Black carbon, a key component of soot, plays a much larger role in global warming than previously estimated, scientists have found.
They cite black carbon as the second largest contributor to global warming after carbon dioxide.
"We've got one more thing to worry about," says Veerabhadran Ramanathan, professor at the Scripps Institution of Oceanography at the US-based University of California at San Diego and co-author of the study.
"We thought it was mainly carbon dioxide emissions [contributing to global warming]. Now we know that there is another major culprit. Our finding adds to the complexity of the issue."
Black carbon is produced from the incomplete combustion of wood, biomass such as crop residues and fossil fuels.
Ramanathan, together with Greg Carmichael at the University of Iowa, United States, combined satellite data with aircraft and surface observations to paint a global picture of the warming effects of black carbon emissions.
Black carbon absorbs solar radiation, warming the atmosphere. The researchers calculated this warming effect as 0.9 watts per square metre, second to carbon dioxide's value of 1.66 per square metre.
But this is much higher than the 0.2–0.4 watts per square metre estimated by the UN Intergovernmental Panel on Climate Change.
Ramanathan told SciDev.Net that reducing black carbon emissions is a real possibility as the atmospheric lifespan of black carbon is short, "in the order of ten days or less". As a result, the benefits will be reaped quickly.
"To cut down carbon dioxideemissions, we are talking about trillions of dollars," he says. "My feeling is black carbon is going to be 10–100 times cheaper. Black carbon emissions in Germany and the United Kingdom have come down by a factor of 2–3 in the last 30–40 years. This shows that we know how to do it."
Ramanathan is undertaking a study in India to look at the cost-effectiveness of biogas plants and encouraging the use of solar cookers instead of burning biomass in villages.
He is currently working with the Chicago Climate Exchange Commission to provide the villagers with carbon credits to sustain these new cooking methods.
A report building on Ramanathan's and Carmichael's analysis, assessing the impact of atmospheric brown clouds — formed from the mixing of black carbon with other aerosols — on China and India will be released by the UN later this year.
The study was published in Nature Geoscience.
Nature Geoscience 1, 221 (2008)