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The melt of '07
Polar experts ponder this summer's dramatic Arctic loss

Horizon

The photo above—showing an isolated patch of multiyear Arctic sea ice (ridges and melt pools) surrounded by thinner ice and open water—was taken near latitude 80°N, longitude 159°W on 26 August by Steven Roberts (NCAR Earth Observing Laboratory) aboard the U.S. Coast Guard icebreaker Healy. Roberts is part of a small group of civilian technicians, managed by the Lamont-Doherty Earth Observatory, that provides technical support for all Healy missions. NCAR enables his participation through NSF's Office of Polar Programs. The Healy sailed north from Alaska to support a NOAA-funded mapping program related to the Law of the Sea Treaty (the international agreement that defines the rights and responsibilities of nations using the world's oceans).

by Bob Henson

On the high seas, it's not so unusual to find yourself tossed by waves two stories tall. When this happens in the western Arctic at 77°N, though, it's a sign of something new. This summer a group of scientists and technicians aboard the U.S. Coast Guard cutter Healy, a polar icebreaker, sailed through vast stretches of open water as close as 1,000 kilometers (600 miles) to the North Pole. (see maps)

Alex Guenther

Steven Roberts with the Coast Guard icebreaker Healy.

Thanks to a blend of unusually sunny, warm weather and surface winds that helped sweep ice toward one side of the basin—all on top of a long-term warming trend—the Arctic Ocean opened up this summer far more than ever before. More than a third of the ocean was free of ice, and the main channel of the fabled Northwest Passage, running along Canada's northern coast, saw more than a month of ice-free conditions.

"The fact there was water so far north was just surreal," says NCAR software engineer Steven Roberts, who helps maintain computer systems aboard the Healy. "Everyone on the ship was talking about the ice—'Where is it?' "

There was more discussion about ice, and the lack of it, at a multiagency workshop on high-latitude climate change that brought 26 experts from five nations to Seattle in October. Though the meeting date had been set months earlier, it proved to be a timely venue for experts to discuss 2007's eye-opening ice loss and implications for future observing and modeling.

Modelers have long been aware of large natural variability as well as anthropogenic forcing in the Arctic, notes James Overland (NOAA Pacific Marine Environmental Laboratory). According to Overland, though, "the magnitude of this summer's ice event was a surprise to everyone at the workshop. It inspires modelers to look more closely at the range of ensemble members coming out of the climate models, because the real-world ensemble is on a fast track."

Healy

At 76°N in the western Arctic, a huge wave breaks over the bow of the Healy. The water temperature was 2.5°C (36.5°F), more than 4°C above the freezing point of seawater. (Photo by Steven Roberts.)

Some of the first ensemble-based model runs to hint at the risk of an ice-free Arctic by mid-century or sooner were generated by the Community Climate System Model in 2005 and analyzed by NCAR's Marika Holland (see "On the Web"). The results stunned many observers. Yet the amount of melting this summer was more than projected by any of the CCSM runs to occur until at least the 2010s (see graphic).

From the beginning of satellite measurements in 1979 through the 1990s, northern sea-ice extent (the area with at least 15% ice coverage) typically reached about 15–16 million square km by March, melting back to around 7 million square km by each September. (See graphic at bottom of page.) That late-summer coating was still enough to keep much of the interior Arctic covered with multiyear ice as thick as 10 meters (33 feet). This year's maximum winter extent reached 14.7 million square km, and the summer extent bottomed out near 4.1 million square km, according to the National Snow and Ice Data Center.

"The atmosphere and ocean appear to be conspiring in multiple ways to foster ice loss, some of which we don't completely understand," says NSIDC's Mark Serreze.

The ice that's left isn't what it used to be, either. Until very recently, ice thickness couldn't be gauged by satellite. Submarines and icebreakers that use sonar and other tools along their limited tracks have found hints that the Arctic's remaining multiyear ice could be as little as half the thickness of several decades ago.

Marika Holland

Marika Holland. (Photo by Carlye Calvin.)

A southern freeze-up

As the Arctic's ice eroded this summer, a far different picture unfolded on the other side of the globe. Frequent shots of polar air brought rare snowfall to places like Buenos Aires and Johannesburg, and the ice sheet that hugs Antarctica extended to more than 16 million square km in area—the largest in the 29-year satellite record. A few climate-change skeptics took this as a counter to the Arctic's dramatic meltback.

