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

AR5 (IPCC, 2013) assessed that global surface temperature increase is close to linearly proportional to the total amount of cumulative CO2 emissions (Collins et al., 2013). The most recent AR6 report reaffirmed this assessment (Canadell et al., 2021). This near-linear relationship implies that for keeping global warming below a specified temperature level, one can estimate the total amount of CO2 that can ever be emitted. When expressed relative to a recent reference period, this is referred to as the remaining carbon budget (Rogelj et al., 2018).

AR6 assessed the remaining carbon budget (RCB) in Chap. 5 of its WGI report (Canadell et al., 2021) for 1.5, 1.7 and 2 °C thresholds (see Table 7). They were also reported in the Summary for Policymakers (Table SPM.2, IPCC, 2021b). These are updated in this section using the same method as last year (Forster et al., 2023).

The RCB is estimated by application of the WGI AR6 method described in Rogelj et al. (2019), which involves the combination of the assessment of five factors: (i) the most recent decade of human-induced warming (given in Sect. 7), (ii) the transient climate response to cumulative emissions of CO2 (TCRE), (iii) the zero emissions commitment (ZEC), (iv) the temperature contribution of non-CO2 emissions and (v) an adjustment term for Earth system feedbacks that are otherwise not captured through the other factors. AR6 WGI reassessed all five terms (Canadell et al., 2021). The incorporation of Earth system feedbacks was further considered by Lamboll and Rogelj (2022). Lamboll et al. (2023) further considered the temperature contribution of non-CO2 emissions, while Rogelj and Lamboll (2024) clarified the reductions in non-CO2 that are assumed in the RCB estimation.

The RCB for 1.5, 1.7 and 2 °C warming levels is re-assessed based on the most recent available data. Estimated RCBs are reported in Table 8. They are expressed both relative to 2020 to compare to AR6 and relative to the start of 2024 for estimates based on the 2014–2023 human-induced warming update (Sect. 7). Note that between the start of 2020 and the end of 2023, about 164 Gt CO2 was emitted (Sect. 2). Based on the variation in non-CO2 emissions across the scenarios in AR6 WGIII scenario database, the estimated RCB values can be higher or lower by around 200 Gt CO2 depending on how deeply non-CO2 emissions are reduced (Lamboll et al., 2023; Rogelj and Lamboll, 2024). The impact of non-CO2 emissions on warming includes both the warming effects of other greenhouse gases such as methane and the cooling effects of aerosols such as sulfates. Updating these pathways increased the estimate of the importance of aerosols, which are expected to decline with time in low emissions pathways (Rogelj et al., 2014; Rogelj and Lamboll, 2024), causing a warming and decreasing the RCB (Lamboll et al., 2023). Structural uncertainties give inherent limits to the precision with which remaining carbon budgets can be quantified. These particularly impact the 1.5 °C RCB. Overall, the 1.5 °C compatible budget is very small and shrinking fast due to continuing high global CO2 emissions.

Updated RCB estimates presented in Table 8 for 1.5, 1.7 and 2.0 °C of global warming are smaller than AR6, and geophysical and other uncertainties therefore have become larger in relative terms. This is a feature that will have to be kept in mind when communicating budgets. The estimates presented here differ from those presented in the annual Global Carbon Budget (GCB) publications (Friedlingstein et al., 2023). The GCB 2023 used the average between the AR6 WGI estimate and the Forster et al. (2023) estimates. The RCB estimates presented here consider the same updates in historical CO2 emissions from the GCB as well as the latest available quantification of human-induced warming to date and a reassessment from AR6 of non-CO2 warming contributions.

The RCB for limiting warming to 1.5 °C is rapidly diminishing. It is important, however, to correctly interpret this information. RCB estimates consider projected reductions in non-CO2 emissions that are aligned with a global transition to net zero CO2 emissions (Lamboll et al., 2023; Rogelj and Lamboll, 2024). These estimates assume median reductions in non-CO2 emissions between 2020–2050 of CH4 (about 50%), N2O (about 20%) and SO2 (about 80%) (Rogelj and Lamboll, 2024) (see Sect. S8 and Table S5). If these non-CO2 greenhouse gas emission reductions are not achieved, the RCB will be smaller (see Lamboll et al., 2023; Rogelj and Lamboll, 2024). This year's update of the 1.5 °C budget uses the historical warming level for the 2014–2023 period of 1.19 °C, with a 0.13 °C future contribution of non-CO2 warming. Assuming a median estimate of 0.45 °C per 1000 Gt CO2, this gives around 400 Gt CO2 from the midpoint of the period, from which we subtract around 200 Gt CO2 (205 Gt CO2 emissions from the middle of the 2014–2023 period and 8 Gt CO2 being the median estimate of the impact of Earth system feedbacks that would otherwise not be covered). This gives an RCB for 1.5 °C with 50% probability of 200 Gt CO2. The full calculation includes the distributions of these values for the uncertainty estimates.

Note that the 50% RCB is expected to be exhausted a few years before the 1.5 °C global warming level is reached due to the way it factors future warming from non-CO2 emissions into its estimate.