Coral Reefs Trace Ancient El Ninos

Oct. 25 -- A study of coral reefs suggests El Niño occurs more frequentlynow than it did in the mid-19th century, before global warming.

Exactly what role, if any, is played by global warming is stillunclear, however.

The study of core samples taken from coral reefs dating back to1840 indicates El Niño happened only about once every 10 years inthe mid-1800s, compared with about once every four years now.

But there was also evidence of cycles as short as three yearsfrom the late 1800s to about 1920. The cycles settled into theircurrent pattern around 1955.

Looking for Trends

El Niño refers to the movement of warm water from the westernPacific to the central and eastern expanse of the ocean. It causeswidespread climate changes, such as heavy rain in the westernUnited States and drought in Australia and southeastern Africa.

Scientists have debated whether global warming from the buildupof greenhouse gases affects how often El Niño appears.

The new study, appearing in Thursday’s issue of the journalNature, suggests any such influence would be complicated, said oneof the researchers.

“It’s not like we can say El Niños are going to be morefrequent,” said Julia Cole of the University of Arizona. “We seea lot of natural variability as well.”

Cole and fellow researchers extracted 3-inch-wide cores fromunderwater coral reefs on the Maiana atoll about 2,500 milessouthwest of Hawaii. Then they measured a rare isotope of oxygenthat is less common in coral when the ocean is warmer and lesssalty — conditions typical of El Niño.

Recent samples matched records of El Niño events since the1950s, making researchers confident that the older core sampleswere also good indicators.

Recent El Niños Unique

In an accompanying article, Robert Dunbar of Stanford said thewarming of the Pacific since the late 1970s “is unique over theentire 155-year record” and suggests global warming is playing arole.