20 years ago, El Niño had an uncorrected impact on the data that went into global warming models, causing puzzling results. Now, those results have been corrected and instead of El Niño impacting global warming, global warming could be impacting El Niño.

Using coral samples from Kiribati, researchers have outlined how El Niño Southern Oscillation (ENSO) cycle has changed over the past 4,300 years - altering the cycles of El Niño and La Niña events that bring extreme drought and flooding to Australia and many other Pacific-rim countries. The new paper suggests that external changes have an impact on the strength and timing of El Niño events.

The researchers also reaffirm what was well known - that natural influences on the Earth's climate, such as those caused by variations in its orbit around the sun, could affect the strength of El Niño events. These natural influences altered seasonal trade winds across the Eastern Pacific and affected the development of El Niño events. The research found that El Niño events in the past started later in the year and were often less intense.

"Our research has showed that while the development of La Niña and El Niño events is chaotic and hard to predict, the strength of these events can change over long time spans due to changes in the global climate," said co-author Dr. Steven Phipps. "For instance, we found that the ENSO cycle was much weaker 4,300 years ago than it is today. This weaker cycle persisted for almost two centuries." 

Credit:  doi:10.1038/ngeo1936 

"We found there was a small strengthening of the regular seasonal trade winds in the Eastern Pacific in response to natural warming cycles in the Earth's orbit around the sun. Remarkably this acted in a big way to stop El Niño events from forming and growing," said lead author Dr Helen McGregor from the University of Wollongong. "This shows us that external factors can influence the ENSO process and that it may have a sustained response to future greenhouse gas changes. Currently 20th Century observations are too short to confirm whether this is occurring now."

These new observations can now be used in climate models to see if these past changes in ENSO processes can be reproduced. 

"Currently, climate models do not agree on how El Niño may change under future global warming scenarios," said Phipps. "With these new observations we can determine which models reproduce the most accurate response to changes in the global climate. This will help us to more accurately forecast the response of ENSO under future global warming scenarios." 

Citation: H. V. McGregor, M. J. Fischer, M. K. Gagan, D. Fink, S. J. Phipps, H. Wong  &  C. D. Woodroffe, 'A weak El Niño/Southern Oscillation with delayed seasonal growth around 4,300 years ago', Nature Geoscience 5 September 2013 doi:10.1038/ngeo1936