Indicators of Global Climate Change 2023: annual update of key indicators of the state of the climate system and human influence

As in Forster et al. (2023), we report best-estimate global mean concentrations for 52 well-mixed greenhouse gases. These concentrations are updated here to 2023.

As in AR6 and Forster et al. (2023), CO2 mixing ratios were taken from the NOAA Global Monitoring Laboratory (GML) and are updated here through 2023 (Lan et al., 2023a). As in Forster et al. (2023), CO2 is reported on the WMO-CO2-X2019 scale, which differs from the WMO-CO2-X2007 scale used in AR6. Prior to the use of NOAA GML data from 1980 onwards, a conversion is applied to the AR6 CO2 time series to take into account the scale change using X2019 = 1.00079 ⋅ X2007 - 0.142 ppm. Other GHG records were compiled from NOAA and AGAGE global networks or extrapolated from literature. Averages of NOAA and AGAGE data were used for N2O, CH4, CFC-11, CFC-12, CFC-113, CCl4, HCFC-22, HFC-134a and HFC-125 (Lan et al., 2023b; Dutton et al., 2024; Prinn et al., 2018), which, along with CO2, account for over 98% of the ERF from well-mixed greenhouse gases. In cases where no updated information is available, global estimates were extrapolated from Vimont et al. (2022), Western et al. (2023) or other literature and were scaled to be consistent with those reported in AR6. Some extrapolations are based on data from the mid-2010s (Droste et al., 2020; Laube et al., 2014; Simmonds et al., 2017; Vollmer et al., 2018) but have an imperceptible effect on the total ERF assessed in Sect. 4 and are included to maintain consistency with AR6. Mixing ratio uncertainties for 2023 are assumed to be similar to 2019, and we adopt the same uncertainties as assessed in AR6 WGI.

The global surface mean concentrations of CO2, CH4 and N2O in 2023 were 419.3 [±0.4] parts per million (ppm), 1922.5 [±3.3] parts per billion (ppb) and 336.9 [±0.4] ppb, respectively. Concentrations of all three major GHGs have increased since 2019, with CO2 increasing by 9.2 ppm, CH4 by 56 ppb and N2O by 4.8 ppb. Increases since 2019 are consistent with those from the CSIRO network (Francey at al., 1999), which are 9.3 ppm, 55 ppb and 5.0 ppb for CO2, CH4 and N2O, respectively. With few exceptions, concentrations of ozone-depleting substances, such as CFC-11 and CFC-12, continue to decline, while those of replacement compounds (HFCs) have increased. HFC-134a, for example, has increased 20% since 2019 to 129.5 parts per trillion (ppt). Aggregated across all gases, PFCs increased from 109.7 to an estimated 115 ppt CF4-equivalent from 2019 to 2023, and HFCs increased from 237 to 301 ppt HFC-134a-equivalent, while Montreal gases declined from 1032 to 1004 ppt CFC-12-equivalent. Mixing ratio equivalents are determined by the radiative efficiencies of each greenhouse gas from Hodnebrog et al. (2020).

Ozone is an important greenhouse gas with strong regional variation in both the stratosphere and troposphere (Szopa et al., 2021). Its ERF arising from its regional distribution is assessed in Sect. 4, but following AR6 convention is not included with the GHGs discussed here. Other non-methane SLCFs are heterogeneously distributed in the atmosphere and are also not typically reported in terms of a globally averaged concentration. Globally averaged concentrations for these are normally model-derived, supplemented by local monitoring networks and satellite data (Szopa et al., 2021).

In this update we employ AR6-derived uncertainty estimates and do not perform a new assessment. Table S2 in Sect. S3 shows specific updated concentrations for all the GHGs considered.