Atmospheric concentrations of the three main greenhouse gases reached record highs in 2025, according to the Greenhouse Gas Bulletin released by the World Meteorological Organization (WMO). The annual average for carbon dioxide reached 426.8 parts per million (ppm) — 153% of the 1750 level, taken as the pre-industrial baseline.
The figure is the average of measurements at 179 monitoring stations around the world. When the bulletin was first published, in 2004, the annual average was 377.5 ppm.
The 2025 numbers
- Carbon dioxide (CO2): 426.8 ppm, or 153% of the pre-industrial level
- Methane (CH4): 1,948 parts per billion (ppb), or 267% of the pre-industrial level
- Nitrous oxide (N2O): 339.1 ppb, or 126% of the pre-industrial level
CO2 rose by 2.8 ppm from 2024 to 2025. That is above the decade's average, though slightly below the jump recorded between 2023 and 2024. The gas's growth rate has tripled since the 1960s: it went from about 0.8 ppm per year to 2.4 ppm per year on average between 2011 and 2020.
Broken down by gas, the warming accumulated since pre-industrial times comes 66% from CO2, 16% from methane, 8% from CFCs, 6% from nitrous oxide and 2% each from HCFCs and HFCs. Methane sources are about 35% natural — wetlands, mainly — and 65% human, between livestock, rice cultivation, fossil fuels, landfills and biomass burning.
Drawing on data from the US agency NOAA, the WMO calculates that radiative forcing from long-lived greenhouse gases — the measure of how much heat they trap — grew 56% between 1990 and 2025, and that CO2 accounts for about 81% of that increase.
The gas nobody sees that lasts a thousand years
The bulletin records that sulphur hexafluoride (SF6) had its largest annual increase on record in 2025. The gas is used mainly in electricity distribution equipment, remains in the atmosphere for more than a thousand years and has a 100-year global warming potential roughly 24,300 times that of CO2. It is a small volume with a disproportionate effect — and one that grows alongside the expansion of power grids.
Hydrogen (H2) concentrations, in turn, rose 5.3% between 2010 and 2024. Hydrogen is not a greenhouse gas, but it causes indirect warming by altering the chemistry of the atmosphere, and leakage from energy systems is flagged by the WMO as a growing concern.
What comes in 2026
"Greenhouse gases are trapping record heat," said Ko Barrett, WMO Deputy Secretary-General, noting that the CO2 already emitted will affect the climate for decades and centuries.
Oksana Tarasova, a senior scientific officer at the organization, attributed the slower growth between 2024 and 2025 to the recovery of land carbon sinks and to the drop in fire emissions after the 2023–2024 El Niño. She warned, however, that concentrations are likely to rise further in 2026, driven by fossil fuel burning and a developing El Niño, and that there is a growing risk the oceans will lose capacity to absorb CO2.
The bulletin is published to inform negotiators at COP31, the UN climate conference, scheduled for Antalya, Türkiye, from 9 to 20 November.
With information from the World Meteorological Organization, Phys.org and Down To Earth.
