From diesel engines to cow-dung cook fires, soot from inefficiently burned fuel has supplanted methane as the second most significant global-warming agent that humans are pumping into the air, according to an exhaustive review of more than a decade's worth of research on black-carbon soot emissions.
Carbon dioxide from burning fossil fuel and from land-use changes remains in the No. 1 spot. But the direct effect of soot on air temperatures, as well as its indirect effect on ice and snow melt and on cloud formation and persistence, are knocking at the door.
Given the uncertainties in the estimates, black-carbon soot may even outpace CO2's warming effect, according to the 232-page study published today in the Journal of Geophysical Research–Atmospheres.
Soot remains in the atmosphere for around seven days – a far shorter time than CO2, which remains in the atmosphere for centuries. This means efforts to reduce soot may apply an important brake to warming in the short term with quick results, the researchers suggest.
Over the long term, however, countries still will have to solve the vexing political and economic challenges of tackling CO2 emissions.
"There's a lot of promise in reducing black carbon" and other relatively short-lived warming agents, such as methane, says Tami Bond, an associate professor of civil and environmental engineering at the University of Illinois at Urbana-Champaign and one of the study's three lead authors. "But there's also a lot of caution."
Properly done, moving to reduce black-carbon soot has immediate climate and public-health benefits, she says. But the uncertainties surrounding some of its climate effects remain large.
Rather than serving as an excuse for inaction, however, the uncertainties should to serve as a guide for research, she adds.
For example, the processes that produce soot also produce not only CO2 emissions but also other particles that can cool the atmosphere, she notes. In other words, emissions from any one source may contain competing influences on global warming.
The fingerprints "of human actions on climate are more complex than just the CO2 story," she says.
Concern over the climate effects of black-carbon soot date back to at least 1971, when interest began to grow in the role small particles, or aerosols, could play in Earth's climate system.
During the past decade or so, however, field studies of soot's effect as a climate warmer typically yielded estimates two to three times higher than the effect seen in climate models, notes Veerabhadran Ramanathan, a professor of atmospheric and climate science at the Scripps Institution of Oceanography in La Jolla, Calif.
With this new study, "we're coming closer to what we think black carbon is doing to the planet's climate," says Dr. Ramanathan, who was not a member of the team that pulled together the new analysis but has been studying the impact of soot on the climate for much of his career.
The soot comes from a mix of sources that varies by region. The study notes that roughly 90 percent of global soot emissions fall into several broad categories: diesel-fueled vehicles; use of coal to heat or cook in homes; small kilns and industrial boilers; burning wood or other biomass for cooking; and open burning of biomass, such as using fires to clear forests for farming.
A study published last year suggested suggested that the use of kerosene lamps was also a significant source. Soot from the lamps contributes about 270 billion tons of soot a year to the atmosphere, representing about 7 percent of warming impact from all energy-related soot, according to the study published last November in the journal Environmental Science and Technology.
The new study estimates that in 2000, humans injected soot into the air at a pace of about 7.5 trillion tons a year globally.
Researchers describe the warming effect in terms of the amount of energy deposited on a patch of Earth one meter square. As of 2005, study notes, the accumulated direct and indirect effects of carbon-dioxide emissions since the dawn of the Industrial Revolution amounted to about 1.56 watts per square meter. Methane was No. 2 at 0.86 watts per square meter. The latest estimate for black-carbon soot now puts it's contribution to the energy warming the planet at 1.1 watts per square meter and perhaps as much as 2.1 watts per square meter.
The effects vary by region as well. Eastern and southern Asia, major sources for the soot, can experience a warming effect from soot 10 times higher than the global average, the researchers estimate. Soot falling on ice and snow in the Arctic has accelerated warming there by speeding the pace at which snow and ice melt during the long hours of summer sun.
Indeed, the mid to high latitudes of the northern hemisphere have seen some of the most pronounced effects of warming from soot, the study notes, because that's where most of the world's population lives.
Scientists also have linked high soot levels to shifts in the regional distribution and intensity of rainfall during Asian monsoons.
Initially, calls from some climate scientists during the past decade to focus first on reducing emissions of shorter-lived but powerful warming agents, such as methane or soot, have been met with polite nods and a resumption of heated debates over CO2 emissions. But the increasing recognition of the adverse health effects of soot, as well as the experience from efforts to control soot, are changing that, some researchers say.
Indeed, "there are clear options" to cut soot emissions, Ramanathan says.
In California, for instance, black-carbon soot emissions fell by half between 1990 to 2008 in response to tighter air-quality regulations affecting diesel emissions, according to a study Ramanathan and colleagues from Scripps and Argonne National Laboratory published in early 2011.
The decline occurred even as "diesel consumption has increased significantly," he adds. The soot pollution "has come down to almost nothing" statewide.
The study on global black-carbon soot released Tuesday notes that focusing initially on diesel sources "appears to offer the most confidence in reducing near-term" warming.
Another opportunity lies in supplying cook stoves that burn biofuels like wood or dung more efficiently, Ramanathan adds.
In the end, putting an immediate focus on short-lived warming agents such as black carbon also cuts into CO2 emissions, notes Brenda Ekwurzel, a climate scientist with the Union of Concerned Scientists in Washington.
"Emitting black carbon also emits carbon dioxide, which has longer-term consequences," she says. "So going after black carbon also helps with the carbon-dioxide emissions associated with incomplete combustion of an ancient tree or critter."