From record floods to crippling droughts and wildfires, a natural swing in Pacific Ocean temperatures can trigger climate chaos around the globe.
The El Niño ocean-weather pattern is linked to droughts in Australia and floods in parts of South America, while its sibling La Niña causes the opposite, with the two phenomena occurring at irregular intervals.
A powerful La Niña triggered record floods in eastern Australia in 2011 and has been blamed for the withering drought in Texas and severe dry spells in South America, hitting corn and soy crops.
Forecasters say an El Niño might develop later in the year.
Following are some questions and answers on El Niño and La Niña and their billion-dollar impacts on economies.
WHAT IS EL NINO?
El Nino, also called "Little Boy" or "Christ Child", is a warming of ocean surface temperatures in the eastern and central Pacific.
Peruvian fishermen noticed the arrival of the warm waters occurred usually around Christmas. The phenomena, which occurs every three to seven years, led to more rains in that part of South America and a drop in the fish catch.
Strong El Ninos can lead to a dramatic weakening of the trade winds that blow west across the Pacific, triggering drought in Southeast Asia and Australia and parts of Africa.
Some El Ninos can also affect the Indian monsoon by reducing rainfall, threatening crops and livelihoods.
WHAT IS THE IMPACT?
Globally, El Niño can trigger above average rains in northern Peru and Bolivia, drought in Southeast Asia, Australia, India and northeast Brazil, cyclones in the central Pacific and stormy weather in southern and western United States.
El Ninos also tend to cut the number of Atlantic hurricanes but boost the number of storms in the eastern Pacific.
In Australia, strong El Ninos can slash wheat crop output, threaten water supplies by cutting river flows, shrink city reservoirs and dry out forests, triggering bush fires.
Major El Ninos occurred in 1982-82 and 1997-98 but the weak El Niño of 2002-2003 also led to severe drought in Australia.
WHAT IS LA Niña?
La Niña, or "Little Girl", is an abnormal cooling of ocean temperatures in the eastern and central Pacific.
This triggers stronger trade winds across the Pacific that pile up very warm warmers in the western Pacific and around northern Australia, triggering above average rains.
Typically, it also boosts the number of cyclones during the November-April Australian cyclone season.
Just like El Nino, the weather impact can be global. In Indonesia and parts of Australia, La Niña can bring flooding rains, affecting wheat, sugar, palm oil and rubber crops as well as coal and iron ore mining.
In Argentina and the U.S. Plains, La Niña can trigger drought, hurting the Argentine corn and soy crops and the main U.S. hard red winter wheat crop.
La Niña tends to lead to wind patterns that favor the formation of more hurricanes in the Atlantic and fewer in the eastern Pacific, potentially meaning a greater threat to U.S. Gulf oil and gas assets and cities in Florida, the Gulf Coast and eastern seaboard.
Major recent La Niña events occurred in 1973-76, 1988-89 and 2010-12.
Heat from the tropics drives the global climate by fuelling ocean and atmospheric patterns that shift the warmth around the globe. Warm tropical waters fuel evaporation and add moisture to the atmosphere needed for clouds to form.
The rising air also drives atmospheric circulation patterns that help shift the moisture and warmth around to other parts of the globe. So disrupting this pattern can alter the climate elsewhere.
Scientists say climate change might also be adding an extra kick to La Niña and El Niño because warmer oceans add more fuel to storms and weather patterns.
(Sources: NOAA, Australian Bureau of Meteorology, National Centre for Atmospheric Research, Reuters)
(Editing by Ed Davies)