Study raises hope of combating global warming by reducing soot
Black-carbon soot is the No. 2 global warming agent released into the atmosphere by human activities. A landmark study in California shows some success in controlling it.
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The study is noteworthy for its comprehensiveness, combining long-term measurements from the ground and from satellites with computer modeling, she notes.Skip to next paragraph
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Funded by the California Air Resources Board, it focuses on emissions from Diesel engines, which have been a focus of California air-pollution control efforts. While the study highlights the reductions in black-carbon soot concentrations since 1989, it also points out that concentrations have fallen some 90 percent since the 1960s, even as Diesel-fuel use has increased fivefold.
It addition, it notes that the Diesel story in California is nuanced. Diesel emissions include not only black-carbon soot, which can heat the surrounding air, but also aerosol particles such as sulfates, which reflect sunlight and can cool the atmosphere.
As more efficient Diesel engines came into wider use, and other emission-reduction measures were implemented, emissions of black-carbon soot declined but emissions of cooling aerosols remained the same. The continued presence of the cooling aerosols was crucial. If both types of aerosols declined, the net effect likely would be warming.
This finally demonstrates in the real world what climate models have suggested – that reducing black-carbon soot would have a net cooling effect, says Dr. Ramanathan.
"This is geoengineering made in heaven," he says, referring to proposals to moderate global warming by injecting cooling aerosols into the atmosphere.
The team holds that curbing emissions of black-carbon soot is critical for limiting global warming over the next 50 years. While black-carbon soot has natural sources, such as wildfires, these are sporadic compared with the constant replenishment from human activity.
Efforts to rein in global warming beyond 50 years still rely on reducing CO2 emissions. Because of CO2's long residence time in the atmosphere, those reductions need to start soon, researchers say.
The team also found that so-called brown carbon soot, most common as the smoldering white smoke following wildfires or when crop residue burns, is not a cooling agent, as previously believed. The team discovered that brown-carbon soot is a warming agent. In the team's study, this brown-carbon soot accounted for about 15 to 25 percent of the atmospheric heating attributed to soot. Current climate models don't account for this, suggesting that models underestimate the warming effects of brown-carbon soot in regions where biomass burning is common.
Overall, the team estimates that the decline in black-carbon soot over California likely cooled the state by about 0.1 degrees C. But Ramanathan puts little stock in the number because it's so small as to get lost in the noise of natural variations in the state's climate. And it comes from only one model. The figure would have more statistical significance if it resulted from multiple runs of several models.
Still, the results are encouraging enough to provide added fodder for a meeting scheduled for October between experts from California and India to what, if anything, in California's approach can be applied to India's considerable black-carbon-soot problem.