Black smogs are reversing the effects of global warming in India, a new study concludes. Pollution across the Indian subcontinent is shielding the sun and depressing the winter temperature by about 0.3掳C.
The scientists say the smog particles are cooling the land by absorbing solar radiation before it reaches the surface. But the bad news is that the particles could be redistributing the heat round the globe 鈥� warming adjacent regions of Asia and beyond.
Smogs are a feature of modern India as the country burns ever more coal and oil in power stations, factories and vehicle engines. 鈥淎ir pollution has more than doubled in the South Asian region during the last 20 years,鈥� says one of the team, Veerabhadi Ramanathan of the Scripps Institution of Oceanography in San Diego, California.
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Black carbon and sulphur dioxide accumulate in the air during the winter months, creating urban smogs and a 鈥渂rown haze鈥� extending out across the Indian Ocean. Winter is when most fuel is burned, but also when there is no rain. The daily rains of the summer monsoon clean the air.
In the shade
The new work is part of an ambitious international investigation of the effect of smogs in southern Asia called the Indian Ocean Experiment. In 2001, Nobel prize-winning atmospheric chemist Paul Crutzen warned that the smogs were blotting out up to 15 per cent of sunlight and threatened 鈥渧ery major consequences鈥� for the region鈥檚 climate.
Now Ramanathan and R Krishnan of the Institute of Tropical Meteorology in Pune, India, have examined climate records going back more than a century. They show that India鈥檚 summer temperatures continue to follow global trends.
But winter temperatures have begun to cool in the past 30 years, coinciding with the smog build-up. Winters are now 0.3掳C cooler than they would be without the smog, they say.
The authors warn that the soot particles that absorb the solar radiation over India may travel outside the region, adding to global warming elsewhere. 鈥淭his is a matter of serious concern to the environment, mankind and the ecosystem at large,鈥� says Krishnan.
鈥淎t present these effects are not generally accounted for in climate model predictions,鈥� says Jeffrey Kiehl of the National Center for Atmospheric Research in Boulder, Colorado. 鈥淏ut we will need to include the absorbing effects of aerosols in future model predictions.鈥�
Journal reference: Geophysical Research Letters (DOI 10.1029/2002GL014687)
