The state of greenhouse gases in the atmosphere using global observations through 2024

This paper highlights the main findings of the twenty-first annual Greenhouse Gas Bulletin (https://library.wmo.int/idurl/4/69654) of the World Meteorological Organization (WMO). The results are based on research and observations performed by laboratories contributing to the WMO Global Atmosphere Watch (GAW) Programme (https://community.wmo.int/site/knowledge-hub/programmes-and-initiatives/global-atmosphere-watch-programme-gaw).

The Bulletin presents global analyses of observational data collected according to GAW recommended practices (https://library.wmo.int/idurl/4/69672) and submitted to the World Data Center for Greenhouse Gases (WDCGG). Bulletins are prepared by the WMO/GAW Scientific Advisory Group on Greenhouse Gases in collaboration with WDCGG.

Observations used for the global analysis are from 179 marine and terrestrial sites for CO₂, 171 for CH₄ and 123 for N₂O. The globally averaged surface mole fractions calculated on the basis of these observations reached new highs in 2024, with CO₂ at 423.9±0.2 ppm, CH₄ at 1942±2 ppb and N₂O at 338.0±0.1 ppb. These values constitute, respectively, 152%, 266% and 125% of pre-industrial (before 1750) levels. The record increase in CO₂ from 2023 to 2024 (3.5 ppm) was most likely due to a combination of natural variability and continued emissions of fossil fuel CO₂. For CH₄, the increase from 2023 to 2024 was lower than that observed from 2022 to 2023 and also lower than the average annual growth rate over the last decade (2014–2023). For N₂O, the increase from 2023 to 2024 was lower than that observed from 2022 to 2023 and slightly lower than the average annual growth rate over the last decade.

The increase of CO₂ in the global surface atmosphere by 3.5 ppm in 2024 was the largest one-year increase in the modern measurement record, exceeding the previous record of 3.3 ppm from 2015 to 2016 and surpassing the increase of 2.4 ppm from 2022 to 2023 by a large margin. Global fossil CO₂ emissions were almost static during 2023–2024 at the record level of 10.2 ± 0.5 GtC/yr. The global terrestrial ecosystems and global oceans are likely responsible for the additional 1.1 ppm/yr (equivalent to 2.34 GtC) in CO₂ growth compared to 2022–2023. Wildfire emissions in the Americas reached historic levels in 2024 and could have contributed to the record CO₂ annual increase.

Current CO₂ emissions to the atmosphere not only impact the global climate today but will continue to do so for millennia, and ongoing CO₂ emissions will ensure that warming continues indefinitely. The removal of anthropogenic CO₂ from the atmosphere depends on exchanges among reservoirs on timescales ranging from years (surface ocean) to hundreds of thousands of years (weathering). The slowed uptake of anthropogenic CO₂ emissions within the global carbon cycle is exacerbated by the slow uptake of heat by the deep oceans, so once CO₂ is emitted to the atmosphere, it affects climate indefinitely. This is different from CH₄, whose atmospheric lifetime is about nine years due to its removal by chemical oxidation. While reducing CH₄ emissions is useful and necessary, climate action urgently needs to focus on reducing fossil fuel CO₂ emissions, which represent the vast majority of overall greenhouse gas emissions.