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Early Arctic Thaw Could Have Chilling Effect

The Denali landscape in winter and in spring
John Kimball at work in the Denali National Park and Preserve in Alaska. JPL maintains a ground station here where researchers monitor biophysical processes in the trees and soil. Spring (lower image) brings a dramatic change to the Denali landscape.
Related Links:
    + Seawinds site
    + QuikScat site
Spring will be coming early next year to the great forests and tundra of the Arctic. Good for the vegetation, but perhaps not so good for the atmosphere.

Spring in the high latitudes has been coming earlier in the past few decades. The early thaw means a longer growing season for the Arctic and the boreal forest, the ring of mostly evergreen trees that stretches across the northern reaches of North America and Eurasia. It also means that more carbon, now stored in the region's usually frozen soils, may be released into the air.

"The spring thaw date in boreal North America has been advancing almost one day a year since 1988," says JPL research scientist Dr. Kyle McDonald. He and his colleagues are using data from NASA's Quikscat and Japan's Midori 2 satellites to determine exactly when and where the thaw occurs. The satellites' radar instrument, the Seawinds scatterometer, can detect changes in water across the landscape from its frozen to liquid state. They used data from previous microwave missions to piece together a historical record of the spring thaw in this large, remote area where few ground measurements exist.

The boreal forest, called the taiga in Russian, is the second largest forest ecosystem on Earth, second only to the tropical rain forest in size. In North America it covers almost 28 percent of the land north of Mexico, covering much of northern Canada and reaching into Alaska.

Scientists are interested in monitoring thaw events in the Arctic and the boreal forest for several reasons. The region is extremely sensitive to change in temperature. "If global climate change is happening, here's where you would expect to see it," McDonald says. The region also plays a major role in Earth's carbon cycle.

spring thaw in Alaska
An animation of QuikScat data shows the spring thaw in Alaska from February through June 2000.
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For most of the year, the temperature of the ground in the Arctic tundra and boreal forest is below 0 degrees Centigrade (32 degrees Fahrenheit) and the vegetation is dormant. But a little bit of warming has a big effect. Once surface soil temperatures edge over freezing, plants rapidly spring into action. "There's a very abrupt change from dormant to productive associated with the spring thaw," says Dr. John Kimball, a research assistant professor at the University of Montana's Flathead Lake Biological Station. "It is like a natural on and off switch."

Over the centuries, this huge area of tundra and forest has taken great quantities of the greenhouse gas carbon dioxide out of the air, released oxygen back into the atmosphere and stored the remaining carbon in vegetation. Eventually plants decompose and the carbon ends up in the soil. "The high latitudes hold roughly 40 percent of the world's soil carbon," says Kimball.

Earlier springs in the boreal forest and Arctic tundra mean a longer growing season with more vegetation taking more carbon out of the atmosphere for longer periods. However, the same warming that releases water and nitrogen in the soil enabling plants to grow is also good for microbes that unleash the carbon stored in the soil back into the atmosphere.

"Frozen soil can store carbon dioxide for thousands of years," says Kimball. "Trees and other vegetation can only store it for up to a few hundred years or less. The residence time is much shorter and more unstable." He also points out that since the 1960s the number of forest fires in the region has increased dramatically, decreasing the length of time that trees and other plants hang on to their stored carbon before releasing it back into the atmosphere.

Could the boreal forest and Arctic tundra change from being a carbon sink, a place where carbon is stored, to a carbon source? That's the big, yet unanswered, question.

In the '50s horror movie "The Thing," warming up a frozen creature discovered in the Arctic brought a monster to life. It may turn out that some other things are better kept frozen.

Rosemary Sullivant
Jet Propulsion Laboratory