Carbon dioxide is building up in the atmosphere faster than what is needed to limit global temperatures to 1.5C above pre-industrial levels, the Met Office has said.
This limit is a key target of the Paris Agreement made in 2015, though continued emissions since then have made it increasingly difficult to achieve.
The Met Office forecasts that CO2 levels will peak at 426.6 parts per million (ppm) in May, higher than at any point in the last two million years.
CO2 levels swing between seasons, with plants in the northern hemisphere absorbing the gas as they grow through spring before dropping their decaying leaves in the autumn which then releases CO2 back into the atmosphere.
The Met Office said that without plants and oceans absorbing CO2, the increase in atmospheric levels would be almost twice as much.
El Nino, which brings hotter sea surface and atmospheric temperatures in multi-year cycles, has reduced the ocean’s ability to absorb as much CO2 as usual, adding to the high concentrations.
Professor Richard Betts, who authored the Met Office forecast, said: “This year’s estimated rise in atmospheric carbon-dioxide concentration is well above all three 1.5C-compatible scenarios highlighted in the IPCC report.
“Even when we compensate for the temporary effects of El Nino, we find that human-induced emissions would still cause the CO2 rise in 2024 to be on the absolute limits of compliance with the 1.5C pathways.”
The rise of CO2 in the atmosphere is entirely down to human beings burning fossil fuels, producing cement and removing forests that would otherwise absorb it.
An observatory in Mauna Loa in Hawaii has been measuring atmospheric CO2 since 1958 and has tracked curbs in its build up that have aligned with reductions in human emissions, such as the global pandemic in 2020.
Prof Betts said: “The atmospheric CO2 rise has accelerated over the last six decades.
“If global warming is to stay below 1.5C, the rate of build-up of CO2 in the atmosphere would need to slow substantially in the coming years, and halt altogether before mid-century. The forecast for 2024 does not show such a slowing.”
The current level of warming is estimated to be around 1.3C above pre-industrial levels, the Met Office said.
Crossing the 1.5C boundary will happen after it becomes the average over a period of around 20 years, so any individual year above that would not mean that the Paris Agreement has failed in its aim.
Last year was confirmed to be the hottest year on record by a large margin, with each successive month from June being the hottest corresponding month.
The Met Office predicts that 2024 could be even hotter because of El Nino adding to the warming caused by greenhouse gas emissions.
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