LONDON Smoke belching from Asia's rapidly growing economies is largely responsible for a halt in global warming in the decade after 1998 because of sulphur's cooling effect, even though greenhouse gas emissions soared, a U.S. study said on Monday.
The paper raised the prospect of more rapid, pent-up climate change when emerging economies eventually crack down on pollution.
World temperatures did not rise from 1998 to 2008, while manmade emissions of carbon dioxide from burning fossil fuel grew by nearly a third, various data show.
The researchers from Boston and Harvard Universities and Finland's University of Turku said pollution, and specifically sulphur emissions, from coal-fueled growth in Asia was responsible for the cooling effect.
Sulphur allows water drops or aerosols to form, creating hazy clouds which reflect sunlight back into space.
"Anthropogenic activities that warm and cool the planet largely cancel after 1998, which allows natural variables to play a more significant role," the paper said.
Natural cooling effects included a declining solar cycle after 2002, meaning the sun's output fell.
The study said that the halt in warming had fueled doubts about anthropogenic climate change, where scientists say manmade greenhouse gas emissions are heating the Earth.
"It has been unclear why global surface temperatures did not rise between 1998 and 2008," said the study published in the Proceedings of the National Academy of Sciences of the United States.
A peak in temperatures in 1998 coincided with a strong El Nino weather event, a natural shift which brings warm waters to the surface of the Pacific Ocean every few years.
Subsequent years have still included nine of the top 10 hottest years on record, while the U.N. World Meteorological Organization said 2010 was tied for the record.
A U.N. panel of climate scientists said in 2007 that it was 90 percent certain that humankind was causing global warming.
Sulphur aerosols may remain in the atmosphere for several years, meaning their cooling effect will gradually abate once smokestack industries clean up.
The study echoed a similar explanation for reduced warming between the 1940s and 1970s, blamed on sulphur emissions before Western economies cleaned up largely to combat acid rain.
"The post 1970 period of warming, which constitutes a significant portion of the increase in global surface temperature since the mid 20th century, is driven by efforts to reduce air pollution," it said.
Sulphur emissions are linked to coal consumption which in China grew more than 100 percent in the decade to 2008, or nearly three times the rate of the previous 10 years, according to data from the energy firm BP.
Other climate scientists broadly supported Monday's study, stressing that over longer time periods rising greenhouse gas emissions would over-ride cooling factors.
"Long term warming will continue unless emissions are reduced," said Peter Stott, head of climate monitoring at Britain's Met Office.