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.