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Scientists Research Impact of Global Warming on Monsoons - 2002-07-27

Rising air temperature isn't the only thing affected by climate change. A new study concludes the Southwest Asian monsoons have gotten stronger over the past 400 years and might continue to intensify as a result of global warming.

Nearly half the world's population in India, Bangladesh, China and other countries of southern Asia is affected by the Southwest Asian monsoons.

The monsoon is caused by a seasonal reversal of winds that blow across the Indian sub-continent from the Arabian Sea and then back again.

The resulting heavy rainfall during the summer months is essential for agriculture.

David Black, an assistant professor at the University of Akron in Ohio, teaches courses on the long-ago behavior of oceans and climate. Mr. Black said a new study, published in the journal Science, for the first time establishes a baseline record for the activity the Southwest Asian monsoon one that reaches back over the past 1,000 years.

"It's a system that affects almost half of the world's population in any given year. And prior to the study we really didn't have any kind of understanding of how that system varied on short time scales, things like decades or centuries," Mr. Black said.

Records of most weather systems are usually much shorter, usually going back no more than about 140 years.

To help them establish the monsoon record, a team American and Indian researchers led by David Anderson of the National Oceanic and Atmospheric Administration in Boulder, Colorado turned to tiny fossilized plankton, called G. bulloides, in the Arabian Sea.

Mr. Anderson said bulloides were brought to the sea by the churning or upwelling of ocean waters caused by the monsoon. "Bulloides, this particular form, is normally found in sub-polar regions. To reconstruct the monsoon, we looked at the abundance of bulloides in the sediment layers. So, stronger winds, more bulloides in the sediments. Weaker winders, less bulloides in the sediments," Mr. Andreson said.

The scientists extracted a sediment core from the bottom of the Arabian Sea and studied the number of bulloides in each two-millimeter slice. The investigators discovered that during periods of known climate cooling, there were fewer bulloides fossils in sediment slices that corresponded to those periods. And there were more fossilized plankton found in sediment slices corresponding to periods of known climate warming, including the past 400 years.

Mr. Anderson said if the trend toward global warming continues, it's possible the Southwest Asian monsoon might become more intense. "There's a general idea here if the warming of the last century was greater at higher latitudes relative to the tropics, that's the type of contrast that would increase the monsoon," he said.

Climate historian David Black said a stronger Southwest Asian monsoon could prove detrimental to the people of the Indian subcontinent. "Every couple of years, you hear about flooding in Bangladesh that kills three-quarters of a million people, which is significant. And potentially, if global warming is correct or is true, and it's a real phenomenon, then this is potentially a problem in the future," Mr. Black said.

The study on the Southwest Asian monsoon is published in the July 25 issue of Science.