But the northern loss and southern growth don't cancel each other out, notes Holland, because the Arctic is losing far more summer ice than Antarctica is gaining during its winter. All told, the net global anomaly in sea ice area dropped to a record 3 million square km this summer, according to Cryosphere Today, a Web site managed by the University of Illinois at Urbana-Champaign. (The UIUC index of sea ice area differs somewhat from NSIDC's measure of sea ice extent.)

The picture gets even more lopsided when it comes to ice thickness. There's little summer sea ice in the south except in the Ross and Weddell seas, which are tucked closer to the South Pole than most other southern waters. Thus, the ice that rings Antarctica each winter is much thinner than the multiyear ice that's predominated in the Arctic until recently.

"I've heard scientists point to Antarctica and say that it balances the Arctic in sea-ice loss. That's an incorrect argument," says Holland.

Healy map

On a 2006 mission, the U.S. Coast Guard cutter Healy circled a polynya, a large gap in summer ice visible just north of Alaska. At the time, scientists wondered whether the hole would recur or not in the summer of 2007. Instead, the region where the polynya had occurred was absorbed into a much larger area of melt by the time of the Healy's late-summer mission in 2007 (right). The Healy's track is shown in red. Ice data are from the Advanced Microwave Scanning Radiometer aboard NASA's Aqua satellite. (Illustrations courtesy Steven Roberts.)

Though the Antarctic's slow growth in sea ice is no match for the massive Arctic loss, it's still a juicy topic for research. Evidence points to the Southern Annual Mode (SAM) as the chief culprit. When the band of strong winds encircling Antarctica tightens poleward, the SAM is in a positive mode, one that's expected to predominate in coming decades as the planet warms. A positive SAM typically means colder weather near the Antarctic coast—good for winter ice growth—but warmer conditions north of about 50°S, perhaps intensifying drought and heat in Australia.

CCSM Illustration

In a set of model runs produced for the 2007 assessment from the Intergovernmental Panel on Climate Change, the Community Climate System Model projected that the Arctic Ocean's summer sea ice would began a sharp decline in coverage during the 2010s. However, the ice's decline in the last several years (blue trace) exceeded the projections of any member of this ensemble. (Illustration courtesy CCSM and Marika Holland.)

Ozone depletion appears to be enhancing this trend by cooling the lower stratosphere and fostering a circulation that promotes cooling over most of Antarctica and warming over the Antarctic Peninsula. "As the ozone hole recovers in the coming decades, we can expect this circulation trend to slow or even reverse, leading to enhanced warming over Antarctica," says Nathan Gillett (University of East Anglia), one of the scientists at the Seattle workshop.

Just wait till next year?

It's too soon to know whether 2007's Arctic ice loss was the start of an ominous new regime or simply a punctuation mark in a more gradual long-term melt. In a letter to Nature on 1 November, Julia Slingo and Rowan Sutton (University of Reading) noted that the abnormal cloud and circulation patterns that fed this summer's Arctic melt were part of a global pattern of anomalies that included La Niña.

Special observing programs in place for the International Polar Year, which runs through March 2009, may help illuminate what's happening to the north (and south). One topic at the Seattle workshop was how best to use the IPY data and whether some observing systems ought to be extended, just as the carbon dioxide measurements established on Mauna Loa in 1957 for the International Geophysical Year became a long-term benchmark that continues today.

Meanwhile, what's next for the Arctic? A straw poll taken in Seattle found most of the attendees expected a small rebound of the ice for at least a year or two, followed by more loss. "Most thought that the ice will never return to conditions of the 1980s," says Overland. "But some thought there will be continued loss next year, and others suggested a major recovery before a major loss later in the century. Most are startled by the realization that the ice may not return to previous conditions and that this happened so fast."

SMMR-SSMI

Data compiled on a month-by-month basis since 1979 from NOAA's scanning multichannel microwave radiometer (SMMR-SSM/I) depict each year's rise and fall in the extent of Arctic sea ice. The longer-term decline is shown as a linear trend (black line) and a 10-year running mean (green line). (Illustration courtesy Walt Meier, National Snow and Ice Data Center.)

On the Web
 

National Snow and Ice Data Center

International Commission on Polar Meteorology
(includes link to Seattle workshop report)

 
 

 

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