By Esme Stallard & Mark PoyntingBBC News Climate & Science
Getty ImagesIt has been confirmed that 2023 was the hottest year on record, driven by human-caused warming but also boosted by a natural weather system called El Niño.
The continuing effects of El Niño mean that 2024 could see even higher temperatures.
What is El Niño?
El Niño is part of the natural climate phenomenon called the El Niño Southern Oscillation (ENSO).
It has two opposite states: El Niño and La Niña, both of which significantly alter global weather.
An El Niño event is typically declared when sea surface temperatures in the tropical eastern Pacific rise to at least 0.5C above the long-term average.
![Sea surface temperatures across the Pacific Ocean in May 2023 compared to the 1991-2020 average in May. Much higher than average sea surface temperatures are present in the East Pacific off the coast of South America. [June 2023]](https://cdn.statically.io/img/ichef.bbci.co.uk/news/240/cpsprodpb/BFE3/production/_130032194_sst_anomaly_may23-2x-nc.png.webp)
In normal conditions, surface water in the Pacific Ocean is cooler in the east and warmer in the west.
The "trade winds" tend to blow east-to-west, and heat from the Sun progressively warms the waters as they move in this direction.
During El Niño events, these winds weaken or reverse, sending warm surface waters eastwards instead.
In La Niña periods, the normal east-to-west winds become stronger, pushing warmer waters further west.
This causes cold water to rise up - or "upwell" - from the depths of the ocean, meaning sea surface temperatures are cooler than usual in the east Pacific.
The phenomenon was first observed by Peruvian fisherman in the 1600s, who noticed that warm waters seemed to peak near the Americas in December.
They nicknamed it "El Niño de Navidad" - Christ Child in Spanish.
How do El Niño and La Niña change the weather?
Not every ENSO event is the same, and the consequences vary from region to region. However, scientists have observed some common effects:
Temperatures
Global temperatures typically increase during an El Niño episode, and fall during La Niña.
El Niño means warmer water spreads further, and stays closer to the surface. This releases more heat into the atmosphere, creating wetter and warmer air.
But the regional effects are complicated, and some places may be both warmer and cooler than expected at different points in the year.
The hottest year on record, 2023, was boosted by El Niño conditions, on top of long-term human-caused warming.
El Niño could mean that 2024 is even hotter, the UK Met Office says, although this is far from certain.
Air temperatures typically peak a few months after El Niño hits maximum strength, as heat escapes from the sea surface to the atmosphere.
In early January 2024, El Niño was considered to be close to maximum strength, but it is expected to weaken in the coming months, according to the US science body NOAA.
![Typical effects of El Nino events on temperature patterns for each region. Key trends include warming in South America, southeast Asia and southern Africa. Cooling in parts of North America. [June 2023]](https://cdn.statically.io/img/ichef.bbci.co.uk/news/240/cpsprodpb/15C23/production/_130032198_el_nino_temperature_impacts_640-nc-2x-nc.png.webp)
Between 2020 and 2022, the world experienced an unusually long period of La Niña conditions, which helped keep a lid on global temperatures.
How UK and Western European temperatures respond to El Niño is complicated and uncertain. It could mean colder than average winters, for example, but that depends on how El Niño unfolds.
Changes to rainfall
During El Niño events, the warmer water pushes the Pacific jet stream's strong air currents further to the south and the east.
This brings wetter weather to southern USA and the Gulf of Mexico.
Tropical regions like southeast Asia, Australia and central Africa typically experience drier conditions.
Under La Niña, the effect is reversed.
![Typical effects of El Nino events on precipitation patterns for each region. Key trends are drying in many equatorial regions (northern South America, central Africa, southeast Asia and Australia). Southern USA generally becomes wetter than normal. [June 2023]](https://cdn.statically.io/img/ichef.bbci.co.uk/news/240/cpsprodpb/E6F3/production/_130032195_el_nino_precipitation_impacts_640-nc-2x-nc.png.webp)
Tropical storms
El Niño also affects atmospheric circulation patterns, which means there are generally more tropical storms in the tropical Pacific, but fewer in the tropical Atlantic, including the southern US.
During La Niña, the reverse is typically true.
Carbon dioxide (CO2) levels
Scientists have also observed that CO2 levels in the atmosphere increase during El Niño events, possibly as a result of warmer and drier conditions in tropical regions.
If plants grow less quickly due to drought, they absorb less CO2, while more wildfires in places like South Asia mean more CO2 is released.
Why do these climate patterns matter?
The extreme weather events worsened by El Niño and La Niña affect infrastructure, food and energy systems around the world.
For example, when less cold water comes to the surface off the west coast of South America during El Niño events, fewer nutrients rise from the bottom of the ocean.
That means there is less food available for marine species like squid and salmon, in turn reducing stocks for South American fishing communities.
Getty ImagesFishing stocks can be affected by the effects of El NiñoThe droughts and flooding caused by the extreme 2015-16 El Niño event affected the food security of more than 60 million people, according to the UN Food and Agricultural Organisation.
A recent study suggested that El Niño events significantly reduce global economic growth, an effect which could intensify in the future.
How often do El Niño and La Niña episodes happen?
El Niño and La Niña episodes typically occur every two to seven years, and usually last nine to 12 months.
They don't necessarily alternate: La Niña events are less common than El Niño episodes.
Is climate change affecting El Niño/La Niña?
In 2021, the UN's climate scientists, the IPCC, said the ENSO events which have occurred since 1950 are stronger than those observed between 1850 and 1950.
But it also said that tree rings and other historical evidence show there have been variations in the frequency and strength of these episodes since the 1400s.
The IPCC concluded there is no clear evidence that climate change has affected these events.
Some climate models suggest that El Niño events will become more frequent and more intense as a result of global warming - potentially boosting temperatures further - but this is not certain.
Graphics by Visual Journalism team.
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