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

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.

For more information and images on the Internet, visit:

A previous, related NASA release, “NASA Finds Soot Has Impact
on Global Climate,” is at: