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Severe La Ninas, linked to both floods and
droughts as well as more landfalls by Atlantic hurricanes, would
happen on average every 13 years in the 21st century if
greenhouse gas emissions keep rising, compared with once every
23 years last century, the researchers said.
"We show that greenhouse warming leads to a significant increase
in the frequency of extreme La Niña events," said the report led
by experts at Australia's Commonwealth Scientific and Industrial
Research Organisation (CSIRO).
La Nina is the opposite of the better-known El Nino weather
event characterized by warmer waters in the tropical Pacific
with knock-on effects that can cause billions of dollars of
damage to food and water supplies around the globe.
La Ninas happen unpredictably every two to seven years, with the
last extreme event, judged by a sharp cooling of Pacific surface
waters, in 1998-99.
The findings, published in the journal Nature Climate Change,
followed another study by lead author Wenju Cai last year that
identified a link between El Nino and global warming.
"We found that extreme El Nino will double in frequency. We
don’t find evidence that the intensity will increase. The
extreme events just occur more frequently," he told Reuters.
The study also indicated that La Ninas were likely to follow El
Ninos more often, in a damaging double dose.
Writing in a comment in Nature Climate Change, Antonietta
Capotondi, of the U.S. National Oceanic and Atmospheric
Administration's Earth System Research Laboratory, said the
findings were "plausible", but urged more study.
Capotondi said it seemed counter-intuitive that La Ninas, linked
to cool waters, might happen more often in a warmer world.
Under La Nina conditions, however, heat over land in the Western
Pacific including Australia and Indonesia would suck winds
westwards off the ocean, in turn drawing waters from the depths
and cooling the ocean surface.
Among other extremes coinciding with the 1998-99 La Nina, river
floods and storms killed thousands of people in China and
Bangladesh. In 1998, Hurricane Mitch killed 11,000 people in
Honduras and Nicaragua.
(Editing by Robin Pomeroy)
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