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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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