"A one-third change in concentration is huge, yet the Earth has only warmed about .8 degrees because the effect is distributed globally," said Charlie Zender, an associate professor of Earth system science at UCI and co-author of the study, which appears in the Journal of Geophysical Research. "A small amount of snow impurities in the Arctic have caused a significant temperature response there."
Dirty snow arises from the soot that escapes from tailpipes, smoke stacks and forest fires to enter the atmosphere and then falls to the ground. While clean, white snow tends to reflect back heat into space and cause cooling, the dark surface of dirty snow causes it to absorb sunlight and thus results in warming.
"When we inject dirty particles into the atmosphere and they fall onto snow, the net effect is we warm the polar latitudes," Zender said. "Dark soot can heat up quickly. It's like placing tiny toaster ovens into the snow pack."According to Zender and his colleagues, dirty snow likely caused the planet's temperature to rise .1 to .15 degree over the past 200 years, accounting for close to 19% of the total warming experienced (.8 degree Celsius) during that time. Throughout that same period, the Arctic warmed by approximately 1.6 degrees, with some estimates showing dirty snow contributing as little as .5 to as much as 1.5 degrees (or up to 94% of the observed increase). The degree to which dirty snow caused increases in temperature was directly related to the number of forest fires every year.
The effect is even worse in some polar areas where impurities in the snow have led to enough melting to expose the underlying sea ice or soil that is much darker and thus more apt to absorb sunlight. This has resulted in polar temperatures rising by as much as 3 degrees Celsius during certain seasons. "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.
Zender thinks policymakers should approach this problem by focusing on reducing industrial soot emissions and switching over to cleaner fuels. Making new snow purer by cutting out impurities would cause an immediate cooling in temperatures.
See also: ::TH Interview: Clif Bar Sets Out to "Save Our Snow" with World Champion Freeskier Alison Gannett, ::Let it Snow, Let it Snow, Let it Snow, ::Aspen Skiing Company Makes Green Snow, ::Big Coal - The Dirty Secret Behind America's Energy Future by Jeff Goodell, ::Arctic Emissaries Head to Washington