Despite a concerted research effort and extensive observational record, uncertainty in climate sensitivity and aerosol forcing, the two largest contributions to future warming uncertainty, remains large. Here we highlight the stark disparity that different aerosol forcing can imply for future warming projections: scenarios compatible with the Paris Agreement can either easily meet the specified warming limits or risk missing them completely using plausible samples from the IPCC Sixth Assessment Report assessed uncertainty ranges.
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D.W.-P. acknowledges funding from the Natural Environment Research Council project NE/S005390/1 (ACRUISE) and from the European Union’s Horizon 2020 research and innovation programme iMIRACLI under Marie Skłodowska-Curie grant agreement No 860100. C.J.S. was supported by a NERC/IIASA Collaborative Research Fellowship (NE/T009381/1). We thank P. Forster, P. Stier, S. Jenkins and A. Williams for useful feedback and discussions while preparing this manuscript.
The authors declare no competing interests.
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Extended Data Fig. 1 The effect of aerosol forcing uncertainty on future temperature projections under high ambition scenarios.
a) The 90% confidence range in global mean surface temperature change depicted in (b) as a function of ERFaer uncertainty and mean ERFaer sampled as described in the methods. b) The global surface mean temperature change relative to 1850 under SSP1-1.9 and sampled from an ensemble of simulations24 consistent with historical temperatures (1850–2019), ocean heat content change (1971–2018) and CO2 concentration (1750–2014) assuming three different reduced uncertainty ERFaer estimates: weak (mauve); medium (orange) and strong (blue). The 90% confidence range for each subset at the end of the century is indicated to the right of the axis. Observed surface temperatures averaged across four available datasets are shown in black. The underlying heatmap shows the average ERFaer of the ensemble members that produce a given temperature change each year where the ensemble density is greater than 10%. The colormap is centred around the median ERFaer in the ensemble and ranges between the 10th-90th percentiles.
Extended Data Fig. 2 The effect of equilibrium climate sensitivity uncertainty on future temperature projections.
a) The 90% confidence range in global mean surface temperature change depicted in (b) as a function of ECS uncertainty and mean ECS sampled as described in the methods. b) The global surface mean temperature change relative to 1850 under SSP1-2.6 and sampled from an ensemble of simulations24 consistent with historical temperatures (1850–2019), ocean heat content change (1971–2018) and CO2 concentration (1750–2014) assuming three different reduced ECS uncertainty estimates: low (mauve); medium (orange) and high (blue). The 90% confidence range for each subset at the end of the century is indicated to the right of the axis. Observed surface temperatures averaged across four available datasets are shown in black. The underlying heatmap shows the average ECS of the ensemble members that produce a given temperature change each year where the ensemble density is greater than 10%. The colormap is centred around the median ECS in the ensemble and ranges between the 10th-90th percentiles.
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Watson-Parris, D., Smith, C.J. Large uncertainty in future warming due to aerosol forcing. Nat. Clim. Chang. (2022). https://doi.org/10.1038/s41558-022-01516-0