Global warming made worse by soot, study says

Soot pouring out of trucks and chimneys could be responsible for a quarter of observed global warming, researchers said yesterday.

The impact of soot on climate is much larger than previously thought, a new study has shown. Soot, mainly composed of black carbon, is the dusty by-product of burning fossil fuels and vegetation.

The Nasa scientists James Hansen and Larissa Nazarenko, from the Goddard Institute for Space Studies in New York, have found that soot is twice as effective as carbon dioxide at raising global surface air temperature.

Levels are highest over China and India, where coal and biofuels are burnt for domestic use, and Europe and North America, where the major source is diesel fuel. Soot warms the Earth by darkening snow and ice, causing it to absorb rather than reflect sunlight. The scientists said high soot emissions might be substantially contributing to earlier spring melts.

It could also be helping glaciers, sea ice and ice sheets to melt at lower temperatures.

Drs Nazarenko and Hansen wrote in the journal Proceedings of the National Academy of Sciences: "This indirect soot forcing may have contributed to global warming of the past century, including the trend toward early springs in the northern hemispheres, thinning arctic sea ice, and melting land ice and permafrost."

They suggested reducing soot emissions - a move that has yet to be considered by the international community.