Thursday, May 2, 2013

With Arctic sea ice vulnerable, summer melt season begins briskly

The Arctic saw a record loss of summer sea ice in 2012, and the 2013 melt is off to a faster start than a year ago. Another record is uncertain, but warming has sapped the ice's staying power.

After a record loss of summer sea ice in the Arctic Ocean last year, the 2013 melt season has begun at the top of the world, with ice vanishing in April at a faster pace than it did this time last year.

Summer sea ice – a key player in Earth's climate system and one whose decline is widely taken as a prominent sign of global warming – has been shrinking in extent since satellites first started to build a consistent record of the ice in late 1978. Ice losses in 2007 set a melt-season record, only to be eclipsed by last year's decline. Ice volume and thickness also have been declining during the past 34 years.

Whether the start to the 2013 thaw presages another record melt is unclear. Sketching expectations for the end of the melt season based on pre-season indicators and one month's activity can be as dicey as predicting that the Boston Red Sox, with the best record in baseball coming out of April, will sweep the World Series next fall.

At the least, ice conditions entering this year's melt season appear to have set the stage for another significant retreat, compared with the 1979-2000 average.

After ice losses set the previous record in 2007, summer ice-extent recovered somewhat in each of the next three years, notes Claire Parkinson, a researcher studying climate and polar sea ice at NASA's Goddard Space Flight Center in Greenbelt, Md.

Still, the long-term decline has increased the ice's vulnerability to sharp summer declines. Resilient multi-year ice floes, once about 60 percent of the Arctic Ocean's summertime ice cover, have dwindled to some 30 percent of summer sea ice, according to the National Snow and Ice Data Center (NSIDC) in Boulder, Colo. It has been replaced by thinner, weaker ice.

This shift in ice quality has helped "precondition" the region for continued losses, Dr. Parkinson says, adding, "the very fact that last year was a record low certainly means [the ice] was further preconditioned."

Much depends on the wind patterns that set up over the spring and summer, she adds. Decades ago, when most of the summer ice was mostly densely packed, multi-year ice, typical spring and summer wind patterns had relatively little effect on ice loss. These days, those same wind patterns can do far more damage because more of the ice is less-densely packed and more fragile.

At the peak of this past winter's freeze in March, Arctic sea ice covered 5.8 million square miles, some 270,000 square miles below the 1979-2000 average, according to NSIDC. That peak occurred after a record rebound in ice extent from last summer – the record set because last summer's melt-back had been so vast.

The ice that returned is so-called seasonal ice – ice with no melt season under its belt yet.

Moreover, by the end of March, the Arctic Ocean had lost additional multi-year ice to the North Atlantic as ice bergs, while multi-year ice elsewhere had migrated to the Beaufort Sea off of the north coasts of Canada and Alaska. This area has become a graveyard for significant amounts of multi-year ice over the past several summers, according to the NSIDC.

In addition, a string of storms in February accelerated the fracturing already occurring in a large region of ice floes in the Chukchi and Beaufort Seas.

Researchers have noted that ice frequently fractures in this area all the time. But this event was unusual for the large extent of ice affected, as well as the length and width of the cracks that opened.

It involved seasonal ice. And prior to the storms, a weather system had stalled over the area, sweeping relatively warm winds out of the southwest across the ice, slowly spinning the collection of relatively large ice floes and driving them en mass like a floating pinwheel toward the northwest corner of Alaska.

Indeed, the vulnerability of single-year, or seasonal, ice was a key factor in last year's record decline in summer ice extent – the combined effect of long-term preconditioning from global warming and a powerful storm in that roared out of Siberia on Aug. 2, and over the central Arctic Ocean, only to weaken over the Canadian Archipelago 12 days later.

In what Parkinson and Goddard colleague Josephino Comiso describe as a polar version of "divide and conquer," the storm in effect bit off a 154,000-square-mile chunk of ice, which broke up and melted. This exposed a new ice edge to rough seas and once the storm passed, to more sun-warmed open water to melt the newly exposed floes to melting from underneath.

The storm, dubbed the Great Arctic Cyclone of 2012, was the strongest summertime storm to hit the region since 1979. It also ranked as the 13th strongest storm ever during that period.

The Goddard duo's analysis of the storm's impact on the ice was published last month in the journal Geophysical Research Letters.

Researchers have been tracking the continuing erosion of summer sea ice because of its potential impact on climate regionally, as well as globally. Less ice exposes more ocean to long hours of sunlight in the summer. The ocean absorbs the radiation and stores it as heat to be slowly released as air temperatures begin to cool.

Scientist have suggested that heat released from this expanding reservoir of relative warmth could well fuel storm systems that pass over the ocean, increasing the risk of storm surges along a coast with far less land-fast ice in the melt season and into the fall than it once had. This could represent an additional challenge to overcome for companies that aim to exploit resources beneath the Arctic coast.

A team led by Jesse Vermaire at Carleton University in Ottawa reported last month an increase in the level of storm-surge activity along the Mackenzie River Delta since the late 19th century. The evidence emerged from 1,200 year's worth of flooding and surge data recorded in sediment cores extracted from small lakes on the outer delta.

Surge activity was low until around 1910. Over the next 50 years, activity increased, then weakened again between 1960 and 1980. After that storm surge activity rose sharply, a change that coincided with rising temperatures and retreating sea ice.

One surge, in 1999, swamped an estimated 1.2 million acres in the low-lying delta, salting the soil, killing vegetation and turning some freshwater lakes brackish. The team notes that a decade later, affected areas still haven't recovered.

While the region is sparsely populated, it is said to contain oil and gas reserves – raising the potential for additional development as the Arctic's climate continues to warm.

The team formally published its results last month in the journal Geophysics Research Letters.

Source: The Christian Science Monitor. Author: Pete Spotts

Photo courtesy of Polar Cruises Flickr (CC BY 2.0)