Dark menace. Black carbon particles in soot have a potent warming effect on the planet.

Cutting Soot Counteracts Warming in California

Eli is a contributing correspondent for Science magazine.

SAN FRANCISCO, CALIFORNIA—New data show that air-pollution regulations in California for trucks running on diesel fuel over 2 decades have cut levels of black carbon, the main component of soot, in half. And while California continues to warm like the rest of the planet, models suggest that cut may also have cut into the warming of the state's climate in an unexpectedly big way, preventing temperatures from climbing even higher.

Soot comes mainly from diesel engines and the burning of wood, charcoal, or other biomass. In recent years, scientists have learned that black carbon, which traps both direct sunlight and heat reflected from the ground, has powerful warming effects: Greenhouse gases methane, ozone in the lower atmosphere, and particulate black carbon together warm the planet as much as carbon dioxide. And as much as half of the loss of snow and ice in the Arctic may be due to black carbon. Breathing black carbon also harms people's health: The United Nations blames soot on 1.5 million deaths per year globally. "The local benefits intersect with the global goods," says atmospheric scientist V. Ramanathan of the Scripps Institution of Oceanography in San Diego, California, who presented the research here at the American Geophysical Union meeting this afternoon.

Publishing their work today in Atmospheric Environment, Ramanathan and colleagues used data from a network of air sensors the state of California maintains to measure how black carbon levels fluctuated from 1988 to 2008. During that period, laws requiring cleaner-burning fuels and catalytic converters led to diesel engines that polluted less. Black carbon levels were slashed in half even though diesel fuel consumption in California rose steadily. "Despite that increase you still got reductions" in black carbon levels, says atmospheric scientist Thomas Kirchstetter of Lawrence Berkeley National Laboratory, who works with Ramanathan on an air pollution measurement project but didn't collaborate on the paper. He called the reduction a "big win" for health regulations.

More unexpected is the calculated effect on the climate. Globally, greenhouse gases "force" warming by trapping between 2 and 4 watts of power from heat given off by the planet per square meter of Earth's surface. Ramanathan's group calculated that on average, the removal of the black carbon prevented an estimated 1.4 watts per square meter of heating, which presumably would have added to the warming that greenhouse gases have caused in California. Ramanathan called that amount "surprising" and said that it shows how powerful a tool controls on black carbon could be. Controlling diesel emissions and making cleaner burning cookstoves, he says, could help limit warming of the planet if scaled up. Because black carbon settles out of the atmosphere in a few weeks, instead of the centuries for which carbon dioxide persists in the air, cutting soot emissions could quickly put the brakes on heating while nations rev up green energy efforts to cut CO2 emissions. "This indeed has major implications for mitigating climate change on a global scale," says Ramanathan. "We have the chance to see a quick global response." Kirchstetter says the climate story is the big message of the paper: "This could provide a big punch."