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SCIENTISTS
CLOSE IN ON MISSING CARBON SINK Forests
in the The
Science article, "Weak northern and strong tropical land carbon uptake
from vertical profiles of atmospheric CO2," was written by an
international team of scientists led by Britton Stephens of the To
study the global carbon cycle, Stephens and his colleagues analyzed air
samples
that had been collected by aircraft across the globe for decades but
never
before synthesized. The team found that some 40 percent of the carbon
dioxide
assumed to be absorbed by northern forests is instead taken up in the
tropics. "Our
study will provide researchers with a much better understanding of how
trees
and other plants respond to industrial emissions of carbon dioxide,
which is a
critical problem in global warming," Stephens says. "This will help
us better predict climate change and identify possible strategies for
mitigating it." The
missing carbon For
years, one of the biggest mysteries in climate science has been the
question of
what ultimately happens to the carbon emitted by motor vehicles,
factories,
deforestation, and other sources. Of the approximately 8 billion tons
of carbon
emitted each year, about 40 percent accumulates in the atmosphere and
about 30
percent is absorbed by the oceans. Scientists believe that terrestrial
ecosystems, especially trees, take up the remainder. To
find this terrestrial carbon sink, scientists have turned to computer
models
that combine worldwide wind patterns with measurements of carbon
dioxide taken
just above ground level. The models indicate that northern forests
absorb about
2.4 billion tons per year. However, ground-based studies have tracked
only
about half that amount, leaving scientists to speculate about a
"missing
carbon sink" in the north. Stephens
and his collaborators set out to test how well the models captured
carbon
sinks, focusing in particular on estimates produced by a recent
international
study into global carbon exchange known as TransCom. They turned to
flasks of
air collected by research aircraft over various points of the globe for
the
past 27 years. The air samples had been analyzed by several labs around
the
world, which used them to investigate various aspects of the carbon
cycle, but
this was the first time that a team of scientists analyzed them to
obtain a
picture of sources and sinks of carbon on a global level. The
research team compared the air samples to estimates of airborne carbon
dioxide
concentrations generated by the computer models. The scientists found
that most
of the models significantly underestimated the airborne concentrations
of
carbon dioxide in northern latitudes, especially in the summer, when
plants
take in more carbon. The aircraft samples show that northern forests
absorb
only 1.5 billion tons of carbon a year, which is almost 1 billion tons
less
than the estimate produced by the computer models. The
scientists also found that intact tropical ecosystems are a more
important
carbon sink than previously thought. The models had generally indicated
that
tropical ecosystems were a net source of 1.8 billion tons of carbon,
largely
because trees and other plants release carbon into the atmosphere as a
result
of widespread logging, burning, and other forms of clearing land. The
new
research indicates, instead, that tropical ecosystems are the net
source of
only about 100 million tons of carbon, even though tropical
deforestation is
occurring rapidly. "Our
results indicate that intact tropical forests are taking up a large
amount of
carbon," Stephens explains. "They are helping to offset industrial
carbon emissions and the atmospheric impacts of clearing land more than
we
realized." Capturing
vertical movements Most
of the computer models produced incorrect estimates because, in relying
on
ground-level measurements, they failed to accurately simulate the
movement of
carbon dioxide vertically in the atmosphere. The models tended to move
too much
carbon dioxide toward ground level in the summer, when growing trees
and other
plants take in the gas, and not enough carbon dioxide up in the winter.
As a
result, scientists believed that there was relatively less carbon in
the air
above mid-latitude and upper-latitude forests, presumably because trees
and
other plants were absorbing high amounts. Conversely,
scientists had assumed a large amount of carbon was coming out of the
tropics
and moving through the atmosphere to be absorbed in other regions. But
the new
analysis of aircraft samples shows that this is not the case. "With
this new information from aircraft samples we see that the models were
overestimating the amount of uptake in the north and underestimating
uptake in
the tropics," says Kevin Gurney of ## Contact: This
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