ENSO variability was high during Pliocene – suggesting warming won’t temper El Niños and La Niñas

ENSO variability during Pliocene - no permanent El NiñoA resonating question among climatologists: how will warming affect the El Niño Southern Oscillation?

Permanent El Niño?

A 2006 Science publication has suggested in warmer water the oscillation could vanish – as during the Pliocene, a warm epoch 5 to 3 million years ago [and just like the Paleocene and Eocene -albeit it much more recent- part of the Tertiary period], the Earth seemed in a permanent El Niño state, with weaker trade winds, so more warm water to the equatorial east and less to the west of the Pacific.

ENSO variability at least as high

A new study however, published in May this year in the journal Paleoceanography, has better looked into the fossil record and concludes – although indeed average temperatures were around current El Niño positive temperature anomaly values, up to 3 degrees warmer – the ‘swings’ did not stop.

The researchers of Oxford and Leeds University got to their conclusions by examining foraminifera shells in the eastern Pacific, which showed signs of frequent temperature shifts, greater than seasonal variability could explain.

In fact, variability could have been bigger than it is today. This is important, as not the average, but the extremes of the ENSO cycle cause the most damage, as the strong 2010 La Niña showed.

The researchers also ran simulations running Hadley Centre’s HadCM3 climate model, which confirmed the results from the fossil record.

The Pliocene paleo comparison

Drawing the parallel from the Pliocene to our current Holocene is quite legitimate, as the epochs lie close on the geological timescale, and the Pacific Ocean’s dimensions and position are almost identical to the present situation.

And if indeed the paleo parallel holds true, the climate lessons are not limited to the ENSO comparison. In the warmest part of the Pliocene the atmospheric CO2 concentration lay around 400 ppm, under the current emissions trend a milestone we’ll pass within 4 years time [meaning we're getting very close to 450 ppm CO2eq as well].

During the Pliocene climatic optimum though the world was on average 2 to 3 degrees hotter than today – which could give us an indication of what lies in store for us once temperature has again settled around a new atmospheric equilibrium greenhouse gas concentration.

Also noteworthy: average sea level height was 25 meters higher during the Pliocene.

© Rolf Schuttenhelm | www.bitsofscience.org

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