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.
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.
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.
Other climate scientists broadly supported Monday’s study, stressing that over longer time periods rising greenhouse gas emissions would over-ride cooling factors.
The study also argues that natural variables become more significant when man-made warming and cooling (sulfur dioxide) trends cancel each other out:
“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.
Indeed, given that scientists now say that the Sun may be headed into a lengthy period of low activity, it will be interesting to see how this plays out.
Global warming advocates will point to the portions of the report stating that warming will re-start once China cleans up its coal industry and the low period of solar activity has ended. Global warming skeptics will point to the end of the warming trend despite increased C02 output as disproving mainstream global warming models. One thing is for sure: climate is not a single equation, but a complicated series of interactions between different forces, and – whatever our beliefs on this issue – we should all strive to make sure that we do more good than harm.