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Next year could be the first where the average global surface temperature is more than 1.5°C warmer than the pre-industrial era, according to a forecast by the UK’s Met Office.

“For the first time, we are forecasting a reasonable chance of a year temporarily exceeding 1.5°C,” says at the Met Office, which is the country’s national weather service.

In 2015, officials from around the world meeting in Paris agreed to try to prevent global temperatures exceeding 1.5°C above pre-industrial levels. “A temporary exceedance of 1.5°C won’t mean a breach of the Paris agreement,” says Dunstone. “But the first year above 1.5°C would certainly be a milestone in climate history.”

There is a 27 per cent chance of 2024 exceeding 1.5°C above the average temperature from 1850 to 1900, according to the forecast. “But I wouldn’t take that number too literally,” says Dunstone. “The chances could be higher.” That’s because 2023 was much hotter than the Met Office predicted for reasons that still aren’t fully understood.

For each of the 10 years prior to 2023, the average global surface temperature has been within the range forecast by the Met Office at the start of the year, says Dunstone. But for the record-smashing year of 2023, the average from January to October is 1.4°C, above the forecast of 1.1°C to 1.3°C made at the end of 2022.

This could be partly due to the ongoing El Niño becoming stronger than expected, says Dunstone. During El Niños, changing winds spread warm waters across the Pacific, temporarily warming the atmosphere.

The 2022 eruption of a submerged volcano in Tonga also injected lots of water vapour into the stratosphere, which has a warming effect. What’s more, the southern hemisphere in particular has been warmer than expected, for reasons that aren’t fully understood.

The forecast for 2024 starts from the currently observed level of warming, says Dunstone, but could be an underestimate if the same factors that made 2023 warmer than expected are still in play. The team can’t correct for these factors until they are sure what they are.

What is clear is that the long-term warming trend is a result of rising greenhouse gas emissions. While the years after 2024 could be cooler due to factors such as the ending of El Niño, it is thought that the long-term average is expected to pass 1.5°C in 2030. This is the generally agreed definition of a breach of the Paris agreement.

When it comes to single months, rather than years, the first on record that exceeded 1.5°C was January 2016, during the last strong El Niño. The month after was even warmer, with an anomaly of 1.64°C, making it the hottest so far. However, this November might exceed it.

Earlier this year, 17 November was the first day where the anomaly exceeded 2°C, according to provisional data. The long-term average is expected to pass 2°C of warming around the 2040s based on current trends.