Sooty black carbon is a far more potent warming influence on the planet's climate than scientists suspected, according to a new study.

The substance, produced by burning fossil fuels and biofuels like wood and dung, is twice as strong a contributor to climate change as earlier research suggested, concluded a group of more than two dozen researchers who spent four years analyzing black carbon's environmental impact.

That makes black carbon the second-largest man-made contributor to climate change, packing about two-thirds the punch of the strongest warmer, carbon dioxide. The finding surprised one of the scientists who led the comprehensive review of black carbon's role in global warming.

"I wasn't expecting it to be that high," said co-lead author Tami Bond, a professor at the University of Illinois, Urbana-Champaign, who studies how energy consumption affects the atmosphere and climate. "I expected it would be higher than people had estimated before, because we knew that in some places the observations indicated a lot more black carbon in the atmosphere than the models were showing."

The new report estimates the warming influence of black carbon is twice as high as the figure given by the U.N. Intergovernmental Panel on Climate Change in its 2007 report.

That could make the substance, which is potent but lingers in the atmosphere for just a few weeks, an attractive target for policymakers seeking to blunt the impacts of man-made climate change.

A 2011 analysis by the U.N. Environment Programme concluded that curbing emissions of black carbon, methane and tropospheric ozone -- all short-lived but potent climate warmers -- could reduce projected warming by just under a degree Fahrenheit by 2070.

A new target for international agreements

"Most international [environmental] agreements ignore black carbon, and this really establishes it as important, that it should not be ignored," said study co-author Mark Jacobson, who studies atmospheric science and energy at Stanford University. "Policy measures should be taken to address it along with greenhouse gases ... not just as a side effect."

Reducing black carbon could also have a significant health benefit, experts said, because tiny particles of black carbon can contribute to or aggravate a host of ailments, including respiratory and cardiac conditions.

Like CO2, black carbon absorbs sunlight and infrared radiation, trapping heat in the atmosphere. When sooty particles fall on snow or ice, their ability to trap heat from the sun helps hasten melting.

Black carbon particles can also promote the formation of clouds that have a cooling or warming influence, depending on their structure and position in the atmosphere.

Complicating the picture, some sources of black carbon also produce substances that can add to its warming impact or counteract it with a cooling effect.

That doesn't mean that cutting black carbon emissions to reduce climate change is impossible, said co-author David Fahey, a research physicist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory. But "it does mean that you have to be very thoughtful about what you're doing," Fahey said.

Diesel engines and cookstoves

The new analysis says the clearest climate benefit would come from reducing emissions from diesel engines, with less certain but likely significant benefits from reducing use of some fuels in residential settings.

That includes cookstoves that burn wood and dung, a potent source of black carbon in emissions in some developing nations, including India.

The cuts could be accomplished with existing technology, said V. Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography. His research shows California's black carbon output fell by half over the last 25 years, in large part due to use of particulate traps on diesel engines.

"What is clear is that black carbon and [substances emitted along with it] significantly change the fundamental forces that drive our weather and climate," Ramanathan said. "It's something we have added. And it's also hurting public health in a major way."