By Alex Kirby
BBC News Online environment correspondent
The effects of soot in changing the climate are more than most scientists acknowledge, two US researchers say.
Soot may be forgotten climate factor
In the Proceedings of the National Academy of Sciences, they say reducing atmospheric soot levels could help to slow global warming relatively simply.
They believe soot is twice as potent as carbon dioxide, a main greenhouse gas, in raising surface air temperatures.
But they say greenhouse gases were the chief cause of last century's global warming, and will probably remain so.
The researchers are Dr James Hansen and Larissa Nazarenko, both of the Goddard Institute for Space Studies, part of the US space agency Nasa, and Columbia University Earth Institute.
In their report, Soot Climate Forcing Via Snow And Ice Albedos, they suggest that trying to reduce the amount of soot produced would be easier than cutting carbon dioxide and other greenhouse gas emissions.
They define soot as "mainly black carbon, the dusty by-product of incomplete combustion of fossil fuels, plants and wood".
Its concentrations vary in time and place, but they are often high over China and India, where coal and organic fuels are used domestically, and over Europe and North America, where the main source is diesel oil.
The authors modelled how soot particles affect climate when they darken snow and ice, causing it to absorb sunlight rather than reflect it.
Clean ice reflects back more solar radiation
It was the results of this modelling that persuaded them that soot is twice as effective as carbon dioxide in raising global surface air temperatures.
The report says high soot emissions may have contributed substantially to global warming over the past century, notably to the growing trend in recent decades for ice, snow and permafrost to melt earlier in the spring.
The authors believe there may be a second effect at work here as well - they suggest soot may cause glaciers, sea ice and ice sheets to melt at lower temperatures than they would otherwise.
This could happen, they say, because the black carbon absorbs more solar energy than clean snow and ice.
Turning back the clock
The authors say: "Environmentalists and climatologists are not sanguine about soot, but they have devoted greater attention to sulphates. We suggest that soot is a more all-round 'bad actor' than has been appreciated."
The Intergovernmental Panel on Climate Change (IPCC) does not include in its evaluations the influence of soot on snow and ice albedo in changing the climate.
Yet the authors say of their estimate for soot's effect on the Arctic and northern hemisphere land areas: "The calculated global warming in an 1880-2000 simulation is about one-quarter of observed global warming."
The effects are widespread
They conclude: "Restoration of snow albedos to something approaching pristine pre-anthropogenic values would have the double benefit of reducing global warming and raising the global temperature threshold at which dangerous anthropogenic interference with climate occurs...
"Technology is within reach that could greatly reduce soot, restoring snow albedo values to near-pristine values, while having multiple other benefits for climate, human health, agricultural productivity, and environmental aesthetics...
"The substantial role inferred for soot in global climate does not alter the fact that greenhouse gases are the primary cause of global warming in the past century and are expected to be the largest climate forcing the rest of this century."
James Hansen is often credited with sparking the current debate on global warming and the extent to which humans are influencing the climate.
He shot to public prominence when he went before a US Senate energy committee in the summer of 1988 to explain the "dust bowl" conditions afflicting much of the American Mid-West at that time.
His comments that "with a high degree of confidence we could associate the warming with the greenhouse effect" received wide coverage and provoked a debate in the scientific community that has now largely come down on his side.
Images courtesy of US National Oceanic and Atmospheric Administration