This study explores the feasibility of limiting increases in global temperature to 1.5°C above pre-industrial levels. A probabilistic simple climate model is used to identify emissions paths that offer at least a 50% chance of achieving this goal. We conclude that it is more likely than not that warming would exceed 1.5°C, at least temporarily, under plausible mitigation scenarios. We have identified three criteria of emissions paths that could meet the 1.5°C goal with a temporary overshoot of no more than 50 years: early and strong reductions in emissions, with global emissions peaking in 2015 and falling to at most 44–48 GtCO2e in 2020; rapid reductions in annual global emissions after 2020 (of at least 3–4% per year); very low annual global emissions by 2100 (less than 2–4 GtCO2e) and falling to zero (or below) in the 22nd century. The feasibility of these characteristics is uncertain. We conclude that the proposed date of review of the 1.5°C goal, set at 2015, may be too late to achieve the necessary scaling up of emissions cuts to achieve this goal.
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All probability estimates are rounded to the nearest 5%.
The range of emissions in 2020 is consistent with the set of 2°C paths developed in Ranger et al. (2009).
Gohar and Lowe (2009) describe a large database of emissions paths. We extract only those 14 paths where Gohar and Lowe estimate a median peak warming of 2°C or below between 2000 and 2200. Each has a different combination of: the year of the peak in global emissions (2014, 2016 or 2020); the annual rate of emissions reductions following the peak (3 to 6% per year, measured in CO2 emissions); and the emissions floor (zero or non-zero emissions in the long-term). The resulting 2020 emissions of these paths are between 44 and 57 GtCO2e and emissions fall to between 3 and 7 GtCO2e by 2100. The paths that have a non-zero emissions floor asymptote to 6 GtCO2e (scenarios B:5, B:6 and B:7) while for the other paths, emissions fall to zero by 2200. For more details see the Supplementary material B
The 2100 radiative forcing simulated in this study for all aerosols is −1.3Wm−2 compared to −5.3Wm−2 for RCP3-PD from the RCP database
The Set B paths tend to require stronger action than for Set A (in terms of both 2020 emissions and the rate of decline post-2020) to meet the same goal. This results from the higher emissions post-2050 for the Set B paths. Indeed, under the low aerosol scenario, no Set B paths are able to meet the goal; hence, action would need to be stronger than demonstrated by the Set B paths.
The temperature reduction seen at the 50th percentile for an emissions based simulation using the RCP3-PD scenario
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The authors wish to thank Brian Hoskins, Andrew Gouldson, Piers Forster, Nicholas Stern and the two anonymous reviewers for their comments. Nicola Ranger, Alex Bowen and Robert Ward were supported during this project by Grantham Foundation for the Protection of the Environment and the UK Economic and Social Research Council (ESRC). Laila Gohar and Jason Lowe were supported during this project by the AVOID programme funded by the UK Department of Energy and Climate Change (DECC) and Department for Environment, Food and Rural Affairs (Defra), under contract DECC/Defra GA0215. The Met Office Hadley Centre provided scientific input to this work but does not advocate particular policy choices.
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Ranger, N., Gohar, L.K., Lowe, J.A. et al. Is it possible to limit global warming to no more than 1.5°C?. Climatic Change 111, 973–981 (2012). https://doi.org/10.1007/s10584-012-0414-8