Editing IPCC:AR6/SR15/Chapter-1 (section)

==== Warming commitment from past emissions of greenhouse gases and aerosols. ====

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Radiative forcing (top) and global mean surface temperature change (bottom) for scenarios with different combinations of greenhouse gas and aerosol precursor emissions reduced to zero in 2020. Variables were calculated using a simple climate–carbon cycle model (Millar et al., 2017a) <sup>[[#fn:r172|172]]</sup> with a simple representation of atmospheric chemistry (Smith et al., 2018) <sup>[[#fn:r173|173]]</sup> . The bars on the right-hand side indicate the median warming in 2100 and 5–95% uncertainty ranges (also indicated by the plume around the yellow line) taking into account one estimate of uncertainty in climate response, effective radiative forcing and carbon cycle sensitivity, and constraining simple model parameters with response ranges from AR5 combined with historical climate observations (Smith et al., 2018) <sup>[[#fn:r174|174]]</sup> . Temperatures continue to increase slightly after elimination of CO <sub>2</sub> emissions (blue line) in response to constant non-CO <sub>2</sub> forcing. The dashed blue line extrapolates one estimate of the current rate of warming, while dotted blue lines show a case where CO <sub>2</sub> emissions are reduced linearly to zero assuming constant non-CO <sub>2</sub> forcing after 2020. Under these highly idealized assumptions, the time to stabilize temperatures at 1.5°C is approximately double the time remaining to reach 1.5°C at the current warming rate.

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Since most sources of emissions cannot, in reality, be brought to zero instantaneously due to techno-economic inertia, the current rate of emissions also constitutes a conditional commitment to future emissions and consequent warming depending on achievable rates of emission reductions. The current level and rate of human-induced warming determines both the time left before a temperature threshold is exceeded if warming continues (dashed blue line in Figure 1.5) and the time over which the warming rate must be reduced to avoid exceeding that threshold (approximately indicated by the dotted blue line in Figure 1.5). Leach et al. (2018) <sup>[[#fn:r175|175]]</sup> use a central estimate of human-induced warming of 1.02°C in 2017, increasing at 0.215°C per decade (Haustein et al., 2017) <sup>[[#fn:r176|176]]</sup> , to argue that it will take 13–32 years (one-standard-error range) to reach 1.5°C if the current warming rate continues, allowing 25–64 years to stabilise temperatures at 1.5°C if the warming rate is reduced at a constant rate of deceleration starting immediately. Applying a similar approach to the multi-dataset average GMST used in this report gives an assessed ''likely'' range for the date at which warming reaches 1.5°C of 2030 to 2052. The lower bound on this range, 2030, is supported by multiple lines of evidence, including the AR5 assessment for the ''likely'' range of warming (0.3°C–0.7°C) for the period 2016–2035 relative to 1986–2005. The upper bound, 2052, is supported by fewer lines of evidence, so we have used the upper bound of the 5–95% confidence interval given by the Leach et al. (2018) <sup>[[#fn:r177|177]]</sup> method applied to the multi-dataset average GMST, expressed as the upper limit of the ''likely'' range, to reflect the reliance on a single approach. Results are sensitive both to the confidence level chosen and the number of years used to estimate the current rate of anthropogenic warming (5 years used here, to capture the recent acceleration due to rising non-CO <sub>2</sub> forcing). Since the rate of human-induced warming is proportional to the rate of CO <sub>2</sub> emissions (Matthews et al., 2009; Zickfeld et al., 2009) <sup>[[#fn:r178|178]]</sup> plus a term approximately proportional to the rate of increase in non-CO <sub>2</sub> radiative forcing (Gregory and Forster, 2008; Allen et al., 2018 <sup>[[#fn:r179|179]]</sup> ; Cross-Chapter Box 2 in this chapter), these time scales also provide an indication of minimum emission reduction rates required if a warming greater than 1.5°C is to be avoided (see Figure 1.5, Supplementary Material 1.SM.6 and FAQ 1.2).

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