To restrict global warming to below the agreed targets requires limiting carbon emissions, the principal driver of anthropogenic warming. However, there is significant uncertainty in projecting the amount of carbon that can be emitted, in part due to the limited number of Earth system model simulations and their discrepancies with present-day observations. Here we demonstrate a novel approach to reduce the uncertainty of climate projections; using theory and geological evidence we generate a very large ensemble (3 × 104) of projections that closely match records for nine key climate metrics, which include warming and ocean heat content. Our analysis narrows the uncertainty in surface-warming projections and reduces the range in equilibrium climate sensitivity. We find that a warming target of 1.5 °C above the pre-industrial level requires the total emitted carbon from the start of year 2017 to be less than 195–205 PgC (in over 66% of the simulations), whereas a warming target of 2 °C is only likely if the emitted carbon remains less than 395–455 PgC. At the current emission rates, these warming targets are reached in 17–18 years and 35–41 years, respectively, so that there is a limited window to develop a more carbon-efficient future.
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We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups (listed in Supplementary Table 1 of this paper) for producing and making available their model output. For CMIP, the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and lead development of the software infrastructure in partnership with the Global Organization for Earth System Science Portals. This work was supported by UK Natural Environment Research Council (NERC) grants NE/P01495X/1 and NE/N009789/1. G.L.F. acknowledges support from UK NERC grants NE/D00876X/2, NE/I005595/1 and NE/P011381/1. E.J.R. acknowledges Australian Laureate Fellowship FL120100050.
The authors declare no competing financial interests.
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Supplementary figures and tables.
Data used to generate Figs. 1–4 and Supplementary Figs. 1–3.
Main code and instructions for the WASP Earth system model.
Functions and forcing scenarios and instructions for the WASP Earth system model as used to perform the experiments in this study.
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Goodwin, P., Katavouta, A., Roussenov, V.M. et al. Pathways to 1.5 °C and 2 °C warming based on observational and geological constraints. Nature Geosci 11, 102–107 (2018). https://doi.org/10.1038/s41561-017-0054-8
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