Curbing soot emissions could also be a life saver, said the study, published Sunday in the British journal Nature.

Each year, more than 400,000 deaths among women and children in India alone, and 1.6 million worldwide, are attributed to smoke inhalation during indoor cooking using biofuels such as wood or dung, one of the primary sources of black carbon, according to the World Health Organisation.

Reviewing dozens of recent scientific studies, two researchers in the United States calculated that black carbon is the second largest contributor to global warming after carbon dioxide, a byproduct of burning fossil fuels.

In addition, the eight million metric tonnes of soot released into the atmosphere every year have created a number of "hot spots" around the world, contributing significantly to rising temperatures.

The plains of south Asia along the Ganges River and continental east Asia are both such hotspots, in part because up to 35 percent of global black carbon output comes from China and India.

Emissions in China alone doubled between 2000 and 2006, according to the study, published in 2006.

Fine black soot settling on snow and ice -- and thus trapping more of the Sun's radiative force -- have also accelerated the melting of glaciers in the Himalayas and ice cover in the Arctic, two regions that have been hit especially hard by climate change in recent decades.

"A major focus on decreasing black carbon emissions offers an opportunity to mitigate the effects of global warming trends in the short term," the authors conclude.

While the presence of black carbon, sometimes in the form of great plumes several kilometres high called atmospheric brown clouds, has been known to scientists for some time, their impact on warming has been hard to assess.

Direct measurement requires multiple aircraft flying over the same domain at different altitudes for an extensive period at the same time.

Significantly cutting back on black carbon emissions is not only possible, but would yield rapid benefits, say the authors, Veerabhadran Ramanathan of the Scripps Institute in San Diego, California, and Greg Carmichael of the University of Iowa.

Forty percent of soot comes from the same sources as greenhouses gases, notably the burning of coal and oil, and will only be reduced as quickly or slowly as economies become less carbon intensive.

But the remaining 60 percent of black carbon in the atmosphere comes from the more easily altered practices of burning biofuels and forests, the authors say.

Also, cutting back soot output would have an almost immediate effect.

Unlike carbon dioxide, which lingers in the atmosphere for 100 years after it is released, black carbon has an atmospheric life cycle of approximately one week.

"Providing alternative energy-efficient and smoke-free cookers, and introducing transferring technology for reducing soot emissions from coal combustion in small industries could have major impacts" on reducing soot's role in global warming, they conclude.

Such measures would result in a 70-80 percent reduction in heating caused by black carbon in south Asia, and a 20-40 percent cut in China, according to the study.

The authors caution, however, that soot reduction can only help delay unprecedented climate change, which is due primarily to CO2 emissions.

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