Fig. 3: Observational constraints on the TCRE and consequences for the design of Paris Agreement-compatible scenarios.

Panel a plots attributed human-induced warming against cumulative emissions of CO₂. The space is shaded by the value of the TCRE (Gaussian distribution in best-estimate and likely range in AR5) and the points are coloured by the decade in which the temperature (relative to 1850–1900 baseline) and cumulative CO₂-fe emissions (relative to 1875) are diagnosed. An ellipse is drawn around central 90% of points. Black lines in panel a depict the 5th, 50th and 95th percentile of the overall observationally constrained TCRE distribution based on the 2011–2020 decade. Panels b–d show the remaining CO₂ and non-CO₂ CO₂-fe budgets from 2020 for each scenario in Fig. 1, coloured by category in the IIASA SR15 database. Panels b and d show remaining budgets to peak warming in each scenario, while panel c shows budgets to 2100 (instead coloured by IAM). In all three panels shading shows budgets compatible with limiting warming to 1.5 °C for the AR5 gaussian TCRE likely range, as in panel a. The solid black lines show the corresponding remaining total CO₂-fe budgets instead of using our observationally constrained TCRE 5th–95th percentile range. In panels b and c the shading, therefore, corresponds to budgets for 0.27 °C remaining warming to 1.5 °C-consistent with 1.23 °C warming in 2020. In panel d the shading refers to budget for 0.42 °C remaining warming to 1.5 °C-consistent with 1.08 °C warming in 2020 (re-baselining historical GMST to 0.85 °C for the decade prior to 2015). Pink horizontal box-whisker plots in panels b and d show estimates of remaining CO₂ budgets for each assumed present-day warming level, assuming a mid-range non-CO₂ budget to peak warming (130 GtCO₂-fe) and plotting the 5th, 33rd, 66th and 95th percentiles (see Methods for information on calculation). In panel c IAM abbreviations correspond to: A/C 2.0/2.1 – AIM/CGE 2.0/2.1; C-R 5.005 – C-Roads 5.005; I 3.0.1 – IMAGE 3.0.1; M V.3 – MESSAGE V.3; M-G 1.0 – MESSAGE-GLOBIOM 1.0; R 1.5/1.7 – REMIND 1.5/1.7; R-M 1.7-3 – REMIND-MAgPIE 1.7–3.0.