Arctic cooked by Man and Nature

Nature and man jointly cook Arctic
Seth Borenstein, AP Yahoo News 3 Jan 08;
By SETH BORENSTEIN, AP Science Writer1 hour, 2 minutes ago

There's more to the recent dramatic and alarming thawing of the Arctic region than can be explained by man-made global warming alone, a new study found. Nature is pushing the Arctic to the edge, too.

There's a natural cause that may account for much of the Arctic warming, which has melted sea ice, ice sheets and glaciers, according to a study published Thursday in the journal Nature.

New research points a finger at a natural and cyclical increase in the amount of energy in the atmosphere that moves from south to north around the Arctic Circle.

But that energy transfer, which comes with storms that head north because of ocean currents, is not acting alone either, scientists say. Another upcoming study concludes that the combination of both that natural energy transfer increase and man-made global warming serve as a one-two punch that is pushing the Arctic over the edge.

Scientists are trying to figure out why the Arctic is warming and melting faster than computer models predict.

The summer of 2007, like the summer of 2005, smashed all records for loss of summer sea ice in the Arctic Ocean and ice sheet in Greenland. In September, the Arctic Ocean had 23 percent less sea ice than the previous record low. Greenland's ice sheet melted 19 billion tons more than its previous record.

The Nature study suggests there's more behind it than global warming because the air a couple miles above the ground is warming more than calculated by the climate models.

Climate change theory concentrates on warming of surface temperatures and explains an Arctic that is warming faster than the rest of the world as mostly because reduced sea ice and ice sheets means less reflecting solar rays.

Rune Graversen, the Nature study co-author and a meteorology researcher at Stockholm University in Sweden, said a shift in energy transfer explains the thawing more, including what's happening in the atmosphere, but does not contradict consensus global warming science.

Oceanographer James Overland, who reviewed Graversen's study for Nature, said the research dovetails with an upcoming article of his which concludes that the Arctic thawing is a combination of the two.

"If we didn't have the little extra kick from global warming then we wouldn't have gone past the threshold for the change in sea ice," said Overland, of the National Oceanic and Atmospheric Administration's lab in Seattle.

Other researchers said Graversen's study underestimates the effect of global warming because it relied on older data that stopped at 2001 and wasn't the most accurate.

Overland and scientist Mark Serreze disagree over which effect — man-made or natural — was the big shove that pushed the Arctic over the edge, but they agreed that overall it's a combined effort.

"Think of it as a boxer that's almost going down for the count ... and that one blow to the noggin comes and he's down for the count," said Serreze, a senior scientist at the government's snow and ice data center in Boulder, Colo.

Shifting heat layers above Arctic to blame for ice crisis: study
Marlowe Hood, Yahoo News 2 Jan 08;

The dramatic loss of the Arctic ice cap may have been triggered by disruption to the thermal layers of atmosphere stacked over Earth's far north, according to Swedish research to be published Thursday.

The study, published in Nature, offers a new explanation for the rise in the Arctic's surface temperature, which over the past century has been nearly two degrees Celsius (3.6 Fahrenheit), or twice the global average.

Until now, the big suspect in "Arctic amplification" has been reflectivity of sunlight.

When the Sun's rays hit snow or ice, most of that solar energy bounces back into space -- but as those melting surfaces give way to dark-blue sea, the heat is absorbed instead.

This self-reinforcing process, called a feedback, is an established factor in accelerating warming in snow and ice.

But Stockholm University scientists led by Rune Graversen believe a possibly bigger cause for Arctic warming could be changes in heat transport in the middle of the troposphere, an atmospheric band that extends 10 kilometers (seven miles) above Earth's surface.

In polar regions, the layers of relative heat above the surface are usually stable. But Graversen says that over the last two decades or so there have been changes in Arctic atmospheric circulation which have brought in heat and moisture.

The moisture is particularly important, as it helps form persistent low cloud over the Arctic.

Moisture-laden clouds at this altitude tend to absorb heat from the Sun, thus bringing a warming effect close to the surface. In contrast, high-altitude clouds, which mainly comprise icy crystals, reflect heat back into space, and thus cool the surface.

The circulatory shifts have an especially big impact in summer, says Graversen.

In 2007, summer sea ice in the Arctic shrank to about four million square kilometers (2.4 million square miles), a 23 percent decrease from the previous record low of 5.3 million square kilometers in 2005.

A second study, also in Nature, meanwhile, shows that the capacity of vegetation to absorb carbon dioxide (CO2) appears to be ebbing, with potentially serious consequences for global warming.

Currently, about 50 percent of all the CO2 produced by burning fossil fuels is soaked up -- "sequestered" -- by land masses, mainly through forests, and by oceans.

Remarkably, that percentage has remained stable even as the output of man-made greenhouse gases has increased.

Up to now scientists have assumed that longer growing seasons were a silver lining of climate change because the warmer temperatures gave photosynthesising plants more time to remove the most important of these gases -- CO2 -- from the atmosphere.

This view has been bolstered by satellite images showing a clearly visible "greening trend," notably in the northern hemisphere.

But Shilong Piao of France's National Centre for Scientific Research (CNRS), found that over the past 20 years the planet's biomass above the 20th parallel released nearly as much CO2 in the fall or autumn as it soaked up during the spring.

"If future autumn warming occurs at a faster rate than in spring, the ability of norther ecosystems to sequester carbon may be diminished earlier than previously suggested," he warns.

While Piao's study draws a clear link between rising temperatures and reduced carbon uptake, projecting future trends is very difficult, cautions John Miller, a expert on carbon cycles at the University of Colorado, in a commentary, also published in Nature.