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Abstract The IPCC Special Report on 1.5 °C concluded that anthropogenic global warming is determined by cumulative anthropogenic CO 2 emissions and the non-CO 2 radiative forcing level in the decades prior to peak warming. We quantify this using CO 2 -forcing-equivalent (CO 2 -fe) emissions. We produce an observationally constrained estimate of the Transient Climate Response to cumulative carbon Emissions (TCRE), giving a 90% confidence interval of 0.26–0.78 °C/TtCO 2 , implying a remaining total CO 2 -fe budget from 2020 to 1.5 °C of 350–1040 GtCO 2 -fe, where non-CO 2 forcing changes take up 50 to 300 GtCO 2 -fe. Using a central non-CO 2 forcing estimate, the remaining CO 2 budgets are 640, 545, 455 GtCO 2 for a 33, 50 or 66% chance of limiting warming to 1.5 °C. We discuss the impact of GMST revisions and the contribution of non-CO 2 mitigation to remaining budgets, determining that reporting budgets in CO 2 -fe for alternative definitions of GMST, displaying CO 2 and non-CO 2 contributions using a two-dimensional presentation, offers the most transparent approach.
Jenkins et al. (Thu,) studied this question.