Dirty Snow: A Global Warming Suspect

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Dirty Snow: A Global Warming Suspect
Midnight EST, January 5, 2003

NASA Press Release

New research from NASA scientists
suggests emissions of black soot alter the
way sunlight reflects off snow. According
to a computer simulation, black soot may
be responsible for 25 percent of observed
global warming over the past century.
Soot in the higher latitudes of the Earth,
where ice is more common, absorbs more
of the sun`s energy and warmth than an icy, white background. Dark-colored
black carbon, or soot, absorbs sunlight, while lighter colored ice reflects
sunlight.
Soot in areas with snow and ice may play an important role in climate change.
Also, if snow- and ice-covered areas begin melting, the warming effect
increases, as the soot becomes more concentrated on the snow surface. "This
provides a positive feedback (i.e. warming); as glaciers and ice sheets melt,
they tend to get even dirtier," said Dr. James Hansen, a researcher at NASA`s
Goddard Institute for Space Studies, New York.

Hansen and Larissa Nazarenko, both of the Goddard Institute and Columbia
University`s Earth Institute, found soot`s effect on snow albedo (solar energy
reflected back to space), which has been neglected in previous studies, may
be contributing to trends toward early springs in the Northern Hemisphere,
thinning Arctic sea ice, melting glaciers and permafrost. Soot also is believed to
play a role in changes in the atmosphere above the oceans and land.
"Black carbon reduces the amount of energy reflected by snow back into
space, thus heating the snow surface more than if there were no black carbon,"
Hansen said.
Soot`s increased absorption of solar energy is especially effective in warming
the world`s climate. "This forcing is unusually effective, causing twice as much
global warming as a carbon-dioxide forcing of the same magnitude," Hansen
noted.

Hansen cautioned, although the role of soot in altering global climate is
substantial, it does not alter the fact greenhouse gases are the primary cause
of climate warming during the past century. Such gases are expected to be the
largest climate forcing for the rest of this century.
The researchers found that observed warming in the Northern Hemisphere was
large in the winter and spring at middle and high latitudes. These observations
were consistent with the researchers` climate model simulations, which showed
some of the largest warming effects occurred when there was heavy snow
cover and sufficient sunlight.
Hansen and Nazarenko used a leading worldwide-climate computer model to
simulate effects of greenhouse gases and other factors on world climate. The
model incorporated data from NASA spacecraft that monitor the Earth`s
surface, vegetation, oceans and atmospheric qualities. The calculated global
warming from soot in snow and ice, by itself in an 1880-2000 simulation,
accounted for 25 percent of observed global warming. NASA`s Terra and Aqua
satellites are observing snow cover and reflectivity at multiple wavelengths,
which allows quantitative monitoring of changing snow cover and effects of
soot on snow.

The research is in the paper "Soot Climate Forcing via Snow and Ice Albedos,"
appearing online this week in the Proceedings of the National Academy of
Sciences.
This research was funded by NASA`s Earth Science Enterprise. The Enterprise
is dedicated to understanding the Earth as an integrated system and applying
Earth system science to improve prediction of climate, weather and natural
hazards using the unique vantage point of space.