Soot is second leading cause of climate change: study

A new U.S. study probing the role of soot emissions in driving global climate change highlights the severe impact that black carbon in the air and dirty snow on the Earth’s surface have in melting Canada’s Arctic sea ice.

During a 10-year investigation detailed in the latest issue of the Journal of Geophysical Research, Stanford University scientist Mark Jacobson isolated the widespread warming effects of soot — the visible residue of burned wood, crops, oil, biomass and other fuels — from the climate impacts caused by greenhouse gases such as carbon dioxide and methane.

He concluded that soot is currently the No. 2 driver of climate change — behind CO2 but ahead of methane — and that curbing emissions of black carbon would produce the fastest, most effective and affordable international response to climate change and the shrinking of the Arctic sea ice.

"Controlling soot may be the only method of significantly slowing Arctic warming within the next two decades," Jacobson, director of Stanford’s atmosphere and energy program, said in a summary of the study. “We have to start taking its effects into account in planning our mitigation efforts, and the sooner we start making changes, the better.”

As of mid-July, the U.S. National Snow and Ice Data Center showed Arctic sea ice is retreating at about the same pace it did in 2007, when an unprecedented summer meltdown sparked alarm among scientists and governments.

Jacobson explained that working to reduce soot’s effects would have a more immediate impact on the Earth’s climate and ice cover because black carbon persists in the environment for a much shorter time than either carbon dioxide or methane. That means reductions in global soot output can quickly curb its heating effects, while the impact of greenhouse gases — which trap heat in the atmosphere — will continue for many decades after CO2- and methane-emission reductions are implemented.

Previous research has identified soot as a significant factor compounding the recent, record-setting Arctic meltdown. Soot-stained snow and ice absorb more of the sun’s heat, reducing the “albedo” effect, which allows light-coloured surfaces to reflect solar energy and keep their cool.

But Jacobson’s research not only accounts for the warming effect of soot as it settles on snow and ice, but also the atmospheric impact as black-carbon particles suspended in the air absorb the sun’s heat and create higher ambient temperatures.

The subsequent loss of sea ice only reinforces warming by replacing frozen ocean with dark stretches of open water, Jacobson notes.

"There is a big concern that if the Arctic melts, it will be a tipping point for the Earth’s climate because the reflective sea ice will be replaced by a much darker, heat absorbing, ocean below," he states. "Once the sea ice is gone, it is really hard to regenerate because there is not an efficient mechanism to cool the ocean down in the short term."

His study concludes the impact of soot on worldwide warming has been seriously underestimated, and notes that reductions in global albedo effect have been greatest “over northern Europe and Canada” — where the ice retreat has been particularly severe in recent years.

Jacobson estimates that eliminating soot from the burning of fossil fuels and solid biofuels alone could reduce Arctic temperatures by up to 1.7 C in the next 15 years — though other factors, without mitigation measures, could continue to drive overall temperatures upward.

In 2007, the U.S. scientists behind another study of soot’s climate impact — also published in the Journal of Geophysical Research — identified Canada as key to any global effort to reduce the effect of black carbon emissions.

One of the co-authors, University of California researcher Charlie Zender, said in 2007 that fallen soot had the effect of “placing tiny toaster ovens into the snow pack."

That study found that about 80 per cent of soot came from man-made sources and 20 per cent from forest fires.

Zender said at the time 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," he stated.

Zender 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,” Zender said, “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."