Atmospheric carbon dioxide reached a global average of 426.8 parts per million in 2025, the World Meteorological Organization (WMO) said, underscoring the scale of greenhouse gas buildup driving long-lasting climate effects.
The WMO's annual Greenhouse Gas Bulletin put the 2025 average at 153% of its estimated 1750 preindustrial concentration. Methane measured 1,948 parts per billion and nitrous oxide 339.1 ppb, both record highs. The reported estimates carry uncertainty margins of ±0.2 ppm for carbon dioxide, ±2 ppb for methane and ±0.1 ppb for nitrous oxide, the bulletin says.
Atmospheric CO2 increased by 2.8 ppm between 2024 and 2025. That gain exceeded the mean yearly rise across 2016-25 but remained below the large increase from 2023 to 2024.
Oksana Tarasova, a WMO senior scientific officer, said land ecosystems recovered some of their ability to absorb carbon, while fire-related emissions declined in 2025 after impacts associated with the strong 2023-24 El Niño. Fossil fuel combustion continued to add substantial CO2, she said. Tarasova expects atmospheric concentrations to rise again in 2026, pointing to ongoing fossil fuel emissions and a powerful El Niño that may affect how much carbon land ecosystems take up.
The bulletin records atmospheric buildup, not emissions released during a particular year. Land ecosystems and oceans absorb about half of annual carbon dioxide output. The measure is distinct from the United Nations Environment Programme (UNEP)'s country-level estimates of emissions changes in 2025, which tracked emissions rather than accumulated concentrations.
Ko Barrett, WMO deputy secretary-general, said carbon dioxide's long atmospheric persistence means its climate influence can last for decades or centuries. She said greenhouse gas warming is adding weather-related risks for economies and communities.
COP31 is scheduled for Nov. 9-20 in Antalya, Türkiye. WMO said negotiators can use the bulletin's updated measurements at the climate conference.
