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Global warming and the Indian monsoon

N. Gopal Raj

"The prediction that 2007 will be the warmest year does not automatically mean more episodes of very heavy rain during the coming monsoon."

GLOBAL WARMING has pushed up average global surface temperatures and the rise has been particularly swift since 1976. The past 10 years, with the exception of 1996, have been the warmest on record. Last week, the U.K. Met Office predicted that 2007 could overtake 1998 to become the warmest year on record.

Average global surface temperatures have risen by about 0.7 degrees Celsius since the start of the 20th century. The United Nations' Intergovernmental Panel on Climate Change has taken the view that average global temperatures could rise by a further 1.4 degrees to 5.8 degrees Celsius by the end of this century.

As a result of global warming, the polar ice caps have shrunk and glaciers that feed many important rivers have retreated.

But the Indian monsoon judged in terms of total nationwide rainfall during the season has remained unaffected. Although there had been a small decrease in India's nationwide monsoon rainfall since 1950, the decline was not statistically significant, according to B.N. Goswami, director of the Indian Institute of Tropical Meteorology in Pune. Moreover, the decrease may well be the result of a multi-decadal cycle and the trend could reverse in the coming years, he added.

However, studies using climate models have indicated that global warming was likely to increase extreme weather events, such as floods and droughts. In a study published last month in the journal Science, Dr. Goswami and scientists at the Indian Institute of Science found evidence that bouts of excessive rain had indeed increased over vast areas of central India in the latter half of the 20th century.

Even if the U.K. Met Office is right in its prediction that 2007 would be the warmest year on record, it is difficult to say how this will impact the monsoon or whether extreme events will become more likely as a result. The monsoon of 1998, currently the warmest year since 1861, saw slightly above average nationwide rainfall while in 2005, the second warmest year, the monsoon ended only whisker below average (but still well within the definition of a `normal' monsoon).

No study has linked changes in the annual average global surface temperature to the outcome of the Indian monsoon, points out K. Krishna Kumar of the Indian Institute of Tropical Meteorology. Moreover, the monsoon was a complex system that was influenced by a multiplicity of factors. However, it was difficult to say how high average global surface temperatures might affect this year's monsoon. On the other hand, if the Eurasian land mass became exceptionally warm in the coming months, it could prove beneficial for the monsoon.

The warming of the oceans and the land could create conditions that were conducive for heavy rain over India, notes V. Venugopal of the Indian Institute of Science, one of the co-authors of the Science paper. However, the actual occurrence of spells of unusually heavy rain was determined by local factors. So a prediction that 2007 would be the warmest year did not automatically mean more episodes of very heavy rain during the coming monsoon, he observed.

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