Soot Plays Big Role in Global Warming

Black carbon, the stuff that gives soot its dirty color, could be the second most important contributor to climate change after carbon dioxide and a key to preventing warming, at least in the short-run, a new study suggests.

Black carbon is a type of aerosol — a small particle suspended in the atmosphere — that is produced in diesel exhaust and when wood, coal or other types of solid fuel are burned.

Like other aerosols, soot particles absorb and scatter the sun's radiation; black carbon is the absorbing component of soot. V. Ramanathan of the Scripps Institution of Oceanography in California and his colleagues reviewed recent studies of black carbon's warming contribution in the March 24 issue of the journal Nature Geoscience.

They also used data from satellites, aircraft and surface instruments to arrive at their own estimate of just how much warming black carbon causes. Their estimates are well above those in the most recent report from the Intergovernmental Panel on Climate Change and suggest that black carbon has a bigger warming effect than previously thought.

Ramanathan suggests that tackling black carbon emissions might be an effective way to prevent some climate warming in the short term, while ways to battle rising carbon dioxide levels are sorted out.

Brown clouds

Aerosols are generally thought to have a cooling effect because many, such as sulfate droplets, reflect solar radiation that hits the Earth back out into space. Aerosols also act as the nuclei of cloud droplets, which also reflect the sun's radiation. So the more aerosols there are in the atmosphere, the more cloud droplets there are, and in this way aerosols can also indirectly have a cooling effect.

Black carbon can mix with other aerosols to form what are called atmospheric brown clouds, which have been observed in regional hotspots over China, India, Southeast Asia, Africa and parts of Central and South America. These brown clouds absorb incoming solar radiation and prevent it from reaching the surface, warming the atmosphere in the process.

Ramanathan's study found that black carbon had a warming effect of about 0.9 watts per meter squared (the average light bulb in a lamp is about 60 watts, so while this effect is a fraction of the heat from a light bulb, that little heater is widespread – there is effectively one on every square meter, about the area of the top of an office desk, across Earth's surface). The most recent IPCC assessment made a lower estimate of the warming effect, between 0.2 and 0.4 watts per meter squared.

Ramanathan and co-author Greg Carmichael of the University of Iowa said these estimates are conservative because they don't take into account the amplification of black carbon's warming effect that occurs when it mixes with other aerosols and because they don't account for warming differences at different altitudes.

The researchers found a significant warming effect at about 6,500 feet (2 kilometers) altitude, where black carbon absorbs not only sunlight, but the light reflected by clouds at lower altitudes. They say this effect is unaccounted for in model-based estimates used in most studies, meaning that black carbon's actual warming contribution has been underestimated. Its warming impact is more than some greenhouse gases, such as methane, Ramanathan contends.

"Overall, the net effect of the black carbon is to warm the climate system globally," Ramanathan said.

But atmospheric scientist Dorothy Koch of NASA's Goddard Institute for Space Studies, who was not affiliated with the study, cautioned that the case wasn't that simple: "There's a lot of uncertainty with black carbon and that makes people kind of reluctant to put it on the table with the greenhouse gases," Koch told LiveScience.

Black carbon doesn't stay in the atmosphere for long, which means it might have a high effect in the short term, but greenhouse gases that stay in the atmosphere longer have a bigger long-term impact. There is also uncertainty as to just how much of the black carbon in the atmosphere comes from human activities, Koch said.

"There's a lot of 'ifs' in there before you can definitely say this is an important thing" in terms of global warming," Koch said.

Ramanathan agreed that there is significant uncertainty in just how much warming black carbon causes globally, but said that there is no doubt that it causes some degree of warming. There is also less uncertainty in the amount regional warming it causes, he said.

Ice melt and public health

Because of its warming impact in the atmosphere and the short time it lingers in the air, Ramanathan thinks that black carbon would make a good target to retard warming in the short term. If steps were taken to switch to cleaner energy sources for cooking fires and heating today to reduce black carbon emissions, "it'll be gone right away," Ramanathan said.

In developing countries such as China and India, many people use solid fuels such as wood, coal or cow dung to cook and heat their homes. Unlike the gas and electric ranges found in North America and Europe, these methods produce a lot of soot because the fuel is burned inefficiently at a low temperature.

The key though is to reduce black carbon without reducing other aerosols that have a cooling effect, at least for the time being, Ramanathan said. He is currently trying to get funding for a study that would provide clean fuel sources for cooking and heating to people in rural India and measure the effects on regional air pollution.

Black carbon has other effects besides adding to global warming. It is taken out of the air by rain and snow and can be deposited on ice surfaces. Scientists think it is contributing to ice melt, especially in the Himalayas, because it makes the ice darker so that it absorbs more radiation.

It also poses a public health risk: Previous studies have estimated that 400,000 deaths of women and children from respiratory diseases each year can be attributed to inhaling the smoke from cooking and heating fires that accumulates in poorly ventilated homes.

"How many more reasons can I give you to get rid of it?" Ramanathan told LiveScience.

Koch agreed.

"There's a lot of good reasons to reduce black carbon," she said, adding though that the impact the reduction would have on global warming is uncertain. She says that reducing black carbon should be investigated because it could be a way to reduce some warming in the short term.

Of course, any efforts to reduce black carbon wouldn't mean that we could ignore carbon dioxide, because it lasts much longer in the atmosphere and its effect will continue if its emissions aren't addressed. "We have to get rid of it," Ramanathan said. "We can't escape from it."

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