| September 29, 2003
Spotlight: Early Arctic Thaw Could Have Chilling Effect
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.
For most of the year, the temperature of the ground in the Arc 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 temperature edge over freezing, plants rapidly
spring into action. "There's 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 fir 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.
Written by Rosemary Sullivant
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Contacts:
Rosemary Sullivant
Jet Propulsion Laboratory
Rosemary.Sullivant@jpl.nasa.gov
Alan Buis
Jet Propulsion Laboratory
(818) 354-0474 |
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Winter and Spring in Denali
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.
Alaskan Thaw
This image of QuikScat data shows the spring thaw in Alaska from February
through June 2000.
Animation
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