Predicting the behavior of the El Niño weather cycle is a challenge for forecasters and climate scientists, and the stakes are high.

El Niño and its counterpart La Niña, driven by changes in sea-surface temperatures in the tropical Pacific Ocean, can have major effects on weather conditions hundreds and thousands of miles away. Studies have linked catastrophic floods, droughts, disease outbreaks, wildfires and even social unrest to the weather cycle, known as the El Niño-Southern Oscillation.

Yet scientists have struggled to understand whether climate change is altering that cycle. Climate models have produced conflicting results, and reliable instrumental records of ENSO events begin in the early 20th century.

Now a new study, which builds on prior efforts to reconstruct El Niño's past behavior by examining coral growth, suggests that El Niño and La Niña events have become more variable and intense over the past several decades.

"We kind of answered the question, is El Niño changing with respect to recent natural variability?" said Kim Cobb, the Georgia Institute of Technology climate scientist who led the research published yesterday in the journal Science. "The answer is yes, tentatively so."

The 7,000-year portrait painted by the corals she examined suggests El Niño may get stronger as the level of carbon dioxide in the atmosphere climbs, Cobb said.

But researchers will need to do more work -- including a concerted effort to fill gaps in the record of El Niño's past behavior -- to prove that connection, for now a tentative one, she said.

"It could be thrown out," Cobb said. "But whatever the result, we will have confidence with more data."

Coral cores tell the story
The new analysis is based on samples that Cobb describes as a data "down payment," tripling the amount of coral-core data available from the past 7,000 years.

Cobb and her colleagues extracted 17 cores from fossilized corals on two tiny islands, Christmas and Fanning, in the Line Islands archipelago in the central Pacific Ocean.

Researchers painstakingly examined each core millimeter by millimeter, using radioactive dating to determine the age of each sample.

To determine how temperature and precipitation had changed during each coral's lifetime, the scientists tracked variations in the ratio of two oxygen isotopes, O-18 and O-16. During warm, rainy El Niño periods, the coral that grows has less O-18. During cool, drier La Niña events, new coral has more O-18.

Cobbling together the data from those 17 coral samples -- each covering a period 20 to 80 years long -- allowed the scientists to reconstruct the ENSO cycle thousands of years into the past.

The results show that the variability and strength of the ENSO cycle was greater during the 20th century than at most points in the 7,000-year fossil record -- but the episode is not unprecedented, the scientists said, pointing to a similar period during the early 17th century.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500