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Clean-air rules on diesel emissions may be helping California keep its cool

Soot, which comes from diesel engines and the burning of biomass, accounts for 1.5 million deaths per year globally.
Soot, which comes from diesel engines and the burning of biomass, accounts for 1.5 million deaths per year globally. (Alamy)
Monday, December 20, 2010; 7:00 PM

New data show that two decades of air-pollution regulations in California for trucks running on diesel fuel have cut levels of black carbon, the main component of soot, in half. And models suggest that the soot reduction may also have cut into the warming of the state's climate in an unexpectedly big way.

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: 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.

Research published in the journal Atmospheric Environment by atmospheric scientist V. Ramanathan of the Scripps Institution of Oceanography and colleagues analyzed how black carbon levels in California 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 the state rose steadily.

More unexpected is the effect on the climate. Globally, greenhouse gases produce warming by trapping two to four 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 they say presumably would have added to greenhouse-gas warming 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 cook stoves, 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 warming while nations rev up efforts to cut CO2 emissions.

- Eli Kintisch

This article was produced by ScienceNOW, the daily online news service of the journal Science, and can be read online at

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