A new study found that emissions of soot, or black carbon, alters the way sunlight reflects off snow and may be responsible for as much as 25 percent of observed global warming over the past century.

The Dirty on Soot
Soot on snow absorbs more of the Sun's energy and heat than icy, white backgrounds, which reflect the Sun's rays. With global warming, many snow- and ice-covered areas are already melting. As can be seen when glaciers and ice sheets melt, they tend to get dirtier, as the soot becomes even more concentrated. Soot thereby adds to the warming effect as ice melts, making icy surfaces darker and absorbing more solar energy.

Soot is generated from traffic, industrial pollution, outdoor fires and household burning of coal and other fuels and is the product of incomplete combustion. Emissions are large in areas where cooking and heating are done with wood, field residue, cow dung, and coal at a low temperature that does not allow for total combustion. The soot particles absorb sunlight very effectively, just as wearing a black shirt outdoors absorbs more solar energy and keeps you warmer than a white shirt.

Research Yields Startling Findings
Dr. James Hansen and Larissa Nazarenko of NASA's Goddard Institute for Space Studies (NASA/GISS) and Columbia University's Earth Institute looked specifically at soot's effect on snow albedo, or reflectivity. Many previous research studies overlooked soot as a potential global warming contributor. Hansen and Nazarenko believe several recently observed changes, such as earlier springs in the Northern Hemisphere and thinning Arctic sea ice may be at least partially linked to soot's role in the atmosphere.

Although the role of soot in altering global climate is substantial, Hansen says it does not change the fact that greenhouse gases will likely continue to be the primary cause of climate warming during this century.

Model Simulations Confirm Warming
Hansen and Nazarenko used a leading worldwide-climate computer model to simulate the effects of greenhouse gases and other factors on world climate. The researchers incorporated into the model data from NASA spacecraft that monitor the Earth's surface, vegetation, oceans and atmosphere. The calculated global warming from soot in snow and ice, by itself, accounted for 25 percent of observed global warming in an 1880-2000 simulation. NASA's Terra and Aqua satellites allow daily monitoring of snow cover and reflectivity, helping scientists better understand the effects of soot on snow.

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.

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