As the world鈥檚 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鈥檚 shifting waters might become more common.
At the moment the Pacific switches between two extreme states, El Ni帽o and La Ni帽a, every two to seven years. This switching, called the El Ni帽o Southern Oscillation (ENSO), affects weather around the world, with both states causing floods and droughts in various parts of the world. A La Ni帽a has , 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 Ni帽o state. This would be bad news for us. Without the periodic upwelling of cold water associated with La Ni帽a, warm water would cover most of the surface of the Pacific, releasing its heat into an atmosphere already warming because of climate change.
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Some evidence that a warming climate might affect ENSO comes from geological studies. For instance, in 2005 of the University of California, Santa Cruz, and colleagues found that the average state of the Pacific was shunted towards an El Ni帽o condition in the Pliocene, 4.5 to 3聽million years ago, when Earth鈥檚 climate was 3聽掳C warmer than today (). Other studies have supported this, but none of them could say whether the system wobbled back and forth as it does today.
Now new evidence from of the University of Oxford, and two other research groups, indicates that ENSO kept working, suggesting that the worst-case scenario 鈥� a fixed El Ni帽o 鈥� probably won鈥檛 happen.
Tropical warmth
Rickaby 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. She 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鈥檚 in line with a recent Japanese study, which looked at fossilised corals and also found evidence of an active ENSO in the Pliocene ().
She has also run climate models which suggested that conditions 3聽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鈥檛 happen.
Into the future
It鈥檚 not clear that what happened in the Pliocene will repeat itself this century, warns 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鈥檚 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 Ni帽o, 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 Ni帽o state. 鈥淚 am confident that this mechanism should [still] work [in future],鈥� DiNezio says.
Even if ENSO keeps happening in the warmer world, however, it may change. In Rickaby鈥檚 model of the past, the switching between El Ni帽o and La Ni帽a 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 Ni帽os in each 200-year period in the Pliocene, compared with three to four in more recent preindustrial times.
Rickaby says we can鈥檛 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.
Journal reference: , DOI: 10.1029/2010pa002097
This article has been updated since first publication to clarify the results of Ana Christina Ravelo鈥檚 2005 study
