The state of greenhouse gases in the atmosphere using global observations through 2023

This paper highlights the main findings of the twentieth annual Greenhouse Gas Bulletin (https://library.wmo.int/records/item/69057-no-20-28-october-2024) 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/activity-areas/gaw).

The Bulletin presents global analyses of observational data collected according to GAW recommended practices 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 146 marine and terrestrial sites for CO₂, 153 for CH₄ and 112 for N₂O. The globally averaged surface mole fractions calculated on the basis of these observations reached new highs in 2023, with CO₂ at 420.0±0.1 ppm, CH₄ at 1934±2 ppb and N₂O at 336.9±0.1 ppb. These values constitute, respectively, increases of 151%, 265% and 125% relative to pre-industrial (before 1750) levels. The increase in CO₂ from 2022 to 2023 (2.3 ppm) was slightly higher than the increase observed from 2021 to 2022 and slightly lower than the average annual growth rate over the last decade, which was most likely partly caused by natural variability, as fossil fuel CO₂ emissions have continued to increase. This increase marked the twelfth consecutive year with an increase greater than 2 ppm.

The Bulletin reports that within-year variability of CO₂ was 2.8 ppm in 2023, the fourth largest within-year annual increase since modern CO₂ measurements started in the 1950s. Such increase may be a result of enhanced fire emissions and reduced net terrestrial carbon sinks. The CO₂ growth rate varies from year to year (between 2.1 and 3.2 ppm during 2014-2023), with the variability mostly driven by the terrestrial biosphere exchange of CO₂, as confirmed by measurements of the stable carbon isotopes ratio, ¹³C:¹²C in atmospheric CO₂. Coincidental with the large CO₂ increase during 2023 was the largest increase in atmospheric carbon monoxide (CO) in the past two decades, suggesting enhanced CO₂ emissions from fires.

The increase of CH₄ mole fraction from 2022 to 2023 (11 ppb) was lower than that observed from 2021 to 2022 but still slightly higher than the average annual growth rate over the last decade. The record rise in atmospheric CH₄ from 2020 to 2022 was accompanied by a significant drop in atmospheric δ¹³C_CH4. The unexpected change in the amount of atmospheric δ¹³C_CH4 is best explained by a transition from fossil fuels to microbial emissions as the dominant driver of increasing CH₄. Moreover, the geographic distribution of CH₄ growth from 2020 to 2022 suggest strong increases in isotopically light emissions from tropical and boreal wetland areas, which is indicative of positive climate feedback on CH₄ emissions in response to climate transition to an El Niño phase in 2023.

In the near future, climate change itself could cause ecosystems to become larger sources or sinks of GHGs. Identifying and tracking the potential climate feedbacks require continued high accuracy observations also at currently undersampled regions.