Record carbon dioxide levels will impact us for centuries, says UN weather agency

This article is published in association with United Nations.

Levels of carbon dioxide (CO2) and other major greenhouse gases in the atmosphere caused by burning fossil fuels reached record highs in 2025, locking in more long-term global warming, the World Meteorological Organization (WMO) said on Tuesday.

The agency’s latest Greenhouse Gas Bulletin indicates an average global concentration of CO₂ at 426.8 parts per million (ppm) last year, or 153 per cent of the 1750 pre-industrial baseline.

“Every additional part per million matters because it traps additional heat. In effect, we are supercharging the ocean and the atmosphere. That means that heat waves, tropical cyclones, and rainstorms are intensifying faster than our historical baselines prepared us for,” WMO Deputy Secretary-General Ko Barrett told journalists in Geneva.

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Equally worrying, levels of methane and nitrous oxide – the second and third most important long-lived greenhouse gases caused by human activities – also hit a new peak in 2025.

“Another year, another new record. Greenhouse gases are trapping an unprecedented amount of heat in the atmosphere, providing energy for more extreme weather and associated risks to our economies and societies,” Ms. Barrett stressed.

“The past 11 years have been the warmest on record and this trend is set to continue. CO₂ has a very long lifetime and so will affect the global climate not just for years but for decades and even centuries.”

When the Bulletin was first published in 2004, the annual average level of CO₂ measured by WMO’s Global Atmosphere Watch network of monitoring stations was 377.5 ppm, compared with nearly 427 ppm today.

El Niño accelerator

Growth rates of CO₂ have tripled since the 1960s, accelerating from an annual average increase of 0.8 ppm per year to 2.4 ppm per year in the decade from 2011 to 2020.

The “exceptional [3.5 ppm] surge” of 2023-2024 coincided with a strong El Niño “which reduced the efficiency of the biosphere through more drought, drier vegetation and more wildfires”, said WMO senior scientific officer Oksana Tarasova.

She noted that although the 2.8 ppm annual increase from 2024-2025 represented a more modest increase, it was nonetheless “a substantial addition of carbon to the atmosphere from fossil fuel burning. We can expect to see a further increase in concentrations in 2026 because of fossil fuel burning and the development of a powerful El Niño which will impact land carbon sinks as it did in 2023-2024.”

CO₂ growth rates triple

The agency’s Bulletin reports on the amount of gas that accumulates in the atmosphere and not new annual emissions. About half of the total CO₂ emitted each year remains in the atmosphere and the rest is absorbed by Earth’s land ecosystems and oceans.

The U.S. National Oceanic and Atmospheric Administration (NOAA) Annual Greenhouse Gas Index shows that from 1990 to 2025, the warming effect on our climate caused by greenhouse gases increased by 56 per cent, with CO₂ accounting for about 81 per cent of this increase, known as “radiative forcing”.

Energy transformation concerns

WMO noted that as countries transform their energy systems towards decarbonization, careful attention should be given to two potent but less abundant trace gases closely related to electricity production and distribution: molecular hydrogen (H₂) and sulphur hexafluoride (SF₆).

Widely used as a spark suppressant in electricity distribution, SF₆ use has risen steadily in the past three decades in line with increased global electrification. “It has an atmospheric lifetime of over 1,000 years and has the highest 100-year global warming potential of approximately 24,300 times that of CO₂. The increase of SF₆ in 2025 was a record high,” the agency’s report notes.

Hydrogen concentration has increased by 5.3 per cent between 2010 and 2024. Hydrogen is being used as an energy storage medium for hard-to-decarbonize sectors such as steelmaking, shipping, and long-duration energy storage, as well as for renewable sources and as an energy source for transport.

“Its leakage into the atmosphere from production facilities, pipelines, storage tanks, and fuel cells poses a growing concern,” WMO said, highlighting that although hydrogen is not a greenhouse gas, “it influences atmospheric chemistry” and has a strong indirect effect on warming, the agency explained.

“Both H₂ leakage and SF₆ persistence represent potential threats that could undermine the climate benefits of the energy transition,” WMO said. “Careful monitoring of the composition of Earth’s atmosphere is essential to detect, understand and prevent unintended consequences of these changes.”.

WMO releases the Bulletin to provide authoritative scientific data for negotiators attending the annual UN Climate conference. The aim of the Conference of the Parties meeting (COP31) in Antalya, Türkiye, from 9 to 20 November is to turn climate commitments into concrete action.