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Northern wildfire smoke may cast shadow on Arctic warming
22 July 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, a publication of the American
Geophysical Union.
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 tmospheric stability, which in turn may keep clouds from
forming," said Stone. "We think that this nfluence 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 that the full climate impact of Arctic aerosols,
including smoke particles, is 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.
On the Web:
http://www.noaanews.noaa.gov/stories2008/20080721_arcticfires.html
Anatta
NOAA Communications, Boulder
Tel. 303-497-6288
Fax 303-497-6554
anatta[at]noaa.gov
www.noaa.gov
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