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Black Soot and Snow: A Warmer Combination

Snowflake under microscope
Image Above: Snowflake under a microscope. Credit: USDA Beltsville Agricultural Research Center.

Global warming might be the last thing on your mind while snuggled up inside during a raging snowstorm. But, scientists say what happens to that snow over the next few days, as its plowed and turns filthy along our roadsides, contributes to climate change.

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.

Clean ice reflect sunlight
Ice cover with soot
Image Above: This is a conceptual animation showing how polar ice reflects light from the sun. As this ice begins to melt, less sunlight gets reflected into space. It is instead absorbed into the oceans and land, raising the overall temperature, and fueling further melting. Darker, soot-covered ice reflects less light as well, part of the warming effect. Click either image to see annimation. Credit: NASA

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

Image Above: Heating Up the Atmosphere (animation) - When soot absorbs sunlight, it heats the air and reduces the amount of sunlight reaching the ground, cooling the Earth's surface. The heated air makes the atmosphere unstable, creating rising air (convection) that forms clouds and brings rainfall to regions that are heavily polluted. The increase of rising air is balanced by an increase in sinking air (subsidence) and drying. When air sinks, clouds and thus rain, can't form, creating dry conditions. Credit: NASA

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.

For more information on this research and high resolution images, please visit:

News Release
NASA Goddard Space Flight Center Topstory Page
NASA Finds Soot Has Impact on Global Climate

Mike Bettwy
NASA Goddard Space Flight Center