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Soot a Culprit in Glacial Melt

New research confirms theories about how so-called black carbon contributes to climate change

Mendenhall Glacier in Alaska

Soot from fossil fuels is causing snow packs, glaciers and ice sheets to absorb heat and melt more quickly, Lawrence Berkeley National Laboratory researchers have confirmed.

Soot, also called black carbon, is produced when fossil fuels are burned inefficiently. Major sources include coal power plants, cooking stoves and vehicles, particularly those powered by diesel. Forest fires also produce soot. Because of its dark color, soot can absorb heat and cause the air to warm. It also accumulates on top of — and inside — the planet’s dwindling frozen water supplies.

In recent years, scientists have begun to understand soot’s powerful influence on the climate, not to mention its role in causing certain cancers and asthma. Carbon dioxide is responsible for as much as 40 percent of global warming, scientists estimated in 2009. Soot is responsible for approximately 18 percent.

A report published this week by Berkeley Lab researchers corroborated widely used theoretical models of the negative impacts that soot can have on snow and ice, causing them to melt and hastening the rise of sea levels.

Cutting back on soot emissions could offer a quick fix for some of the planet’s climate woes, because once released, the particles quickly break down or fall out of the atmosphere — often within a week. Carbon dioxide, by contrast, can persist in the atmosphere for a century.

Recognizing the benefits of keeping black carbon and other fast-acting but short-lived greenhouse gases, such as methane, out of the atmosphere, Secretary of State Hillary Clinton last month announced that the United States, Bangladesh, Canada, Mexico, Sweden and Ghana had struck an agreement to work together to curb emissions.

“We know that in the principal effort necessary to reduce the effects of carbon dioxide, the world has not yet done enough,” Clinton said during the Feb. 16 announcement in Washington, D.C. “So when we discover effective and affordable ways to reduce global warming — not just a little, but by a lot — it is a call to action.”

Unlike carbon dioxide, which spreads around the globe, black carbon mainly affects local climates where it is produced or where it has been carried by winds and jet streams, said Surabi Menon, the San Francisco-based director of research at the nonprofit ClimateWorks Foundation, which seeks to help reign in climate-changing emissions through partnerships around the world.

“It traps the heat,” Menon said. “It doesn’t allow as much sunlight to reach the surface because it’s absorbing that heat.”

When soot mixes with falling snow, it can darken layers of snow and ice on the ground, reducing the reflection of sunshine back into space, trapping heat on the earth's surface and making the snow melt faster.

A group of scientists reported in January in the journal Nature Geoscience that carbon found on Alaskan glaciers was in fact soot produced by the burning of fossil fuels — not by a natural process, as some had suspected. Peter Hernes, a scientist at the University of California, Davis, who was involved with the research, said a lack of wood cells in the carbon was a clue that it was not from natural sources.

In preparing an upcoming climate assessment, the Intergovernmental Panel on Climate Change has been relying on theoretical models about the relationship between soot levels and the amount of sunlight reflected by snow and ice.

On Sunday, Berkeley Lab researchers announced in the journal Nature Climate Change that four years' worth of research has corroborated those models.

“The more black carbon, or soot, that we put into snow, the lower the reflectance of the snow,” said Thomas Kirchstetter, an environmental and energy scientist at the Berkeley Lab.

As less sun is reflected away from the snow, more heat is absorbed and the melting process speeds up, according to Kirchstetter. That leads to global warming and rising seas.

Kirchstetter and a colleague, Odelle Hadley, blended soot at varying concentrations with water droplets at freezing temperatures to study just how much black carbon affects the absorption of sunshine by snow.

At concentrations of 100 parts per billion, which represents typical levels of pollution in places such as the Himalayas and the Arctic, the tainted snow absorbed 5 to 10 percent more sunshine than pure white snow, the researchers discovered. That might not sound like much, but the researchers point out that it equates to multiple times more heat than that absorbed by pure white powder.

At milder concentrations of 10 to 20 parts per billion, believed to be slightly higher than pollution levels in the Sierra Nevada, where snowmelt flows to reservoirs and the Pacific Ocean each year, about 1 percent more heat is absorbed than by pure white snow.

Kirchstetter said the laboratory findings “compared reasonably well” with the theoretical models and estimates that scientists and the climate change panel have relied on.

“Up until now, there have been very few checks on the estimates,” Kirchstetter said. “This is the most extensive verification to date.”

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