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Wildfire Smoke May Cast Shadow on Arctic Warming
Northern Wildfire Smoke May Cast Shadow
on Arctic Warming
July 21, 2008
 The Arctic may get some temporary relief
from global warming if the annual North American
wildfire season intensifies, according to a new study by researchers
at the University of Colorado and NOAA.
Smoke transported to the Arctic from northern forest fires
may cool the surface for several weeks to months at a time,
according to the most detailed analysis yet of how smoke influences
the Arctic climate relative to the amount of snow and ice
cover.
"Smoke in the atmosphere temporarily
reduces the amount of solar radiation reaching
the surface. This transitory effect could partly offset some
of the warming caused by the buildup of greenhouse
gases and other pollutants," said Robert Stone,
an atmospheric scientist with the university and NOAA Cooperative
Institute for Research in Environmental Sciences (CIRES)
and lead author of the study, which appears this week in the
Journal of Geophysical Research.
How much solar energy is prevented from reaching the surface
depends on the smoke's opacity, the elevation of the sun above
the horizon, and the brightness of the surface, according
to the study.
Stone and his research colleagues analyzed the short-term
climate impact of numerous wildfires that swept through Alaska
and western Canada in 2004. That summer, fires burned a record
10,000 square miles of Alaska's interior and another 12,000
square miles in western Canada.
A NOAA climate observatory near Barrow, Alaska,
provided the data for the study. Smoke observed at Barrow
was so thick that at times visibility dropped to just over
one mile. The aerosol optical depth (AOD),
a measure of the total absorption and scattering of solar
radiation by smoke particles, rose a hundredfold from typical
summer values.
Smoke in the atmosphere tends to cool the snow-free tundra
while warming the smoke layer itself, the authors found. Smoke
has an even greater cooling effect over the darker, ice-free
ocean and less over bright snow.
 "The heating of the smoke layer and cooling of the surface
can lead to increased atmospheric stability, which in turn
may keep clouds from forming," said Stone. "We think
that this influence of smoke aerosol on clouds further affects
the balance of radiation reaching the surface in the Arctic."
Research observatories as far away as Greenland and the Svalbard
archipelago north of Norway also recorded elevated AOD values
over several weeks during the 2004 summer, suggesting that
the climate footprint of the North American wildfires was
far-reaching. Smoke from the same fires also was observed
as far south as the Gulf of Mexico.
To conduct their analysis, Stone and colleagues
looked at how a range of smoky conditions might change the
amount of solar radiation reaching the Earth’s
surface. Models showed that the cooling caused by future forest
fires would depend on the severity of the fire season and
on the geographic dispersion of smoke.
The authors cautioned, however, that the full climate impact
of Arctic aerosols, including smoke particles, is still not
entirely clear. For one thing, smoke particles captured within
clouds or deposited on snow may change the brightness of these
objects, further affecting the amount of solar radiation
absorbed by the surface.
Also, aerosols such as smoke affect the absorption and scattering
not only of solar radiation, but also of longwave or thermal
radiation within the atmosphere. The impact of aerosols on
longwave radiation, which dominates at night and during the
long, dark winter season in the Arctic, has yet to be quantified.
NOAA understands and predicts changes in the Earth's environment,
from the depths of the ocean to the surface of the sun, and
conserves and manages our coastal and marine resources.
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