WASHINGTON, D.C.—Carbon dioxide is the elephant in the room for any discussion of how to stem global warming. But a new report suggests that tackling emissions of two other short-lasting pollutants—methane and the black component of soot—could slow expected warming by a full 0.5˚C beyond what targeting CO2 alone could accomplish by 2070.
The report, which will be discussed here Sunday at the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW), includes a lot of uncertainty. But it fits with what scientists have learned about these pollutants in recent years. Methane is a more potent warming agent than CO2, although its duration in the atmosphere is measured in decades rather than centuries. Methane also contributes to asthma-causing pollution. Black carbon, the product of burning wood or other carbon-based fuels, heats the air directly, accelerates the melting of any snow it lands on, and creates so-called brown clouds that warm the sky. Although harmful to the hundreds of millions who breathe it each day, black carbon settles out of the atmosphere in a few weeks. So reducing emissions of it would have a nearly immediate impact on global temperatures.
In the study, an international team of researchers first examined 2000 different pollution-control measures for the two pollutants and chose 16. The measures include stemming methane leaks from coal mines or landfills and stopping black carbon pollution from primitive stoves and diesel construction equipment. The scientists then ran two separate climate models to learn how the rate of global warming might change if the 16 measures were deployed, with and without carbon dioxide controls.
Under control runs without any pollution controls, the global temperature rose by 2.5˚C—plus or minus about 0.7˚C—by 2070. Stemming CO2 (to an atmospheric level of 450 ppm) reduced that warming by about 0.5°C. Deploying the 16 controls reduced the warming by an additional 0.5˚C, again with big error bars. (Since 1850, scientists estimate Earth's temperatures have risen by 0.7˚C.)
"I was surprised when we looked through 2000 control measures and it turned out that doing just 16 of them could make a big dent in climate," said Drew Shindell, a researcher at NASA Goddard Institute for Space Studies in New York City who co-coordinated an international writing team on the report.
"It's the first time a group has picked out actual measures that might improve forcing by short-lived pollutants," says pollution-control expert Tami Bond of the University of Illinois, Urbana-Champaign, who was not an author on the study. "Previously, the analyses have focused only on one pollutant, or on entire economic sectors, but this is not how policy is done."
David Fahey, an atmospheric scientist at the National Oceanic and Atmospheric Administration in Boulder, Colorado, said that the researchers will need to do additional analyses to reduce the "significant uncertainties associated with the role of black carbon in the climate." But he said that the promise of reducing emissions and improving human health made the analysis an "important" contribution to the field.
The report did not undergo formal peer review, but Bond and Fahey were among hundreds of outside scientists who submitted comments before the study was released. On Wednesday of next week, environment ministers from around the world meeting at the United Nations will receive the preliminary version of the full report and begin to discuss how to implement some of its findings, said a spokesperson for the United Nations Environment Programme, which sponsored the report along with the World Meteorological Organization.