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Paris - La Nina, a weather phenomenon that periodically causes devastating droughts and storms, will likely occur more frequently and more violently this century as a result of global warming, researchers said on Monday.
An exceptionally harsh La Nina like the 1998-9 event which killed thousands of people and displaced millions, will become almost twice as common in the 21st century, according to their statistical modelling.
If emissions of Earth-warming greenhouse gases continue unabated, an "extreme" La Nina (Spanish for The Girl) will occur once every 13 years on average, compared to once every 23 years in the previous century, the team wrote in the journal Nature Climate Change.
"It means more occurrences of devastating weather events, and more frequent swings of opposite extremes from one year to the next, with profound socio-economic consequences," they warned.
Las Ninas sometimes occur the year after an extreme El Nino (The Boy), which the researchers said may also become more damaging and frequent.
The two phases are part of a natural cycle of weather variability called the El Nino Southern Oscillation or ENSO, which can be disruptive around the Pacific rim and further afield.
Contrary to El Nino, La Nina is characterised by cold sea-surface conditions in the central equatorial Pacific Ocean and a sharper contrast with land temperatures in western Pacific countries like Indonesia, Malaysia, the Philippines, Singapore, Bangladesh and Vietnam.
This "temperature gradient", which could increase with global warming, influences atmospheric air flow and precipitation patterns -- a steeper difference brings wetter conditions to the western Pacific countries and drier weather for the Americas to the east, said the authors.
The extreme 1998-9 La Nina, which followed an exceptional 1997-8 El Nino, changed droughts into floods in western Pacific countries, and wet weather into severe drought in the southwestern United States.
More than 200 million people were displaced by floods and storms in China, and Bangladesh experienced one of the most destructive floods in modern history.
The new study used 21 climate models to simulate the probability of extreme La Ninas from 1900 to 2099.
"On average, for the 2000-2099 century, we should get about eight such (extreme) events," study co-author Cai Wenju of the Ocean University of China in Qingdao told AFP.
There is no reliable data on La Nina before 1950, but two extreme events were recorded in the 50 years to 1999 - in 1988 and 1998.
The team based their calculations on a worst-case warming scenario which mirrors an unbraked rise in greenhouse-gas emissions.
In its latest, Fifth Assessment Report, the UN's Intergovernmental Panel on Climate Change (IPCC) said this scenario would cause warming this century of about 3.7°C. The UN targets warming of no more than 2°C over pre-industrial levels.
Whether ENSO will be amplified by man-made global warming is a big debate among scientists. A January 2014 study said there did not appear to be a link between climate change and the frequency and volatility of El Nino events.
According to the US National Oceanic and Atmospheric Administration, ENSO events occur on average every three to five years, but since 1975, Las Ninas have been only half as frequent as Los Ninos.
