Extreme El Nino Events To Double In Number Even With 1.5-Degree Warming: Study

Fairfax - Peter Hannam

Extreme El Nino events will more than double in frequency, even with the most ambitious goals to curtail global warming, exposing large regions to severe droughts and placing coral reefs in peril, a team of scientists including Australians say.
In 2015, almost 200 nations signed the Paris Climate accord, agreeing to curb greenhouse gas emissions to prevent global temperatures warming more than 1.5-2 degrees compared with pre-industrial times. National pledges so far point to warming of closer to 3 degrees.
El Ninos are marked by abnormally warm temperatures in the tropical eastern Pacific that has global impacts. Photo: NASA
Even at the lower end of that range, which implies atmospheric levels of carbon dioxide peak by about 2040-2050 before starting to decline, big El Ninos will still become twice as common as their natural frequency to an average of about 10 per century and continue to rise further, according to research published in Nature Climate Change.
During El Ninos, the eastern equatorial Pacific is unusually warm, triggering a reversal of trade winds and a shift in rainfall patterns that often have consequences - such as droughts in Australia and the Horn of Africa, heavy rain in South America - well beyond the region. During extreme events, most recently in 2015-16, impacts can intensify.
"[During] big blockbuster El Ninos, if they happen more frequently, we can expect more damage and loss of life because of that," said Michael McPhadden, a senior scientist at the US National Oceanic and Atmospheric Administration and an author of the report that analysed 13 climate models.
"The number [of El Nino] might not necessarily increase but the stronger ones will get stronger," he said.
Cai Wenju, a principal research scientist at the CSIRO and one of the paper's lead authors, said the frequency of extreme El Ninos is beginning to emerge from the natural variability. There have been four such events since 1950 compared with one in the previous half century.
"To our surprise, if you stabilise GMT at 1.5 degrees, the extreme El Ninos continue to rise," Dr Cai said. "After that, the frequency of extremes rises another 40 per cent" to as often as one in every seven years before finally stabilising.
Aiming for that 1.5-degree warming cap is still worthwhile not least because the frequency of severe El Ninos will increase even faster if temperatures climb higher, Dr Cai said.
A young Somali girl displaced by drought wears mock spectacles cut from an antibiotics medicine box at a relief camp near the capital, Mogadishu, in March 2017. Photo: AP
Moreover, the flipside pattern, extreme La Ninas, do not increase in frequency at 1.5 degrees, the researchers found, but do so at higher warming levels. Complex circulation patterns mean the threshold for extreme El Nino events is more easily crossed with on-going warming than La Ninas.
"El Ninos are more sensitive to greenhouse forcing than La Nina but you'll reach a point where radiative forcing from excess greenhouse gas where both El Ninos and La Ninas will experience more extreme events," Dr McPhadden said.
Coral reefs, already struggling as water temperatures rise, will face severe threats during heat spikes, scientists say. Photo: WWF-Aus/BioPixel
Heat goes on
Apart from the regional impacts, El Ninos typically drive surface temperatures higher as the Pacific absorbs less of the excess heat being collected in the atmosphere from the additional greenhouse gases.
The past three years – 2014, 2015, and 2016, each broke annual records as the Pacific flirted and then fell into a full-blown El Nino. Even 2017 is likely to be among the hottest, with the first six months the second warmest on record, trailing only last year, NOAA said last week. (See NOAA chart below.)

La Ninas have their own negative impacts, including above-average numbers of cyclones in northern Australia, but the global effects of El Ninos tend to be worse, Dr Cai said. That's even taking into account big floods in China in 1998 during a record strong La Nina.
"El Ninos tend to have huge areas of drought and the impacts may be more long-lasting than flood," Dr Cai said. With droughts, farmers may miss out on several planting seasons and natural eco-systems can take longer to recover compared with floods.
El Nino years have also seen severe coral bleaching and mortality as the corals respond to excess heat by expelling the algae that provide them with the bulk of their energy. As much as half of the Great Barrier Reef corals have died during the past two summers.
The rising background temperatures from climate change mean the heat spikes during El Ninos can be expected to make coral bleaching more common in the future.
"You  can envisage these kinds of impacts with the more frequent [extreme El Nino] events in the future," Dr McPhadden said.
The recent bleaching "was shocking how extensive it was, and how it affected so severely areas that had not been affected before", he said.
"Can they adapt? If not, there are some hard times ahead."


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