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Estimating the timing of geophysical commitment to 1.5 and 2.0 °C of global warming

Abstract

Following abrupt cessation of anthropogenic emissions, decreases in short-lived aerosols would lead to a warming peak within a decade, followed by slow cooling as GHG concentrations decline. This implies a geophysical commitment to temporarily crossing warming levels before reaching them. Here we use an emissions-based climate model (FaIR) to estimate temperature change following cessation of emissions in 2021 and in every year thereafter until 2080 following eight Shared Socioeconomic Pathways (SSPs). Assuming a medium-emissions trajectory (SSP2–4.5), we find that we are already committed to peak warming greater than 1.5 °C with 42% probability, increasing to 66% by 2029 (340 GtCO2 relative to 2021). Probability of peak warming greater than 2.0 °C is currently 2%, increasing to 66% by 2057 (1,550 GtCO2 relative to 2021). Because climate will cool from peak warming as GHG concentrations decline, committed warming of 1.5 °C in 2100 will not occur with at least 66% probability until 2055.

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Fig. 1: Constrained FaIR ensemble global temperature projections.
Fig. 2: Committed warming and scenario warming following SSP2–4.5.
Fig. 3: Committed warming and scenario warming relative to 1850–1900 for all SSPs.

Data availability

All data necessary to interpret, verify and extend the research in this article are available to download from the online repository Zenodo48.

Code availability

The FaIR model is available to download from the public code repository GitHub (https://github.com/OMS-NetZero/FAIR). All other code used to used to set up model simulations, analyse model output and create figures are available to view and download from GitHub49.

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Acknowledgements

The authors acknowledge funding from the following sources: National Science Foundation Grant AGS-1752796 (M.T.D., K.C.A.), Alfred P. Sloan Research Fellowship FG-2020-13568 (K.C.A.), NOAA Grant UWSC12184 (C.P.), NERC/IIASA Collaborative Research Fellowship NE/T009381/1 (C.J.S.) and NSF grant AGS-1665247 (D.M.W.F.).

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Authors and Affiliations

Authors

Contributions

M.T.D., K.C.A. and C.P. designed the study. M.T.D. performed the analysis. K.C.A., C.P., D.M.W.F., M.B.B. and C.J.S. made suggestions to the analysis and helped interpret the results. M.T.D. wrote the manuscript with edits from all other authors.

Corresponding author

Correspondence to M. T. Dvorak.

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The authors declare no competing interests.

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Nature Climate Change thanks Chris Jones, Andrew MacDougall and Alexander MacIsaac for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Prior and posterior distributions of climate response metrics.

Posterior estimates of ECS (a) and TCR (b) are 2.9C [1.8-4.7C, 90% confidence] and 1.7C [1.2-2.5C], respectively.

Extended Data Fig. 2 Prior and posterior distributions of Held two-layer model variables.

The global radiative feedback parameter, λ (a), ocean heat exchange coefficient, γ (b), and deep ocean efficacy factor, ε (e). Note that neither γ nor ε are well constrained by the observational record. See Supplementary Figure S3 for a sensitivity test of the effect of uncertainty in these variables on results.

Extended Data Fig. 3 Prior and posterior distributions of radiative forcing for main GHGs and aerosols, with the 5th, 50th and 95th percentiles indicated.

CO2 (a), CH4 (b), N2O (c), and aerosol (d) forcing in 2018 relative to 1765. Total ERF (e) is the 2006-2019 mean relative to the 1850-1900 average. Note that the posterior median total ERF of 2.1 Wm−2 corresponds well with the observational value of 2.2 W m−2, σ = 0.43 W m−2. Median aerosol forcing agrees well with the AR6 estimate of -1.1 W m−2 [-2.0 to -0.4 W m−2] for the same period.

Extended Data Fig. 4 Prior and posterior distributions of carbon cycling parameters.

R0 (a) represents the airborne fraction of CO2 during the preindustrial, and rt (b) and rc (c) capture the decreasing absorption efficacy of land and ocean carbon sinks with rising global temperatures and CO2 concentrations, respectively. Note that rc and rt are not well-constrained by the observational record. The posterior mean r0 is 33.8 years, which is between that of Millar et al.’s (2017) value of 32.4 years, and Smith et al.’s (2018) value of 35 years.

Extended Data Fig. 5 Observational constraint results in a closer reproduction of the historical temperature record from 1850-2020 relative to 1850-1900.

Prior (300,000 member) (a) and posterior (6,729) (b) modeled global temperatures. The observed temperature (overlaid in black) is the ensemble mean from the HadCRUT5 blended air and sea surface temperature dataset (49). Shading represents the 90% confidence interval.

Extended Data Fig. 6 Modeled radiative forcing for the period 2000-2100 relative to 1765 for each SSP scenario.

CO2 (a), Aerosol (b), and total (c) radiative forcing. Shading represents the 90% confidence interval.

Extended Data Fig. 7 Abrupt emissions cessation results in less warming relative to linear phase-out scenarios.

Modeled global temperature anomaly relative to 1850-1900 (a) and total radiative forcing relative to 1765 (b) for a phase-out of anthropogenic emissions as compared to the abrupt cessation shown in the main paper (‘abrupt’) following SSP2-4.5. Legend indicates the number of years over which the phase-out occurred, beginning in 2021, where emissions of all gases decrease linearly to zero (GHGs) and to 1765 levels (all other gases), with no net-negative CO2 emissions.

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Description of contents, Figs. 1–8 and Tables 1–6.

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Dvorak, M.T., Armour, K.C., Frierson, D.M.W. et al. Estimating the timing of geophysical commitment to 1.5 and 2.0 °C of global warming. Nat. Clim. Chang. 12, 547–552 (2022). https://doi.org/10.1038/s41558-022-01372-y

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