22:00 22 December 03
NewScientist.com news service
Soot particles may be twice as bad as the greenhouse gas carbon dioxide in contributing to global warming, suggests a new study.
Grains of soot deposited in snow have also caused about one-quarter of the observed rise in global surface temperature since 1880, suggests the model by James Hansen and Larissa Nazarenko. The pair examined how soot particles affect the atmosphere when they darken snow and ice.
Darkened snow traps more solar energy than pristine snow. The effect could explain why sea ice and glaciers are melting faster than expected from the increase in surface temperature alone, says Hansen of the NASA Goddard Institute for Space Studies in New York.
"The effect of soot on snow is unambiguous," he told New Scientist. "It causes a strong warming effect."
The findings could be good news on climate as soot emissions may be easier to cut than carbon dioxide. Hansen says technology exists to more cleanly burn fossil fuels without releasing large quantities of soot. The "payback" in terms of climate would be faster by cutting soot than carbon dioxide gas emissions.
But Hansen cautions: "It does not change the need to slow down the growth rate of carbon dioxide and eventually stabilize the atmospheric amount."
Incident light
Soot's effects on the atmosphere are complex and not well understood. The emissions are a combination of black carbon and organic compounds which have different effects on surface temperatures. Black carbon causes warming, but the organic compounds cause cooling. The effects on ice and snow are simple in comparison, but had largely been overlooked.
Fresh snow reflects more than 90 percent of incident light - both the organic and black-carbon components of soot increase its absorption. Local soot concentrations in snow vary widely, but Hansen estimates that it reduces light reflection by 1.5 percent in the Arctic and by 3 percent over land in the Northern hemisphere.
The extra absorbed energy helps melt snow and ice. This creates positive feedback - and speeds melting - because wet snow absorbs more light than dry snow, and liquid water absorbs about 90 percent of the incident light.
This may also explain another climate puzzle - observations of earlier springs and melting ice sheets. "There is no way to account for the rapid retreat of ice globally based only on global warming," Hansen says. The link to melting ice is critical because rising sea level is a key danger posed by global warming.
Hansen stresses greenhouse gases remain the main cause of global warming. But soot does not stay as long in the atmosphere as carbon dioxide, and emissions may be easier to control, he says.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.2237157100 -- ----------------------------------------------
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