Carbon budgets, which define the total allowable CO2 emissions associated with a given global climate target, are a useful way of framing the climate mitigation challenge. In this paper, we review the geophysical basis for the idea of a carbon budget, showing how this concept emerges from a linear climate response to cumulative CO2 emissions. We then discuss the difference between a “CO2-only carbon budget” associated with a given level of CO2-induced warming and an “effective carbon budget” associated with a given level of warming caused by all human emissions. We present estimates for the CO2-only and effective carbon budgets for 1.5 and 2 °C, based on both model simulations and updated observational data. Finally, we discuss the key contributors to uncertainty in carbon budget estimates and suggest some implications of this uncertainty for decision-making. Based on the analysis presented here, we argue that while the CO2-only carbon budget is a robust upper bound on allowable emissions for a given climate target, the size of the effective carbon budget is dependent on the how quickly we are able to mitigate non-CO2 greenhouse gas and aerosol emissions. This suggests that climate mitigation efforts could benefit from being responsive to a changing effective carbon budget over time, as well as to potential new information that could narrow uncertainty associated with the climate response to CO2 emissions.
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Here, values from 1750–2011 from Myhre et al.  were extended to 2015 using observations from the NOAA greenhouse gas index (http://www.esrl.noaa.gov/gmd/aggi/aggi.html) and adopting RCP6 percentage trends over 2011–2015 for the other anthropogenic forcing agents ; from 2011 to 2015 greenhouse gas forcing increased by 5% and non-greenhouse forcings changed very little.
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H.D.M. and J.-S.L. acknowledge support from the Natural Sciences and Engineering Research Council of Canada (NSERC). A.-I.P. was supported by a research grant from Emil Aaltonen foundation.
This article is part of the Topical Collection on Carbon Cycle and Climate
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Matthews, H.D., Landry, J., Partanen, A. et al. Estimating Carbon Budgets for Ambitious Climate Targets. Curr Clim Change Rep 3, 69–77 (2017). https://doi.org/10.1007/s40641-017-0055-0
- Carbon Budgets
- Cumulative CO2 Emissions
- Climate Targets
- Climate Mitigation