Tefnut's Environmental and Drought News Article
Tuesday, 5 July 2011 Gerard Wynn Reuters
Researchers believe coal-fuelled growth in Asia was responsible for the cooling effect.
Smoke from Asia 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 US study claims.
The paper, which appears this week in the Proceedings of the National Academy of Sciences, raises the prospect of more rapid, pent-up climate change when emerging economies eventually crack down on pollution.
Global temperatures did not rise significantly between 1998 and 2008, despite human-induced emissions of carbon dioxide growing by nearly a third, various data show.
The researchers from Boston and Harvard Universities and Finland's University of Turku say pollution, and specifically sulphur emissions, from coal-fuelled 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's authors write.
They claim natural cooling effects include a declining solar cycle after 2002, meaning the Sun's output fell.
The researchers believe this halt in warming has fuelled doubts about anthropogenic climate change, which implies human-induced greenhouse gas emissions are heating the Earth.
"It has been unclear why global surface temperatures did not rise between 1998 and 2008," they write.
A peak in temperatures in 1998 coincided with a strong El Niño 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 UN World Meteorological Organization says 2010 was tied for the record.
A UN panel of climate scientists said in 2007 that it was 90 per cent certain that humankind was causing global warming.
History repeating
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," according to the researchers.
Sulphur emissions are linked to coal consumption which in China grew more than 100 per cent 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 support the 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," says Peter Stott, head of climate monitoring at Britain's Met Office.
Source: ABC Science News
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