A natural weather event known as El Niño has begun in the Pacific Ocean, likely adding heat to a planet already warming under climate change.
US scientists confirmed that El Niño had started. Experts say it will likely make 2024 the world's hottest year.
They fear it will help push the world past a key 1.5C warming milestone.
It will also affect world weather, potentially bringing drought to Australia, more rain to the southern US, and weakening India's monsoon.
The event will likely last until next spring, after which its impacts will recede.
For months, researchers have been increasingly confident that an El Niño event was set to emerge in the Pacific Ocean.
"It's ramping up now, there have been signs in our predictions for several months, but it's really looking like it will peak at the end of this year in terms of its intensity," said Adam Scaife, head of long-range predictions at the UK Met Office.
"A new record for global temperature next year is definitely plausible. It depends how big the El Niño turns out to be - a big El Niño at the end of this year, gives a high chance that we will have a new record, global temperature in 2024."
This natural phenomenon is the most powerful fluctuation in the climate system anywhere on Earth.
The El Niño Southern Oscillation, or ENSO, as it is properly called, has three different phases: Hot, cold or neutral.
The hot phase, called El Niño, occurs every two to seven years and sees warm waters come to the surface off the coast of South America and spread across the ocean pushing significant amounts of heat up into the atmosphere.
Record warm years, including 2016, the world's hottest on record, usually happen the year after a powerful El Niño event.
Weather agencies around the world use different criteria to decide when this hot phase is upon us.
For scientists in the US, their definition requires the ocean to be 0.5C hotter than normal for a month, the atmosphere must be seen to be responding to this heat and there must be evidence the event is persisting.
These conditions were met in the month of May. In a statement, US National Ocean and Atmospheric Administration (NOAA) said that "El Niño conditions are present".
"This is a very weak signal. But we believe that we're starting to see these conditions and that they will continue to intensify," said Michelle L'Heureux, a scientist with NOAA.
"Our weekly value is actually 0.8C this past week, which is even stronger."
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El Niño planet-warming weather phase has begun
Storms, droughts and record high temperatures lie ahead as US scientists confirm that El Niño has arrived.
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