Charlie Zender ... grab a shovel!

Canada has dirty snow. Who knew, and what horse did he ride in on?

A team of U.S. scientists has found that "dirty snow" is a surprisingly significant contributor to global warming, and is urging Canada -- as "custodian" of a vast, snowbound nation -- to lead an international cleanup effort.

The researchers have measured, in the first comprehensive study of its kind, how snowy landscapes tainted by carbon particles from inefficiently burned fuels and forest fires are absorbing more of the sun's heat than the less sooty snow cover of centuries past.

"Snow becomes dirty when soot from tailpipes, smokestacks and forest fires enters the atmosphere and falls to the ground," the team explains. "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."

Even a slightly darkened surface impairs the natural reflective properties of snow crystals, say the scientists, who calculated that dirty snow accounts for a startling one-third of rising temperatures in the Arctic over the past two centuries.

"When we inject dirty particles into the atmosphere and they fall onto snow, the net effect is we warm the polar latitudes," says Charlie Zender, a University of California atmospheric physicist and co-author of a study published in the latest Journal of Geophysical Research. "Dark soot can heat up quickly. It's like placing tiny toaster ovens into the snow pack."

About 80 per cent of "black carbon" pollutants are man-made, the researchers estimate, with forest fires accounting for the rest.

Mr. Zender said that although all nations contribute to the problem of snow impurity through the long-range transport of pollutants, Canada bears particular responsibility to push for cleaner-burning fuels and reduced industrial emissions of soot.

"Just as Brazil is the custodian of the Amazon, a world resource whose deforestation has all sorts of negative consequences, so is Canada a custodian of an important swath of snow-covered land that helps to cool the planet," said Mr. Zender.

He also raised a red flag about increased ship traffic through the Northwest Passage -- widely viewed as a potential economic boon for Canada in the coming decades -- as a result of the melting Arctic ice pack.

"One implication," Mr. Zender said of his team's research, "is that any increase in shipping through the Arctic Ocean -- for example, the Northwest Passage -- is likely to exacerbate these effects by putting soot emissions right in the middle of the remaining snow and sea ice. We must think very carefully about this."

In their NASA-funded project, Mr. Zender and three colleagues from UC-Irvine and the University of Colorado calculated that dirty snow caused the Earth's temperature to rise 0.1 to 0.15 C, or up to 19 per cent of the total warming of 0.8 C over the past 200 years.

In that time, the Arctic has warmed about 1.6 C, and dirty snow there has caused at least 0.5 C of the warming, the team found.

"The global warming debate has focused on carbon dioxide emissions," the scientists note. But their research has "determined that a lesser-known mechanism -- dirty snow -- can explain one-third or more of the Arctic warming primarily attributed to greenhouse gases."

While the international Kyoto strategy to cut greenhouse gases is deemed essential to fight climate change, Mr. Zender's team says targeting sources of dirty snow could pay faster dividends in curbing some of the increase in global temperatures.

"Carbon dioxide lives in the atmosphere for a century, so cutting back on emissions can prevent further warming but does not produce immediate cooling," they said. "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. New snow falls each year, and if it contained fewer impurities, the ground would brighten and temperatures would cool."

The researchers warn that dirty snow not only results in gradual warming but can also create temperature spikes where surface snowmelt is accelerated enough to expose the deeper, darker layers of previous years' snowfall, or even the soil lying below.

A map produced by the team shows a minimal effect across most of mainland North America and extensive areas of significant heating from darkened snow throughout the High Arctic latitudes.

© The Ottawa Citizen 2007

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