Concentrations of the three main long-lived greenhouse gases in the atmosphere reached new record highs in 2025, the World Meteorological Organization (WMO) said on Tuesday, October 27, warning that the build-up locks in further long-term warming. The findings come in the agency’s annual Greenhouse Gas Bulletin, released ahead of the COP31 United Nations climate conference.

The globally averaged surface concentration of carbon dioxide (CO2) was 426.8 parts per million (ppm) in 2025, or 153% of the pre-industrial level of 1750, according to the bulletin. Methane reached 1,948 parts per billion (ppb), 267% of the 1750 level, and nitrous oxide reached 339.1 ppb, about 26% above pre-industrial levels.

“Another year, another new record. Greenhouse gases are trapping an unprecedented amount of heat in the atmosphere,” said Ko Barrett, the WMO’s deputy secretary-general. According to the WMO, the past 11 years have been the warmest on record.

Growth slowed slightly from 2024’s surge

CO2 concentrations rose by 2.8 ppm between 2024 and 2025, the WMO said. That was slightly less than the exceptional jump between 2023 and 2024 but above the average annual growth of 2.4 ppm recorded in the decade from 2011 to 2020. In the 1960s, concentrations rose by about 0.8 ppm a year, meaning the growth rate has roughly tripled. When the first bulletin was published in 2004, the annual average was 377.5 ppm.

The WMO attributed the slightly slower rise to a recovery of land carbon sinks, the land ecosystems that absorb part of the CO2 emitted into the atmosphere, and to lower carbon emissions from fires in 2025. During the 2023–2024 El Niño, drought and drier vegetation had reduced how efficiently land absorbed carbon and had increased wildfires.

“The more modest growth rate from 2024 to 2025 reflects recovery of the land carbon sinks,” said Oksana Tarasova, a senior scientific officer at the WMO. She added: “We can expect to see a further increase in concentrations in 2026 because of fossil fuel burning.” The agency also pointed to the development of a powerful El Niño, which it said will affect land carbon sinks as it did in 2023–2024, and said that “the risks of reduced CO2 uptake by the ocean are also increasing in our changing climate.”

Where the warming effect comes from

According to the WMO, citing the Annual Greenhouse Gas Index of the U.S. National Oceanic and Atmospheric Administration (NOAA), radiative forcing by long-lived greenhouse gases, their warming effect on the climate, rose by 56% between 1990 and 2025. CO2 accounted for about 81% of that increase.

Since pre-industrial times, CO2 has contributed 66% of the increase in radiative forcing, methane 16%, nitrous oxide 6%, chlorofluorocarbons (CFCs) 8%, and hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) 2% each, the WMO said. About 35% of methane comes from natural sources such as wetlands and about 65% from human activities including cattle, rice farming, fossil fuel exploitation, landfills and biomass burning. Nitrous oxide comes from sources including biomass burning, fertilizer use and industrial processes.

The bulletin also reported a record increase in sulfur hexafluoride (SF6), an industrial gas that stays in the atmosphere for more than 1,000 years and has a warming potential over 100 years about 24,300 times that of CO2. Concentrations of molecular hydrogen rose 5.3% between 2010 and 2024.

How the data are gathered

The bulletin is an annual analysis by the WMO’s Global Atmosphere Watch program, which coordinates observations of greenhouse gases and other atmospheric constituents. The CO2 figures are based on observations from 179 stations, and the data are archived at the World Data Centre for Greenhouse Gases, hosted by the Japan Meteorological Agency. The bulletin is an agency report based on measurement networks rather than a paper in a peer-reviewed journal.

Why it matters

Because CO2 remains in the atmosphere for a very long time, the WMO said, the levels already reached will affect the climate for decades and even centuries. Even with the slower rate of increase in 2025, concentrations of all three main gases still reached new highs.

The WMO describes the bulletin as a flagship publication intended to give scientific information to negotiators at the UN climate conference. COP31 is scheduled to take place in Antalya, Türkiye, from November 9 to 20, 2026.

What to watch

The WMO expects concentrations to rise further in 2026, driven by fossil fuel burning, and said the developing El Niño could again weaken land carbon sinks.

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