Dirty snow may warm Arctic as much as greenhouse gases - cleaner fuels needed

The global warming debate has focused on carbon dioxide emissions, but scientists at UC Irvine have determined that a lesser-known mechanism – dirty snow – can explain one-third or more of the Arctic warming primarily attributed to greenhouse gases.

Snow becomes dirty when soot from fossil fuel burning and forest fires enters the atmosphere and falls to the ground. Soot-infused snow is darker than natural snow. Dark surfaces absorb sunlight and cause warming, while bright surfaces reflect heat back into space and cause cooling, a process known as the albedo effect. Burning cleaner fuels would be a ready solution to brighten snow and thus lower temperatures, the scientists say.

Because of their emission profiles, biofuels may contribute to solving part of the problem:

• Both first-generation biodiesel and ultra-clean synthetic biofuels show dramatic reductions in particulate matter and soot emissions compared to fossil fuels.
• Likewise, the co-combustion of biomass with coal in power plants shifts particle size distribution from fine particles to coarse particles, which can be captured by dust collection systems, thus preventing soot and particulate matter emissions.
• Moreover, around 2 billion people in the developing world rely on primitive biomass for energy (basically burning wood or dung on open fires). Introducing modern, clean biofuels amongst this vast mass of people would reduce soot and particulate matter emissions drastically.
• Finally, burning of field residues in agriculture releases vast amounts of soot and fine particles. If these resources were used for power generation and to offset fossil fuels, once again the build up dirty snow could be reduced (see an example from Egypt).

On the other hand, if biofuel production were to imply the burning of forests or field based residues in the open air, they may actually worsen the situation.

The study, appearing this week in the Journal of Geophysical Research, shows dirty snow has had a significant impact on climate warming since the Industrial Revolution. In the past 200 years, the Earth has warmed about 0.8 degree Celsius. Charlie Zender, associate professor of Earth system science at UCI, graduate student Mark Flanner, and their colleagues calculated that dirty snow caused the Earth’s temperature to rise 0.1 to 0.15 degree, or up to 19 percent of the total warming.

In the past two centuries, the Arctic has warmed about 1.6 degrees. Dirty snow caused 0.5 to 1.5 degrees of warming, or up to 94 percent of the observed change, the scientists determined (map, click to enlarge).

The amount of warming by dirty snow varied from year to year, with higher temperatures in years with many forest fires. Greenhouse gases, which trap outgoing energy, are primarily responsible for the remaining temperature increase and are considered the Earth’s most important overall climate changing mechanism. Other human influences on Arctic climate change are particles in the atmosphere, including soot; clouds; and land use:
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Humans create the majority of airborne soot through industry and fuel combustion, while forest and open-field fires account for the rest. Because of human activity, greenhouse gas levels have increased by one-third in the last two centuries.

"A one-third change in concentration is huge, yet the Earth has only warmed about 0.8 degrees because the effect is distributed globally," Zender said. "A small amount of snow impurities in the Arctic have caused a significant temperature response there."

Previous studies have analyzed dirty snow’s effect on climate, but this is the first to take into account realistic emissions from forest fires in the Northern Hemisphere and how warming affects the thickness of the snow pack.

In some polar areas, impurities in the snow have caused enough melting to expose underlying sea ice or soil that is significantly darker than the snow. The darker surfaces absorb sunlight more rapidly than snow, causing additional warming. This cycle causes temperatures in the polar regions to rise as much as 3 degrees Celsius during some seasons, the scientists say.

"Once the snow is gone, the soot that caused the snow to melt continues to have an effect because the ground surface is darker and retains more heat," Zender said.

Dirty snow is prevalent in East Asia, Northern Europe and Northeastern United States.

Zender believes policymakers could use these research results to develop regulations to mitigate global warming. Limiting industrial soot emissions and switching to cleaner-burning fuels would leave snow brighter, he says. New snow falls each year, and if it contained fewer impurities, the ground would brighten and temperatures would cool. Carbon dioxide lives in the atmosphere for a century, so cutting back on emissions can prevent further warming but does not produce immediate cooling.

UCI scientist James Randerson and Philip Rasch, a scientist at the National Center for Atmospheric Research in Boulder, Colo., also worked on the study. The National Science Foundation and NASA funded this research.

Image: Map showing the annual mean temperature change due to dirty snow in degrees Celsius.

More information:
Mark G. Flanner, Charles S. Zender, James T. Randerson, Philip J. Rasch, "Present-day climate forcing and response from black carbon in snow", Journal of Geophysical Research, Vol. 112, D11202, doi:10.1029/2006JD008003, 2007