UN: Carbon Dioxide Hit 426.8 ppm in 2025 as All Three Major Greenhouse Gases Set Records

Greenhouse gas concentrations in the atmosphere rose to record highs in 2025, locking in climate impacts that will persist for centuries, the United Nations said Tuesday. The World Meteorological Organization said the three main greenhouse gases, carbon dioxide, methane and nitrous oxide, all reached unprecedented levels, which will drive more long-term temperature increases.

"Another year, another new record," said WMO Deputy Secretary-General Ko Barrett. "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." She noted that the past 11 years have been the warmest on record and that the trend is set to continue.

The agency's 22nd annual Greenhouse Gas Bulletin measures concentrations, the amount of each gas in the atmosphere, rather than annual emissions. In 2025 carbon dioxide stood at 426.8 parts per million, methane at 1,948 parts per billion and nitrous oxide at 339.1 parts per billion. That is 153%, 267% and 126% of pre-industrial levels from before 1750. Carbon dioxide accounts for about 66% of the warming effect on the climate, and around half of the CO2 emitted each year stays in the atmosphere while the rest is absorbed by land and oceans.

The global average concentration of CO2 rose by 2.8 ppm from 2024 to 2025. That is smaller than the record 3.6 ppm jump from 2023 to 2024, but still above the average growth rate of the past decade, 2.6 ppm. In the 1960s the figure was 0.8 ppm. WMO senior scientific officer Oksana Tarasova said the smaller rise reflected the recovery of land carbon sinks and reduced fire emissions in 2025 after the strong 2023-2024 El Nino. "However, a 2.8 ppm annual increase is still a substantial addition of carbon to the atmosphere from fossil fuel burning," she said.

Tarasova warned that concentrations should climb further in 2026 because of continued fossil fuel burning and the development of a powerful El Nino, which will affect land carbon sinks as the last one did. Barrett stressed that carbon dioxide's long lifetime means it will shape the global climate "not just for years but for decades and even centuries."

Methane accounts for about 16% of the warming effect of long-lived greenhouse gases but has a much shorter lifetime than CO2. Around a third of methane released comes from natural sources such as wetlands, while the rest comes from human sources including cattle, rice farming, landfills and biomass burning. The bulletin said that as countries move their energy systems toward decarbonization, more attention should be paid to two potent but less abundant gases. One is hydrogen, whose levels have risen over the past decade for reasons that are not known, though it has begun to be used as an energy storage medium. The other is sulfur hexafluoride, used mainly as a spark suppressant in electricity distribution.

The bulletin lands ahead of the COP31 U.N. climate summit in Antalya, Turkey, which runs from Nov. 9 to Nov. 20.