March 13, 2006 — A new study of Earth's bouts of global warming two to five million years ago show more evidence than ever that the fluttering climate patterns of today can settle into a very long El Niño pattern. If so, the rather predictable global climatic effects known to accompany El Niño have a chance of becoming the norm as the planet heats up.

"People assume that the current temperature pattern we have in the tropics is really stable over long time periods," said paleoclimatologist Christina Ravelo of the University of California at Santa Cruz.

She was referring to today's rather regular oscillations among "normal" El Niño and La Niña wind and ocean temperature patterns.

"We're showing that, in fact, it's not stable. It can't quite decide which way to be," she said.

Ravelo and her colleagues have been piling up evidence of past climate shifts by studying the temperature-sensitive chemistry of microscopic foraminifera shells buried in ocean sediments from the Pliocene, an epoch that stretches 1.8 to 5.3 million years ago.

The "foram" shells contain a record of the temperature of the surface waters where they lived, and therefore an ancient archive of the warming of equatorial Pacific surface waters that signal the El Niño shift.

The weather effects of El Niños show up most clearly during winter. El Niño winters are mild over western Canada and parts of the northern United States, and wet over the southern United States from Texas to Florida.

The forams say, quite clearly now, that global warming in the Pliocene turned El Niño on and made it the norm, reported Ravelo and her colleagues in the current issue of GSA Today.

Pliocene climate changes are particularly applicable to studying today's climate, said Ravelo, because the oceans and continents were in virtually the same positions as today, which is critical for studying how the global climate works. It means the Pliocene can probably tell us about climate extremes that still might be possible.

The Pliocene is also the perfect testing ground for today's many global climate models, said Ravelo. If a model is any good, it ought to be able to predict climate situations that actually happened during hot periods in the Pliocene, like the clicking on of El Niño.

"What's so interesting is that if you look at all the models, they don't agree on what the tropics will be doing (with global warming)," said Ravelo. "They just don't agree."

To address that, Ravelo and her colleagues are working with climate modelers who have models that can, at least, reproduce the "paleoclimate" of permanent El Niños during the Pliocene.

"The real tests for the models are the paleoclimate," agreed George Philander, a veteran El Niño modeler and the director of Atmospheric and Oceanic Sciences program at Princeton University. "The focus (of modelers) is a bit too much on the recent past."

Modelers need to step back and look a few million years into the past to get a genuine perspective of the climate extremes that are possible, he said.