U. LEEDS (UK) — Swings between the two climatic extremes El Niño and La Niña appear to have occurred more frequently in the past than previously thought and may increase in regularity in the future.
Researchers say El Niño Southern Oscillation (ENSO), the sloshing of the warmest waters on the planet from the West Pacific towards the East Pacific every 2-7 years, continued during the Earth’s last great warm period, the Pliocene.
It is these extreme ENSO events that cause droughts, forest fires, and flooding—like that in Pakistan last year and in Queensland, Australia this year, as well as changes in fishery production.
Reporting in the journal Paleoceanography, researchers use the Pliocene as a past analog and predictor of the workings of Earth’s future climate.
The Pliocene (which lasted from 5 to 3 million years ago) had carbon dioxide levels similar to the present day, with global mean temperatures about 2-3ºC higher, so it is a useful test-ground for climate research.
“We know from previous studies that the mean state of the Pacific during the warm Pliocene was similar to the climate patterns observed during a typical El Niño event that we see today,” says Nick Scroxton from the University of Oxford.
“However, until recently it was believed that a warmer Pacific would reduce the climate swings that cause the dramatic weather extremes throughout the region leading to a permanent state of El Niño. What we didn’t expect was that climatic variability would remain strong under these warmer conditions.”
Scientists combined experiments performed on the Met Office Hadley Centre climate model, HadCM3, with analyses of the chemical composition of foraminifera, individual shells of small organisms.
The foraminifera were collected from a deep sea sediment core in the East Equatorial Pacific and provided a record of temperature in the upper layer of the ocean through time.
Researchers discovered that the range of temperatures experienced by these organisms during the Pliocene, was higher than what would be expected from a typical seasonal cycle.
The extra variation in temperature can be explained by the additional extreme temperature swings provided by the El Niño/La Niña system. The researchers say the agreement in findings from both ocean data and modelling leaves little doubt that ENSO will persist in a warmer world.
“It is reassuring that ENSO kept on ticking during the warmer Pliocene as opposed to switching into some novel mode, which if it reoccurred in the future would have massive consequences for regional and global weather patterns and climate,” says co-author Alan Haywood from the University of Leeds.
“Previously, many scientists had forewarned that a warmer world may experience a permanent El Niño state, a condition which could act to amplify warming.”
Earlier this year a team from Japan studying corals from the same period showed climatic variability in the western Pacific on a similar scale to today, questioning the idea of a permanent El Niño during the Pliocene.
The new study goes a step further, showing that the oscillation is Pacific-wide, and is likely to be caused by the El Niño/La Niña, suggesting that a warmer future will continue to be dogged, maybe even more regularly, by extreme climatic events.
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