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El Niņo's impact may extend to the shores of East Africa

People living along the Pacific coast have learned to respect the awesome power of El Niņo ­ the periodic warming and cooling of the Pacific Ocean that can bring torrential rainfall or extreme drought every three to seven years.

Now researchers analyzing coral samples drilled off the African coast have discovered that El Niņo's influence may stretch well beyond the Pacific into the waters of the Indian Ocean.

Robert B. Dunbar, Stanford professor of geological and environmental sciences, and University of Arizona professor of geosciences Julia E. Cole describe their findings in the Jan. 28 issue of the journal Science.

Using underwater drills, Dunbar's team removed 12-foot-long core samples from a living colony of coral growing in the waters of Kenya's Malindi Marine Park (see photo 1).

Like trees, coral skeletons form annual growth rings that accumulate over hundreds of years. The Malindi core samples provide a 194-year record of growth dating back to 1801 (see photo 2).

As they grow, coral skeletons take up a mixture of different types of the chemical element oxygen. One type is heavier than the other, and as the coral experiences seasonal and long-term changes in water temperature, the relative proportion of heavy and light oxygen in the skeleton also changes.

By measuring how much heavy oxygen is present in each growth ring, scientists can determine past changes in water temperature.

When Dunbar and Cole analyzed the quantity of heavy oxygen in the Malindi samples, they made a remarkable discovery: Surface temperatures in the western Indian Ocean have risen and fallen roughly every 10 years in a pattern almost identical to the periodic warming and cooling that occurred in the tropical Pacific.

"It brings home the message that the Pacific Ocean exerts a tremendous impact on the global climate," Dunbar says. "This is strong evidence of a big influence from the Pacific on a decadal time scale."

The Malindi corals also revealed that the surface temperature of the Indian Ocean has risen approximately 2.3 degrees F (1.3 C) since 1801.

"This is a big increase," Dunbar says, noting that the largest rise occurred in the late 20th century ­ possibly the result of an artificial greenhouse effect caused by people burning gasoline and other fossil fuels.

Dunbar and his colleagues plan to continue their research to determine if El Niņo has any effect on the severity of monsoon rainfall along the Indian Ocean coastline. Eventually scientists may be able to predict severe monsoon seasons months in advance, a breakthrough that could benefit millions of coastal residents who face severe flooding in Kenya, India and nearby countries.

Two other researchers co-authored the Science article: Timothy R. McClanahan of the Wildlife Conservation Society and Nyawira A. Muthiga of the Kenya Wildlife Service.


By Mark Shwartz

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