New data from taken from coral records suggests that future El Nino events will be 'unpredictable'.
New evidence extracted from corals, some of them 7000 years old, suggest that El Nino events in the past have been highly variable, which means that they are likely to remain so in the future.
While the ENSO events in the twentieth and twenty-first centuries are more intense than normal, ENSO strength has varied so much in the past, we can’t be sure that current intensification is due to changes in our climate as a result of rising carbon dioxide levels.
"The level of ENSO variability we see in the 20th century is not unprecedented," says Kim Cobb, an earth and atmospheric scientist at the Georgia Institute of Technology.
"But the 20th century does stand out, statistically, as being higher than the fossil coral baseline."
Her team at the Georgia Institute of Technology analysed 15,000 coral samples taken from 17 rocks that had rolled onto beaches of Pacific Islands.
"We are able to count back in time, following the seasonal cycles locked in the coral skeleton, as long as the core will allow us," Cobb explains.
The researchers were able to put dates on the coral by looking at ratios of the radioactive isotopes present. They then used X-ray and chemical analysis to track changes in the coral skeleton giving them accurate records of the global temperatures and rainfall over 650 year periods.
"We looked at the long-term variability of ENSO in the climate models and asked how it compares to the long-term variability of ENSO in the real world," says Cobb.
"We show that they actually match fairly well. This project sets the stage for conducting more detailed data-model comparisons from specific time intervals to test the accuracy of ENSO characteristics in the various models."
The team hope that their report, published in the journal Science, will help to make future climate models and thus predictions more accurate.
“Prior to this publication, we had a smattering of coral records from this period of interest,” adds Cobb.
“We now have tripled the amount of fossil coral data available to investigate these important questions. We have been able to provide a comprehensive view of recent variations in ENSO.”