The Greenhouse Gas Bulletin has been published annually since 2004, this publication presents the latest analysis of observations from the WMO Global Atmosphere Watch (GAW) Programme on concentrations of long-lived greenhouse gases in the atmosphere for 2023.

The Bulletin reports globally averaged surface mole fractions of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). It compares these values to those of the previous year and pre-industrial levels. Additionally, it provides insights into changes in radiative forcing — the warming effect on the atmosphere by long-lived greenhouse gases and details the contribution of individual gases to this effect. The Greenhouse Gas Bulletin is released yearly to inform the United Nations Climate Change negotiations, the annual Conference of the Parties (COP). In the course of 2023, large vegetation fire CO₂ emissions and a possible reduction in carbon absorption by forests combined with stubbornly high fossil fuel CO₂ emissions from human and industrial activities to drive the increase, according to the WMO’s annual Greenhouse Gas Bulletin No. 20. The globally-averaged surface concentration of CO₂ reached 420.0 parts per million (ppm), methane 1 934 parts per billion and nitrous oxide 336.9 parts per billion (ppb) in 2023.

These values are 151%, 265% and 125% of pre-industrial (before 1750) levels, it said. These are calculated on the basis of the long-term observations within the Global Atmosphere Watch network of monitoring stations. The 2023 increase of CO₂ in the atmosphere was higher than that of 2022, although lower than that of the three years before that. The annual increase of 2.3 ppm marked the 12th consecutive year with an increase greater than 2 ppm. The WMO Greenhouse Gas Bulletin is one of WMO’s flagship publications released to inform the UN Climate Change conference, COP, and is now in its 20th issue. During that time the CO₂ level has increased by 11.4 % (42.9 ppm) above the level of 377.1 ppm recorded in 2004 by WMO’s Global Atmosphere Watch network of monitoring stations. The Greenhouse Gas Bulletin reports on greenhouse gas concentrations, rather than on emission levels. Analysis of data shows that just under half of CO₂ emissions remain in the atmosphere.

Just over one quarter are absorbed by the ocean and just under 30% by land ecosystems – although there is considerable year-to-year variability in this because of naturally occurring phenomena like El Niño and La Niña. The Greenhouse Gas Bulletin complements the UN Environment Programme’s Emissions Gap report. Both were published ahead of COP29 in Baku, Azerbaijan. During El Niño years, greenhouse gas levels tend to rise because drier vegetation and forest fires reduce the efficiency of land carbon sinks. “The Bulletin warns that we face a potential vicious cycle. Natural climate variability plays a big role in carbon cycle. But in the near future, climate change itself could cause ecosystems to become larger sources of greenhouse gases. Wildfires could release more carbon emissions into the atmosphere, whilst the warmer ocean might absorb less CO₂. Consequently, more CO₂ could stay in the atmosphere to accelerate global warming.

These climate feedbacks are critical concerns to human society,” said WMO Deputy Secretary-General Ko Barrett. From 1990 to 2023, radiative forcing – the warming effect on our climate – by long-lived greenhouse gases increased by 51.5%, with CO₂ accounting for about 81% of this increase, according to the National Oceanic and Atmospheric Administration Annual Greenhouse Gas Index cited in the WMO Bulletin. As long as emissions continue, greenhouse gases will continue accumulating in the atmosphere leading to global temperature rise. Given the extremely long life of CO₂ in the atmosphere, the temperature level already observed will persist for several decades even if emissions are rapidly reduced to net zero. The last time the Earth experienced a comparable concentration of CO₂ was 3-5 million years ago, when the temperature was 2-3°C warmer and sea level was 10-20 meters higher than now.

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Greenhouse gas concentrations surge again to new record in 2023