News Archive

posted 12/23/03

Black Soot and Snow: A Warmer Combination

Soot also affects melting of alpine glaciers. Some scientists believe the snow cap of Mount Kilimanjaro will be gone in two decades. Researchers say the ice fields on Africa's highest mountain shrank by 80 percent in the past century. The snow cap formed some 11,000 years ago. The Landsat satellite captured these images of Kilimanjaro February 17, 1993 and February 21, 2000. Credit: NASA/USGS

New research from NASA and Columbia University 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, and a member of the Earth Institute at Columbia University.

Hansen and Larissa Nazarenko, both of the Goddard Institute and the 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.

The figure on the left shows how polar ice reflects light from the sun. As this ice begins to melt, less sunlight gets reflected into space, and is instead absorbed into the oceans and land, raising the overall temperature, and fueling further melting.The figure on the right shows darker, soot-covered ice which reflects less light as well, part of the warming effect. Credit: NASA

"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.

A previous, related NASA release, "NASA Finds Soot Has Impact on Global Climate," is at: http://www.gsfc.nasa.gov/topstory/2003/0509pollution.html

The Earth Institute at Columbia University is among the world’s leading academic centers for the integrated study of Earth, its environment, and society. The Earth Institute builds upon excellence in the core disciplines—earth sciences, biological sciences, engineering sciences, social sciences and health sciences—and stresses cross-disciplinary approaches to complex problems. Through its research, training and global partnerships, it mobilizes science and technology to advance sustainable development, while placing special emphasis on the needs of the world’s poor.