Levels of carbon dioxide (CO2) and other major greenhouse gases in the atmosphere reached unprecedented highs in 2025, locking in more long-term temperature increase for the planet, according to a new report from the World Meteorological Organization (WMO).

The WMO Greenhouse Gas Bulletin reported that the globally averaged surface concentration of CO2 in 2025 was 426.8 parts per million, or 153% of the 1750 pre-industrial baseline. Concentrations of methane and nitrous oxide – the second and third most important long-lived greenhouse gases related to human activities – also rose to a new record.

“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. The past 11 years have been the warmest on record and this trend is set to continue. CO2 has a very long life-time and so will affect the global climate not just for years but for decades and even centuries,” said WMO Deputy Secretary-General Ko Barrett.

WMO releases the Bulletin, one of its flagship annual publications, to provide authoritative scientific information for negotiators at the UN Climate conference. The COP 31 meeting in Antalya, Türkiye, from 9-20 November is seeking to turn climate commitments into concrete actions.

CO2 growth rates tripled

The Bulletin reports on greenhouse gas concentrations – namely 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.

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.

The National Oceanic and Atmospheric Administration (NOAA) Annual Greenhouse Gas Index shows that from 1990 to 2025, radiative forcing – the warming effect on our climate – by long-lived greenhouse gases increased by 56%, with CO2 accounting for about 81% of this increase.

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. From 2024 to 2025, the global average concentration of CO2 rose by 2.8 ppm – above the decadal average but slightly less than the exceptional surge of 2023/2024.

“The more modest growth rate from 2024 to 2025 reflects recovery of the land carbon sinks and reduced fire carbon emissions in 2025 after the strong 2023/2024 El Nino, which reduced the efficiency of the biosphere through more drought, drier vegetation and more wildfires,” said WMO senior scientific officer Oksana Tarasova.

“However, a 2.8 ppm annual increase is still 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. The risks of reduced CO2 uptake by the ocean are also increasing in our changing climate,” said Oksana Tarasova.

Methane and nitrous oxide

Methane accounts for about 16% of the warming effect on our climate by long-lived greenhouse gases but it has a much shorter lifetime than CO2. Approximately 35% of methane is emitted into the atmosphere by natural sources (for example, wetlands) which are sensitive to climate as well, and about 65% comes from anthropogenic sources such as cattle, rice farming, fossil fuel exploitation, landfills and biomass burning.

The globally averaged methane concentration in 2025 was 1948 parts per billion (ppb) – or 267% of the 1750 level.

Nitrous oxide is the third most important long-lived greenhouse gas and comes from both natural sources and due to human activities such as biomass burning, fertilizer use and various industrial processes.

The globally averaged concentration of N2O reached 339.1 ppb in 2025, 126% above 1750.

Smart transition to clean energy

WMO’s Global Atmosphere Watch network monitors carbon dioxide, methane, nitrous oxide and other long-lived gases of potential concern.

As countries respond and take steps to 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).

SF6 has been rising steadily over the past three decades in proportion to increased global electrification, since SF6 is used primarily as a spark suppressant in electricity distribution.

It has an atmospheric lifetime of over 1000 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.

Hydrogen concentration has increased by 5.3% 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 an energy source for transport. Yet its leakage into the atmosphere from production facilities, pipelines, storage tanks, and fuel cells poses a growing concern. Hydrogen is not a greenhouse gas, but it influences atmospheric chemistry and so has a strong indirect effect on warming.

SF6 represents the challenge of irreversible, long-lived greenhouse gas accumulation from industrial processes, while H2 illustrates the subtle but potentially significant indirect climate effects of a gas used as an energy carrier. Both H2 leakage and SF6 persistence represent potential threats that could undermine the climate benefits of the energy transition. Careful monitoring of the composition of Earth’s atmosphere is essential to detect, understand and prevent unintended consequences of these changes.

WMO, through its Global Atmosphere Watch program, coordinates global observations of both gases and supports a stable and internationally recognized calibration scale for atmospheric SF6 and H2 measurements. The robust WMO-GAW framework allows for joint data utilization across different programs around the world, enabling researchers to confidently track changes in atmospheric SF6 and H2 concentrations and investigate changes in their sources and sinks.