Study raises hope of combating global warming by reducing soot

Between 1989 and 2008, clean-air rules in California virtually halved the concentrations of black-carbon soot in the state's skies, in effect reducing the state's carbon footprint by the equivalent of cutting carbon dioxide emissions by 21 million tons a year, according to new analysis.

That would represent about 5 percent of the state's CO2 emissions in 2009, according to the report. Though the data have yet to be fed into global climate models to see if California's results can be replicated elsewhere, they bolster the hope that focusing on black-carbon soot could be an effective way to begin to address global warming.

During the past decade, atmospheric scientists have focused increasing attention on black-carbon soot, the tiny particles found in Diesel exhaust as well as the emissions from wood and dung fires. The soot, which absorbs sunlight and re-radiates it as heat, has edged out methane as the second most-abundant greenhouse-agent released into the atmosphere by human activities. But unlike carbon dioxide or methane, soot takes only days or weeks to settle out of the atmosphere, compared with decades to centuries for methane and CO2.

This has led some scientists to conclude that by focusing near-term emissions reductions on global soot, in addition to other shorter-lived warming agents such as methane and ozone, humanity could slow the expected rate of global warming during the next several decades.

Last year, for instance, a study by NASA's Goddard Institute for Space Studies in New York suggested that such controls could reduce warming expected by 2050 by about 0.5 degrees Celsius, or about 1 degree Fahrenheit. That is a significant amount, given the international interest in ensuring that global temperatures don't increase by more than 2 degrees C by 2100, compared with preindustrial levels.

The study released Thursday marks the first time researchers have been able to measure the climate-related effects of long-term reductions in black-carbon soot over a region, the research team says. Until now, scientists have had to rely on limited field studies and computer simulations to estimate such effects.

Estimates of how large an effect soot controls could have on global warming will have to wait until models chew over the new data. But the report's lead author is willing to hazard an educated guess.

"I'm speculating that it could cut down the rate of warming by 15 percent" over the near term, says Veerabhadran Ramanathan, a researcher at the Scripps Institution of Oceanography in La Jolla, Calif. He worked with researchers from the University of California at San Diego and from national laboratories in California and Washington State.

Others are a bit more cautious, noting that the effects could vary widely, based on regional differences in atmospheric circulation patterns as well as population density and predominant sources of soot.

Still, the study represents a landmark in efforts to understand the impact of emission-control efforts, according to Yan Feng, a researcher at the Argonne National Laboratory in Argonne, Ill., who helped craft the proposal for the study three years ago but did not take part in the execution of the study.

The study is noteworthy for its comprehensiveness, combining long-term measurements from the ground and from satellites with computer modeling, she notes.

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.