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June
6, 2007
DIRTY
SNOW MAY
WARM ARCTIC AS MUCH AS GREENHOUSE GASES
The
global warming debate has focused on carbon dioxide emissions, but
scientists
at UC Irvine have determined that a lesser-known mechanism –
dirty snow – can
explain one-third or more of the Arctic
warming primarily attributed to greenhouse gases.
Snow becomes dirty when soot from tailpipes, smoke stacks and forest
fires
enters the atmosphere and falls to the ground. Soot-infused snow is
darker than
natural snow. Dark surfaces absorb sunlight and cause warming, while
bright
surfaces reflect heat back into space and cause cooling.
"When we inject dirty particles into the atmosphere and they fall onto
snow, the net effect is we warm the polar latitudes," said Charlie
Zender,
associate professor of Earth system science at UCI and co-author of the
study.
"Dark soot can heat up quickly. It’s like placing tiny
toaster ovens into
the snow pack."
The study appears this week in the Journal of Geophysical
Research.
Dirty snow has had a significant impact on climate warming since the
Industrial
Revolution. In the past 200 years, the Earth has warmed about .8 degree
Celsius. Zender, graduate student Mark Flanner, and their colleagues
calculated
that dirty snow caused the Earth’s temperature to rise .1 to
.15 degree, or up
to 19 percent of the total warming.
In the past two centuries, the Arctic
has
warmed about 1.6 degrees. Dirty snow caused .5 to 1.5 degrees of
warming, or up
to 94 percent of the observed change, the scientists determined.
The amount of warming by dirty snow varied from year to year, with
higher
temperatures in years with many forest fires. Greenhouse gases, which
trap outgoing
energy, are primarily responsible for the remaining temperature
increase and
are considered the Earth’s most important overall climate
changing mechanism.
Other human influences on Arctic climate change are particles in the
atmosphere, including soot; clouds; and land use.
Humans create the majority of airborne soot through industry and fuel
combustion, while forest and open-field fires account for the rest.
Because of
human activity, greenhouse gas levels have increased by one-third in
the last
two centuries.
"A one-third change in concentration is huge, yet the Earth has only
warmed about .8 degrees because the effect is distributed globally,"
Zender said. "A small amount of snow impurities in the Arctic
have caused a significant temperature response there."
Previous studies have analyzed dirty snow's effect on climate, but this
is the
first to take into account realistic emissions from forest fires in the
Northern Hemisphere and how warming affects the thickness of the snow
pack.
In some polar areas, impurities in the snow have caused enough melting
to
expose underlying sea ice or soil that is significantly darker than the
snow.
The darker surfaces absorb sunlight more rapidly than snow, causing
additional
warming. This cycle causes temperatures in the polar regions to rise as
much as
3 degrees Celsius during some seasons, the scientists say.
"Once the snow is gone, the soot that caused the snow to melt continues
to
have an effect because the ground surface is darker and retains more
heat," Zender said.
Dirty snow is prevalent in East Asia, Northern Europe and Northeastern United States.
Zender believes policymakers could use these research results to
develop
regulations to mitigate global warming. Limiting industrial soot
emissions and
switching to cleaner-burning fuels would leave snow brighter, he says.
New snow
falls each year, and if it contained fewer impurities, the ground would
brighten and temperatures would cool. Carbon dioxide lives in the
atmosphere
for a century, so cutting back on emissions can prevent further warming
but
does not produce immediate cooling.
UCI scientist James Randerson and Philip Rasch, a scientist at the National Center
for Atmospheric Research in Boulder, Colo.,
also worked on the study.
The National Science Foundation and NASA funded this research.
##
Contact:
Jennifer
Fitzenberger
University of California -
Irvine
949-824-3969
jfitzen@uci.edu
This text derived from:
http://today.uci.edu/news/release_detail.asp?key=1621
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