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Climate Variability Increases as the World Warms

By Stephen Luntz

The variability in seasonal conditions brought on by El Niño and La Niña events is growing more intense as the world warms, according to a new study from the University of NSW Centre of Excellence for Climate System Science.

The behaviour of the El Niño Southern Oscillation has proven a challenge for climate modellers, with different models predicting contrasting results. While some foresee greater variability as the world warms, others anticipate reductions and still others a shift towards more El Niños and fewer La Niñas.

Dr Shayne McGregor drew together proxies indicating the temperature and rainfall at different locations around the planet over the past 600 years. “There are error bars on the timing of events, so sometimes one proxy can suggest an

El Niño in one year and another the year after,” McGregor says.

However, when the researchers looked at variability, rather than seeking a direct measure of whether the world was in an

El Niño or La Niña at a particular time, they found a more consistent pattern.

“During the cooling period from about 1600–1800, variability was lower,” McGregor says. “That could mean events were less frequent, or less intense, or both. We can’t tell at this distance.”

The proxies indicate that the past 30 years has seen the most intense variability over the past 600 years, McGregor reports in Climate of the Past, with particularly strong evidence from 1590 when proxies are more numerous.

“Climate models have been assessed on their ability to reproduce what has been observed over the past 100 years. We will now be able to compare them against a longer period, but this has not been done yet,” McGregor says.

The variability method used did not allow McGregor to see if a colder world produced a surfeit of one state over the other, and he says it remains hard to predict the balance in a hotter climate. “We still don’t understand why,” McGregor says. “One of the things that drives El Niño is the temperature gradient along the Equator. That could weaken or strengthen as the world warms.”

Dr Agus Santoso of the same institution has revealed in Nature that since the 1970s a new type of El Niño has appeared, whereby warming starts in the west Pacific and spreads east, reversing the traditional pattern. He attributes this to weakening of the westward equatorial currents in the Pacific.

“These currents are well represented in a number of climate models,” he says. “Using these models we confirmed, even under modest global warming scenarios, these unusual El Niño events doubled in frequency.”