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Monsoon Intensity Increasing as Earth Warms

Dumping untold gallons of rain on 65 percent of the earth¿s population each year, the Asian southwest monsoon is one of the most important climate systems in the world. The seasonal storms arise from a disparity in temperature between the hot Tibetan plateau and the cooler Indian Ocean, and they impact the food supplies and livelihoods of millions of people: too little rain may lead to famine, and too much can cause flooding, crop damage and death. Scientists have wondered how changes in the earth¿s temperature affect monsoon intensity. Now findings published today in the journal Science may provide some answers. Using historical abundances of a fossilized marine microorganism called Globigerina bulloides, researchers have pieced together a 1,000-year record of monsoon intensity.

G. bulloides thrive in disturbed waters. During monsoon season, high winds produce oceanic upwelling, which in turn spurs the growth and reproduction of the aquatic creature. Bigger storms produce faster winds, and consequently more G. bulloides. Over time, an extensive fossil record of G. bulloides abundance at different points in history has accumulated on the ocean floor. David Anderson of the University of Colorado at Boulder and his colleagues extracted cores of this fossil-filled sediment from the floor of the Arabian Sea to reconstruct monsoon intensity over the past 1,000 years. The team found that storm force has indeed increased over time, especially during the past century. The scientists then compared their findings to global temperatures over the same time period. "We have seen a substantial century-to-century variation on these fossil layers," Anderson reports, "but the monsoon wind strength has increased during the past four centuries as the Northern Hemisphere has warmed."

The researchers say that either the increase in snow cover over Eurasia or the escalation in greenhouse gas concentration, volcanic aerosols and solar output are to blame. All of these phenomena generate a greater contrast between land and ocean temperatures, the cause of the monsoon. "Either interpretation is consistent with the hypothesis that the southwestern monsoon strength will increase during the coming century as the greenhouse gas concentrations continue to rise and northern latitudes continue to warm," the authors write. Adds Anderson: "What we are seeing is consistent with the effects of global warming."

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