Pacific shouldn't amplify climate change

As the world's climate warms, will the Pacific Ocean make matters worse by dumping extra heat into the atmosphere? Ancient fossils and the latest modelling both suggest such fears may be unfounded. But there is a downside: extreme weather caused by the Pacific's shifting waters might become more common.

At the moment the Pacific switches between two extreme states, El Niño and La Niña, every two to seven years. This switching, called the El Niño Southern Oscillation (ENSO), affects weather around the world, with both states causing floods and droughts in various parts of the world. A La Niña has just finished, after bringing record-breaking floods to Australia earlier this year.

Climatologists have long suspected that the ENSO might shut down as the climate warms, with the Pacific shifting into a permanent El Niño state. This would be bad news for us. Without the periodic upwelling of cold water associated with La Niña, warm water would cover most of the surface of the Pacific, releasing its heat into an atmosphere already warming because of climate change.

Some evidence that a warming climate might shut down ENSO comes from geological studies. For instance, in 2005 Ana Christina Ravelo of the University of California, Santa Cruz, and colleagues said that there had been a fixed El Niño in the Pliocene, 4.5 to 3 million years ago, when Earth's climate was 3 °C warmer than today (Science, DOI: 10.1126/science.1112596).

But according to Ros Rickaby of the University of Oxford, and two other research groups, that worst-case scenario probably won't happen.

Tropical warmth

Rickaby has long questioned the evidence for a permanent El Niño in the Pliocene. In 2005, one of her studies suggested that, if anything, the Pacific was trapped in a La Niña state during the epoch (Science, DOI: 10.1126/science.1104666).

Rickaby now thinks she can explain the conflicting interpretations of Pliocene climate. She looked at the fossils of foraminifera, tiny marine animals that live just weeks and whose fossils carry a record of the temperatures they lived in. Rickaby collected 700 to 800 fossils from the Pliocene and found that they had experienced a wider range of temperatures than the changing seasons alone would have produced. There must have been an extra source of variation in the Pliocene climate, which she thinks was ENSO.

That's in line with a recent Japanese study, which looked at fossilised corals and also found evidence of an active ENSO in the Pliocene (Nature, DOI: 10.1038/nature09777). Rickaby says the earlier studies may have picked up past El Niños or La Niñas, but could not spot the switching.

She has also run climate models which suggested that conditions 3 million years ago would have allowed ENSO to function. In fact, she says, climate models have struggled to produce a warm world without an ENSO, and that may simply be because it doesn't happen.

Into the future

It's not clear that what happened in the Pliocene will repeat itself this century, warns Pedro DiNezio of the National Oceanic and Atmospheric Administration in Miami, Florida. He says the climate is now changing much faster than it did back then, so the oceans may respond differently.

However, DiNezio's own modelling work also suggests that ENSO will continue in a warmed world: although the rise in temperatures pushes the Pacific towards a permanent El Niño, the ocean pushes back. Even as the surface warms, the deeps remain cool, and this cold water will continue to periodically push the ocean out of the El Niño state. "I am confident that this mechanism should [still] work [in future]," DiNezio says.

Even if ENSO keeps happening in the warmer world, however, it may change. In Rickaby's model of the past, the switching between El Niño and La Niña was more regular than it is now, and big events were up to twice as common. In her model, there were six to eight big El Niños in each 200-year period in the Pliocene, compared with three to four in more recent preindustrial times.

Rickaby says we can't be sure if the ENSO will become more frequent in the near future. If it does speed up, both sides of the Pacific can expect more extremes of rainfall, and more floods and droughts.

Source New Scientist