Each fall, predictions about the upcoming winter's weather rely heavily on whether there is an El Niño or La Niña pattern brewing. These extremes of a phenomenon known as the El Niño-Southern Oscillation (ENSO) affect the temperature of the surface waters in the Pacific Ocean, which in turn influence storm and temperature patterns across the globe.

In recent years, ENSO seems to have become more variable, with stronger and possibly more common El Niño events, leading some experts to speculate that climate warming has a direct impact on the weather-altering oscillation.

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To test that idea, researchers used fossil coral records to piece together a 7,000-year history of ENSO patterns from the tropical Pacific. By analyzing weighted oxygen isotopes in dated coral skeletons, the team was able to determine what the temperature and rainfall conditions were like in the region over thousands of years.

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Results confirmed that the ENSO became more variable in the late 20th century than it had been for hundreds of years before, researchers report today in the journal Science. But our most recent era is not the only time in history that the oscillation showed that kind of shift. The early 17th century, for example, was also particularly extreme.

The new findings don’t rule out the possibility that global warming could be behind recent El Niño and La Niña trends. But the historical perspective makes it impossible to conclude that human-caused climate change is to blame for the recent shifts. It may be that ENSO is just naturally variable, regardless of external conditions.

"Relatively robust 20th-century ENSO variability may reflect a sensitivity to anthropogenic greenhouse forcing," the researchers wrote. "But definitive proof of such an effect requires much longer data sets than are currently available, given the large range of natural ENSO variability implied by the available fossil coral data."

Photo: Fossil coral "rocks" located on beaches in the Line Islands provide detailed records of temperature and precipitation useful for understanding ENSO events. Credit: Jordan Watson