By Tim Radford, Climate News NetworkThis Creative Commons-licensed piece first appeared at Climate News Network.

LONDON — El Niño, the cyclic Pacific weather phenomenon that periodically brings global devastation in its wake, is not the only thing likely to grow more extreme with global warming.

A team of international scientists now predicts that its cool little sister, La Niña, is liable to turn nasty more often too — every 13 years, which is twice as often as the historic record.

Both are observed fluctuations in mid-ocean temperatures in the Pacific that are the signal for changes in the climate pattern: both are natural, both occur as part of a cycle, and both can be traced back through human history.

Mobile blister

El Niño is a mobile blister of Pacific Ocean heat that then affects winds and currents, and was first dubbed “The Child” by Peruvian fishermen, who noticed that it tended to arrive around Christmas.

A powerful El Niño is accompanied by drought and forest fire on the western side of the Pacific, and torrential rain and floods on the normally dry eastern Pacific coasts.

Meteorologists then amended the name to label opposite phase of what they call the “El Niño southern oscillation”.

With La Niña, unseasonally cold sea surface temperatures in the Pacific create a temperature gradient that can intensify droughts in the American south-west, trigger floods in the western Pacific, and hurricanes in the Atlantic.

The CSIRO team has established “a plausible scenario of changes that may have very serious implications for society”

A year ago, Wenju Cai, a climate researcher for Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), warned that the frequency of extreme El Niño events could double with climate change.

Now Dr Cai and colleagues report once again in Nature Climate Change that the same is true for what one might call the oscillation’s downside: global warming is likely to double the frequency of extreme La Niña events, too. And 75% of these are likely to follow immediately after an extreme El Niño.

The paradox is that global warming could also increase the intensity of not just hotter-than-usual seasons but also cool or cold episodes that would trigger unusual or extreme weather responses far from the ocean’s cool centre.

So some parts of the world are likely to experience blazing drought, followed by catastrophic floods, while across the ocean, other nations will have torrential rain and then unseasonal drought, every 13 years or so.

Mild La Niña events are recorded every two to seven years. El Niño, too, can develop gently, with no great consequences.

The CSIRO study was concerned with simulating the probability of extreme events. The planet has already experienced notorious an up-and-down cycle: 1997-1998 marked an extreme El Niño, immediately followed by an extreme La Niña in 1998-1999.

Destructive floods

In 1998, as the La Niña developed, flash floods and landslides killed 25,000 in Venezuela, and floods and storms displaced 200 million people in China. Bangladesh experienced one of the most destructive floods in its history, with more than half of the country under water.

The 1998 Atlantic hurricane season spawned Hurricane Mitch, which claimed 11,000 lives in Honduras and Nicaragua. The South-western US, meanwhile, experienced one of the most severe droughts in its history

Such things used to happen once a generation — on average, every 23 years. But now, according to Dr Cai and an international team of scientists and climate modellers from China, Australia, Hawaii and New Jersey in the US, Peru, Paris and Exeter in the UK, such a double whammy could occur every 13 years.

Antonietta Capotondi, a climate scientists at the University of Colorado in the US, commenting in Nature Climate Change, says that climate models are the only tools scientists have for understanding the intricacies of global warming.

And despite the uncertainties, she says, the CSIRO team has established “a plausible scenario of changes that may have very serious implications for society”.

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