What is El Nino and how can the Pacific Ocean climate cycle cause extreme weather?

Forecasters have declared the return of the El Nino weather pattern, increasing the likelihood the world will see a new heat record within the next two years.

El Nino is one of the three phases of a Pacific Ocean phenomenon known as the El Nino-Southern Oscillation (ENSO) that is closely tied to global temperatures.

El Nino is the hottest of the three cycles and US forecasters have now declared it has returned after a three-year period in a neutral phase.

Forecasters believe it will last until next spring.

The temperature cycle in the Pacific can affect weather in Britain and around the world with the hottest year on record, 2016, being an El Nino year.

Experts have warned that El Nino could see the crucial 1.5C temperature barrier breached as global temperatures soar. This refers to the global average temperature exceeding 1.5C above pre-industrial levels.

The 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.

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What is El Nino?

El Nino is the best-known part of El Nino-Southern Oscillation climate cycle, with El Nino events seeing the Pacific Ocean up to 3C warmer than normal.

During La Nina – the other part of the cycle – the ocean is up to 3C colder.

There is also occasionally a neutral phase.

Temperatures around the world increase by about 0.2C during El Nino, and fall about 0.2C during La Nina.

While 2022 was the fifth-hottest year on record, that was modulated by the fact that the Northern Hemisphere has seen three La Nina events in a row.

"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 told the BBC.

"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."

El Nino events can have knock-on effects on weather in the UK, with El Nino years linked to a risk of colder winters in Britain and heatwaves in summer.

The Met Office said: "El Nino is felt strongly in the tropical eastern Pacific with warmer than average weather.

"The effects of El Nino often peak during December. It's name, 'the boy', is thought to have originated as El Nino de Navidad centuries ago when Peruvian fishermen named the weather phenomenon after the newborn Christ."

What is the difference between El Nino and La Nina?

La Nina refers to the cooling of ocean surface temperatures coupled with winds and rainfall in the Pacific.

It can have knock-on effects on weather around the world, often having the opposite impact on the global climate as the better-known El Nino.

The Met Office said: "La Nina, or 'the girl', is the term adopted for the opposite side of the fluctuation, which sees episodes of cooler-than-average sea surface temperature in the equatorial Pacific.

"The conditions for declaring La Nina differ between different agencies, but during an event sea temperatures can often fall 3C to 5C below average. Cooler, drier than average weather is experienced in the tropical eastern Pacific."