humid: Full Forecast | Tropical trouble? *

Daily sea ice extent through June 21, 2010. Credit: NSIDC.

Summer is off to a hot start -- not just in the Washington area, but also in the Far North, where if recent trends continue, Arctic sea ice could reach a new record low that surpasses the foreboding milestone set in 2007. (That's a pretty big if, however). The recent sharp declines are consistent with a world that is warming unevenly, with the Arctic experiencing about twice the rate of warming in the past several decades as the lower-48 states, for example.

As indicated by measurements of sea ice extent -- which the National Snow and Ice Data Center (NSIDC) defines as "the total area covered by some amount of ice, including open water between ice floes," the month of May proved to be a punishing one for the shifting sea ice pack. After going into the melt season with more ice over a larger area than recent years, sea ice extent plummeted by a daily rate of 26,000 square miles per day during May, which was the highest rate of loss ever observed for the month since satellite records of sea ice began in 1979 (edging out May 1980). By the end of the month, sea ice extent was near an all-time low for the month.

Despite the steep drop, there are still eight Mays on record with a lower average ice extent.

"But Andrew," you may be wondering, "doesn't sea ice normally, um, melt during the melt season? What's the big deal about this year?"

Ah, yes, of course the sea ice normally melts during the Arctic summer, with its 24-hour sunshine and all. But this melt season has already proven noteworthy for the rate of the melt and the potential for a new record low that, should it occur, would likely generate a media frenzy. It comes at a time when the decline of Arctic sea ice has become one of the most visible manifestations of global climate change.

It can sometimes be difficult to put the sea ice decline into perspective. Nick Sundt of the environmental group WWF framed it well when he wrote on May 29, "Since reaching a seasonal maximum of approximately 14,407,344 km² on March 31st, the extent of sea ice has fallen a staggering 3,245,156 km² or 2,016,446 square miles. That is an area roughly half the size of the entire United States (including Alaska) and represents a decline of roughly 55,950 km² per day (34,766 square miles per day)." [bold is original emphasis]

The rapid melt this May is especially remarkable considering that ice had reached its maximum extent much later than it typically does, after "a late-season spurt in ice growth" in several parts of the Arctic, according to NSIDC. Part of the reason for the abnormally rapid melt is that sea ice cover today is much thinner and younger than it used to be, after successive years of depleted sea ice cover at the end of the melt season. This trend is occurring because of feedback mechanisms inherent in the Arctic climate system.

For example, as sea ice melts it exposes more areas of dark, open ocean to the sun's rays. The water absorbs far more energy than the bright ice would have, which leads to warming. This warming in turn melts more ice, which induces more warming.... and so forth. This is known as the "sea ice-albedo feedback."

Younger and thinner ice is more susceptible to spells of above-average temperatures, which is exactly what occurred across the Arctic in May and into June. As noted here previously, 2010 is on track to be the warmest year globally since instrumental records began, and much of that warmth can be found in the Far North, including Canada, Alaska and Greenland.

"In sharp contrast [to April], ice extent declined rapidly during the month of May. Much of the ice loss occurred in the Bering Sea and the Sea of Okhotsk, indicating that the ice in these areas was thin and susceptible to melt," NSIDC stated. The long-term thinning trend is evident from observations, as well as computer modeling of the Arctic climate.

A NASA study published last year found that Arctic sea ice thinned about 0.17 meters (seven inches) per year between 2004 and 2008, for a total of 0.68 meters (2.2 feet) over four winters. Also, the total area covered by thicker "multi-year" ice shrank by 42%. Other studies have documented longer-term declines in sea ice thickness, due to warming temperatures as well as winds and ocean currents that have transported thicker ice out of the Arctic and into the North Atlantic.

In the image below, one can see the sharp negative anomalies of sea ice volume this year, which is taking place against the backdrop of a general decline in sea ice volume since 1979.

Arctic sea ice volume anomaly and trend from the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS). Credit: University of Washington Polar Science Center.

Whether or not a new record is set this year will depend on the weather conditions throughout the rest of the summer.

A group of leading polar researchers is expected to release its sea ice outlook any day now, which may provide additional clues as to the likelihood of another record melt season. However, seasonal predictions of sea ice melt have not shown much skill, due to the inconsistent historical records upon which to base predictions as well as the high degree of variability in weather conditions and sea ice movement.

Still, one scientific group from the University of Washington's Polar Science Center, which issues sea ice forecasts that are based on statistical models, has steadily shifted its predictions for how much sea ice will remain at the end of this melt season toward the low end of the spectrum.

Prediction of the total sea ice extent in September. Credit: University of Washington, Polar Science Center.

The views expressed here are the author's alone and do not represent any position of the Washington Post, its news staff or the Capital Weather Gang.


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