Framing Climate Goals in Terms of Cumulative CO 2 ‐Forcing‐Equivalent Emissions

Abstract The relationship between cumulative CO 2 emissions and CO 2 ‐induced warming is determined by the Transient Climate Response to Emissions (TCRE), but total anthropogenic warming also depends on non‐CO 2 forcing, complicating the interpretation of emissions budgets based on CO 2 alone. An alternative is to frame emissions budgets in terms of CO 2 ‐forcing‐equivalent (CO 2 ‐fe) emissions—the CO 2 emissions that would yield a given total anthropogenic radiative forcing pathway. Unlike conventional “CO 2 ‐equivalent” emissions, these are directly related to warming by the TCRE and need to fall to zero to stabilize warming: hence, CO 2 ‐fe emissions generalize the concept of a cumulative carbon budget to multigas scenarios. Cumulative CO 2 ‐fe emissions from 1870 to 2015 inclusive are found to be 2,900 ± 600 GtCO 2 ‐fe, increasing at a rate of 67 ± 9.5 GtCO 2 ‐fe/yr. A TCRE range of 0.8–2.5°C per 1,000 GtC implies a total budget for 0.6°C of additional warming above the present decade of 880–2,750 GtCO 2 ‐fe, with 1,290 GtCO 2 ‐fe implied by the Coupled Model Intercomparison Project Phase 5 median response, corresponding to 19 years' CO 2 ‐fe emissions at the current rate.