Soot causes twice as much global warming as previously thought

Soot is the second largest man made contributor to global warming after CO2 and its influence on climate has been greatly underestimated, according to the first comprehensive analysis of the problem.

The landmark study, co-led by the University of Leeds, says the direct warming effect of soot, could be about twice previous estimates.

Black carbon is believed to have a warming effect of about 1.1 Watts per square meter, approximately two thirds of that of the largest man made contributor to global warming, carbon dioxide, and greater than that of methane.

The figures indicate that there may be a greater potential to curb warming by reducing soot emissions than previously thought, according to the study published in the Journal of Geophysical Research Atmospheres.

"There are exciting opportunities to cool climate by cutting soot emissions, but it is not straightforward. Reducing emissions from diesel engines and domestic wood and coal fires is a no-brainer, as there are tandem health and climate benefits," study co-author Piers Forster said.

"If we did everything we could to reduce these emissions, we could buy ourselves up to half a degree less warming or a couple of decades of respite," Forster said in a statement.

The 232-page report, which took four years to complete, is the first comprehensive and quantitative analysis of the role of black carbon in the climate system.

Its best estimate of the direct climate influence from black carbon is about a factor of two higher than most previous work, including the estimates in the last IPCC Assessment in 2007.

"This study confirms and goes beyond other research that suggested black carbon has a strong warming effect on climate, just ahead of methane," said researcher David Fahey from the US National Oceanic and Atmospheric Administration (NOAA).

Huge quantities of man made soot enter the atmosphere every year, with about 7.5 million tonnes emitted in 2000 alone. These emissions come from a variety of sources.

The largest global cause is the burning of forest and savannah grasslands, but diesel engines account for about 70 per cent of emissions from Europe, North America and Latin America.

Residential fires contribute 60 to 80 per cent of Asian and African emissions and coal fires are also a significant source of soot in China, some Eastern European countries and the former USSR.