What is El Niño and why could it mean record temperatures?
A natural weather pattern called El Niño - which could bring extreme weather to many parts of the world - has begun, US scientists say.
The National Oceanic and Atmospheric Administration (NOAA) said that El Niño conditions are likely to strengthen over the rest of 2026. Many forecasts suggest it could be one of the strongest El Niños ever recorded.
Coming on top of decades of human-caused warming, 2027 may be the hottest year on record, with disruption to weather, food supplies and economies.
What happens during an El Niño and why could this one be strong?
An El Niño develops in the Pacific Ocean and the atmosphere above it.
When the winds that typically blow east-to-west weaken or reverse, warmer water can spread across the central and eastern tropical Pacific.
Scientists at NOAA announced that a new El Niño phase had started after observing sea surface temperatures more than 0.5C above average in the central tropical Pacific.
They also noticed a switch in atmospheric conditions, with a drop in pressure over the central Pacific compared with the western Pacific.
The Japanese Meteorological Agency also said that El Niño conditions are present.
Some scientists have warned that this El Niño could be especially strong, partly because the water beneath the surface of the Pacific is unusually warm.
These waters have been about 6C above average in places, according to the UN's World Meteorological Organization.
Deep-sea heat is often followed by warmer waters at the surface.
A "very strong" or so-called "super" El Niño is when the warming of the central tropical Pacific Ocean surface reaches 2C or more over an extended period. These events have only happened a few times since 1950.
The NOAA said there was a 63% chance of this El Niño ending up "very strong". That would "rank among the largest El Niño events in the historical record going back to 1950", it added.
It is expected to last at least into early 2027.
The El Niño phenomenon was first observed by Peruvian fishermen in the 1600s, who nicknamed it El Niño de Navidad - Christ Child in Spanish.
How could a strong El Niño affect the weather?
A strong El Niño event would "exacerbate drought and heavy rainfall and increase the risk of heatwaves both on land and in the ocean", said World Meteorological Organization secretary general Celeste Saulo.
During El Niño, the ocean will transfer heat to the air, making it warmer.
Coupled with higher global temperatures from human-caused climate change, this could make 2027 the hottest year on record for the planet.
The exact impact on the weather depends on where you are and what time of year it is.
No two El Niños are the same, but a strong event typically fuels hot, dry weather in parts of South America, South East Asia and Australia, raising the chances of droughts and wildfires.
It can also weaken the Indian monsoon. In the southern US, heavier rainfall can increase the risks of flooding.
El Niño tends to bring more tropical storms in the eastern and central Pacific, but fewer in the tropical Atlantic, including the south-east US.
The way UK weather is affected is complicated, and can vary. But El Niño may increase the chance of a mild start and cold end to UK winter, according to the Met Office.
What impact could El Niño have on people?
The UN's secretary general, António Guterres, has warned the world to prepare.
"El Niño conditions will pour fuel on the fire of a warming world. Impacts will hit even harder, travel even farther, and cross borders with devastating speed," he said.
Droughts in parts of South America and South East Asia could hit crops at a time when the effective closure of the Strait of Hormuz is already disrupting the distribution of fertiliser. This could mean smaller harvests, reduced food supply and higher prices.
For fishing communities in South America, there is the risk of smaller catches. During El Niño, less cold, nutrient rich water comes to the surface, reducing food availability for marine species such as anchovies.
Some scientists are drawing comparisons with the 2015-16 El Niño, one of the strongest ever recorded.
At that time there were water shortages in the Caribbean, a record-breaking storm season in the central Pacific, and drought in the Horn of Africa.
The combination of storm events and widespread drought - at least partly the result of El Niño - led to food shortages affecting millions of people around the world, according to the UN's Food and Agricultural Organization.
s climate change affecting El Niño?
El Niño events since 1950 have been stronger than those between 1850 and 1950, according to the UN's climate scientists, the IPCC.
But it said that tree rings and other historical evidence show there have been variations in their frequency and strength since the 1400s.
The IPCC said there is no clear evidence that climate change has affected El Niño events.
Some climate models suggest that El Niño episodes could become more frequent and more intense as a result of global warming - although this is a complex and uncertain area of science with no clear consensus.
But the impacts of El Niño will occur on top of those from long-term climate change - which could fuel increasingly severe weather extremes.
What is La Niña?
El Niño has a sister weather pattern called La Niña.
During these events, the temperature of the surface of the sea in the central-eastern Pacific is cooler - the opposite of what is seen during El Niño.
Atmospheric pressure is also higher than usual in the central Pacific, and lower than usual in the west.
La Niña typically brings wetter conditions to parts of Australia, Indonesia and equatorial South America, and drier conditions to the southern US.
El Niño and La Niña often alternate, but sometimes we can have two of the same event back to back. On average, they occur every two to seven years.
bbc.com
