Forecasters have warned it is almost certain that this year's El Nino will intensify into a very strong event in the coming months and last until February next year.
A very strong El Nino can significantly shift rainfall and temperature patterns worldwide and fuel extreme weather.
The climate pattern is already firmly established and is expected to strengthen over the next few months, fuelled by exceptionally warm ocean conditions across the tropical Pacific, the World Meteorological Organisation (WMO) said in an update on Thursday.
The body expects El Nino to peak towards the end of this year, though the climate impacts will continue well into next year.
"This exceptional El Nino demands exceptional preparation and response," said WMO Secretary-General Celeste Saulo.
She warned that the climate pattern "has the potential to deliver a massive blow to communities and economies across the world".
"We are already seeing disruption and devastation from droughts and floods, and we expect these impacts to increase as El Nino intensifies," she added.
UN Secretary-General António Guterres said: "El Nino is being supersized before our eyes. The science leaves no room for doubt: the planet is in uncharted waters, and those waters are heating up.
"Sea surface temperatures are rising, temperatures keep climbing, and the world is in the danger zone of extreme weather. The race now is between rising risks and our commitment to take climate action and protect people. We must win that race."
What is El Nino?
El Nino typically takes place irregularly every two to seven years and usually lasts nine to 12 months.
The natural phenomenon describes the unusual warming of the central and eastern tropical Pacific Ocean. It is associated with widespread changes in the climate.
It is part of a larger, irregular climate cycle called the El Nino-Southern Oscillation (ENSO), which includes a cooling phase, La Nina, and a neutral phase, researchers said.
It increases the risk of drier conditions in parts of Indonesia, Australia, India and central and equatorial South America, while wetter conditions are likely in some other regions of South America and the southern United States.
A warming in the Pacific alters atmospheric circulation, including the position of the jet stream and the distribution of tropical rainfall - and these changes can shift storm tracks and influence temperature and rainfall thousands of miles away.
What effects could a strong El Nino have?
The effects of El Nino are already being felt in parts of the world.
On Wednesday, an official from Indonesia's palm oil association GAPKI warned that the country's output of palm oil would be lower than expected due to prolonged dry conditions linked to the El Nino weather pattern.
For the UK, the Met Office expects El Nino to increase the chances of wetter, stormier conditions in autumn and early winter.
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That will all change in summer, however, as El Nino events also drive an increase in global average temperature, something that Dr Nick Dunstone says means "2027 is very likely to replace 2024 as the warmest year on record".
Professor Adam Scaife, head of long-range forecasting at the Met Office, said this year's El Nino was "an unprecedented event".
He added that: "I have never seen an El Nino signal this intense in our forecasts... if the forecast is accurate - and other forecast systems show similar extreme values - then 2026 will far exceed our recent experience of El Nino and its worldwide climate influences."
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