The state of greenhouse gases in the atmosphere using global observations through 2021

This paper highlights the main findings of the eighteenth annual Greenhouse Gas Bulletin (https://library.wmo.int/index.php?lvl=notice_display&id=22149) 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 more than 100 marine and terrestrial sites worldwide for CO₂ and CH₄ and at a smaller number of sites for other greenhouse gases. The globally averaged surface mole fractions calculated from this in situ network reached new highs in 2021, with CO₂ at 415.7 ± 0.2 ppm, CH₄ at 1908 ± 2 ppb and N₂O at 334.0 ± 0.1 ppb. These values constitute, respectively, 149%, 262% and 124% of pre-industrial (before 1750) levels. The increase in CO₂ from 2020 to 2021 was equal to that observed from 2019 to 2020 and larger than the average annual growth rate over the last decade. For CH₄, the increase from 2020 to 2021 was higher than that observed from 2019 to 2020 and considerably higher than the average annual growth rate over the last decade. For N₂O, the increase from 2020 to 2021 was slightly higher than that observed from 2019 to 2020 and also higher than the average annual growth rate over the last decade.

The Bulletin highlights the exceptional growth of CH₄ in 2020 and 2021. The causes of these exceptional increases are still being investigated though analyses of measurements of atmospheric CH₄ abundance and its stable carbon isotope ratio ¹³C/¹²C indicate that the increase in CH₄ since 2007 is associated mostly with biogenic processes, but the relative contributions of anthropogenic and natural sources to this increase are unclear.

The Bulletin further highlights that the accuracy of emissions estimates from atmospheric measurements depends on the geometry of the surface network, pointing to the large observational gaps in tropical regions and the interior of the Asian continent. The tropics accommodate not only highly uncertain emissions from natural wetlands, but also the atmospheric hydroxyl radical sink of CH₄, which is largest there. Surface measurements provide limited information to distinguish between increasing surface emissions and decreasing atmospheric sinks, which could both explain the increasing atmospheric CH₄ abundance.

WMO is working with the broader greenhouse gas community to develop a framework for sustained, internationally coordinated global greenhouse gas monitoring. These efforts are envisaged to result in an internationally coordinated approach to observing network design and acquisition, international exchange and use of the observations. It is foreseen that this will result in the expansion of the in-situ network, especially in currently undersampled regions and lead to reduced uncertainties in the quantification of the net atmospheric balance of CH₄ and other greenhouse gases.