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Coral reefs hide El Niño secrets

  • Written by  Katie Burton
  • Published in Oceans
Coral reefs hide El Niño secrets
01 Jul
2019
Analysis of coral cores, extracted from coral reefs in the Pacific Ocean, has allowed scientists to create a 400-year record of El Niño events

El Niño events present scientists with a major problem. These monumental climate cycles only take place every two to seven years, making it extremely difficult to build up a long-term picture. But for Dr Mandy Freund, a researcher at the University of Melbourne, one method in particular struck her as worth a try. Using coral cores taken from reefs in the Pacific Ocean, she was able to extract a 400-year record of El Niño events. This new data tells us more than ever before about the changing nature of El Niños, which cause profound weather effects across the globe from increased rainfall and tropical storms in Peru, Chile and Ecuador to drought in Australia, southern Africa and southeast Asia.

Coral drilling, Christmas IslandUnderwater drilling of corals off Christmas Island (underwater team: Jennie Mallela, Oscar Branson; surface team: Jessica Hargreaves, Nerilie Abram)

‘Corals have been used before, and a lot of the records are really high resolution, but nobody was really using the differences in the seasonal evolution of the corals,’ explains Freund. ‘When I asked my supervisors why no one looks at those differences, no one could really tell me.’ Though it had previously been considered impossible, Freund and her colleagues were able to test isotopes within the coral cores and thereby build up this 400-year seasonal picture. The record is available because corals take in chemicals (which are influenced by the climate) and accumulate them within their skeletons as they grow over many centuries. After carefully refining the technique to reconstruct the signature of an El Niño, the team were able to compare recent coral results with the existing instrumental record. Freund found a strong agreement between the coral cores and already recorded events -this confirmation allowed her to extend the record back in time to periods were instrumental records don’t exist. 

The results of the research show that there has been an unprecedented increase in the number of El Niños forming in the Central Pacific over the past 30 years, compared to all 30-year periods in the past 400 years. At the same time, the last three Eastern Pacific El Niños (which took place in 1982, 1997 and 2015) were the most intense ever recorded. Though it’s not yet possible to call this a pattern, or determine exactly why it is happening, Freund says that the change does align with some climate-change models. ‘When we compared the intensity of the last three El Niño events with the instrumental evidence, we’ve seen from the early 1990s, they stand out. And if we then look at the corals, they still stand out. But it’s only three events. Climate model scientists say this could happen, that those El Niño events become more intense, but basically three events are not yet enough.’ 

coral3Corals take in chemicals and accumulate them within their skeletons as they grow over many centuries [Image: Corey Huber]

The increased frequency of the Central Pacific El Niños is a more clear-cut trend and all the more so for the evidence provided by this elongated record. The hope is that it can now serve scientists when it comes to looking forward. ‘Having a better understanding of how different types of El Niños have affected us in the past and present will mean we are more able to model, predict and plan for future El Niños and their wide-ranging impacts,’ says Freund.

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