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Vinson Kurian

THIRUVANANTHAPURAM, July 26

EVEN as India ponders on how to come to grips with what for all practical purposes looks like a case of a failed monsoon, researchers have come out with evidence suggesting that the Asian southwest monsoon has in fact been gaining in intensity over a period of centuries, and will continue to do so in future.

In a paper which will be published in the forthcoming issue of the journal Science, Dr David Anderson of the University of Colarado, Dr Jonathan Overpeck of the University of Arizona and Dr Anil K. Gupta, an associate professor at the Indian Institute of Technology Kharagpur, suggest that the Asian monsoon, which affects the livelihood of millions of people, has increased in intensity during the last four centuries, perhaps as a result of warming in the Northern Hemisphere, and will tail the trend in the times to come.

Dr Anderson says that sediment cores from the Arabian Sea near Oman indicate the wind strength of the monsoon has increased as the Northern Hemisphere has warmed. Dr Anderson and his colleagues hypothesise the southwest monsoon strength will continue to increase during the coming century as greenhouse gases rise and northern latitudes continue to warm.

"Our goal is to understand the natural variability of the climate in hopes of determining whether we are influencing the climate of the planet,'' says Dr Anderson, a research associate at CU-Boulder's Institute of Arctic and Alpine Research. "Although there are several scenarios as to why the monsoon is increasing, what we are seeing is consistent with the effects of global warming.''

Roughly 65 per cent of the world's people live in monsoon regions, says Prof Peter Webster of CU-Boulder's Program for Oceanic and Atmospheric Sciences, who was not involved in the study. Because people in monsoon regions are largely dependent on agriculture, too little rain during weak monsoons can cause significant crop failures while too much rain during monsoons can cause flooding, crop damage and population displacement.

A monsoon is the seasonal reversal of wind fields that causes dramatic increase in moisture during the warm season. The two key ingredients to a monsoon are a hot land mass and a cooler ocean.

In the case of the Asian southwest monsoon, the key sources are the Tibetan Plateau which Dr Anderson likened to an "empty frying pan on the stove'' — and the Indian Ocean to the south.

The winds blowing along the coast of the Arabian Sea during the monsoon cause upwelling of the water, which dramatically increases the abundance of a tiny marine organism known as G. bulloides.

The stronger the monsoon, the more G. bulloides are produced. Over time, a unique fossil record of the species has built up on the sea floor.

The scientists sampled sea sediment cores containing the fossils in 2-millimeter layers from along the coast of Oman and dated them with radiocarbon techniques. The results have provided valuable data on wind speeds during the summer southwest Asian monsoon going back more than 1,000 years.

"We have seen a substantial century-to-century variation on these fossil layers,'' says Dr Anderson. From 1200 to 1400, for example, the monsoon was strong.

The sediments showed the monsoons to be weaker during the 1600s, when a period of cooling in Europe known as the Little Ice Age is believed to have occurred.

"But the monsoon wind strength has increased during the past four centuries as the Northern Hemisphere has warmed,'' says Dr Anderson.

There is an apparent link between decreased Eurasian snow cover and increasing monsoon strength. Alternately, climate-forcing events such as greenhouse gases, volcanic aerosols and solar output may be increasing the strength of the monsoon.

"Either interpretation is consistent with the hypothesis that the south-western monsoon strength will increase during the coming century as greenhouse gas concentrations continue to rise and northern latitudes continue to warm,'' the researchers wrote in Science.

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