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

=== 1.2.4 Geophysical Warming Commitment ===

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It is frequently asked whether limiting warming to 1.5°C is ‘feasible’ (Cross-Chapter Box 3 in this chapter). There are many dimensions to this question, including the warming ‘commitment’ from past emissions of greenhouse gases and aerosol precursors. Quantifying commitment from past emissions is complicated by the very different behaviour of different climate forcers affected by human activity: emissions of long-lived greenhouse gases such as CO <sub>2</sub> and nitrous oxide (N <sub>2</sub> O) have a very persistent impact on radiative forcing (Myhre et al., 2013) <sup>[[#fn:r153|153]]</sup> , lasting from over a century (in the case of N <sub>2</sub> O) to hundreds of thousands of years (for CO <sub>2</sub> ). The radiative forcing impact of short-lived climate forcers (SLCFs) such as methane (CH <sub>4</sub> ) and aerosols, in contrast, persists for at most about a decade (in the case of methane) down to only a few days. These different behaviours must be taken into account in assessing the implications of any approach to calculating aggregate emissions (Cross-Chapter Box 2 in this chapter).

Geophysical warming commitment is defined as the unavoidable future warming resulting from physical Earth system inertia. Different variants are discussed in the literature, including (i) the ‘constant composition commitment’ (CCC), defined by Meehl et al. (2007) <sup>[[#fn:r154|154]]</sup> as the further warming that would result if atmospheric concentrations of GHGs and other climate forcers were stabilised at the current level; and (ii) and the ‘zero emissions commitment’ (ZEC), defined as the further warming that would still occur if all future anthropogenic emissions of greenhouse gases and aerosol precursors were eliminated instantaneously (Meehl et al., 2007; Collins et al., 2013) <sup>[[#fn:r155|155]]</sup> .

The CCC is primarily associated with thermal inertia of the ocean (Hansen et al., 2005) <sup>[[#fn:r156|156]]</sup> , and has led to the misconception that substantial future warming is inevitable (Matthews and Solomon, 2013) <sup>[[#fn:r157|157]]</sup> . The CCC takes into account the warming from past emissions, but also includes warming from future emissions (declining but still non-zero) that are required to maintain a constant atmospheric composition. It is therefore not relevant to the warming commitment from past emissions alone.

The ZEC, although based on equally idealised assumptions, allows for a clear separation of the response to past emissions from the effects of future emissions. The magnitude and sign of the ZEC depend on the mix of GHGs and aerosols considered. For CO <sub>2</sub> , which takes hundreds of thousands of years to be fully removed from the atmosphere by natural processes following its emission (Eby et al., 2009; Ciais et al., 2013) <sup>[[#fn:r158|158]]</sup> , the multi-century warming commitment from emissions to date in addition to warming already observed is estimated to range from slightly negative (i.e., a slight cooling relative to present-day) to slightly positive (Matthews and Caldeira, 2008; Lowe et al., 2009; Gillett et al., 2011; Collins et al., 2013) <sup>[[#fn:r159|159]]</sup> . Some studies estimate a larger ZEC from CO <sub>2</sub> , but for cumulative emissions much higher than those up to present day (Frölicher et al., 2014; Ehlert and Zickfeld, 2017) <sup>[[#fn:r160|160]]</sup> . The ZEC from past CO <sub>2</sub> emissions is small because the continued warming effect from ocean thermal inertia is approximately balanced by declining radiative forcing due to CO <sub>2</sub> uptake by the ocean (Solomon et al., 2009; Goodwin et al., 2015; Williams et al., 2017) <sup>[[#fn:r161|161]]</sup> . Thus, although present-day CO <sub>2</sub> -induced warming is irreversible on millennial time scales (without human intervention such as active carbon dioxide removal or solar radiation modification; Section 1.4.1), past CO <sub>2</sub> emissions do not commit to substantial further warming (Matthews and Solomon, 2013) <sup>[[#fn:r162|162]]</sup> .

Sustained net zero anthropogenic emissions of CO <sub>2</sub> and declining net anthropogenic non-CO <sub>2</sub> radiative forcing over a multi-decade period would halt anthropogenic global warming over that period, although it would not halt sea level rise or many other aspects of climate system adjustment. The rate of decline of non-CO <sub>2</sub> radiative forcing must be sufficient to compensate for the ongoing adjustment of the climate system to this forcing (assuming it remains positive) due to ocean thermal inertia. It therefore depends on deep ocean response time scales, which are uncertain but of order centuries, corresponding to decline rates of non-CO <sub>2</sub> radiative forcing of less than 1% per year. In the longer term, Earth system feedbacks such as the release of carbon from melting permafrost may require net negative CO <sub>2</sub> emissions to maintain stable temperatures (Lowe and Bernie, 2018) <sup>[[#fn:r163|163]]</sup> .

For warming SLCFs, meaning those associated with positive radiative forcing such as methane, the ZEC is negative. Eliminating emissions of these substances results in an immediate cooling relative to the present (Figure 1.5, magenta lines) (Frölicher and Joos, 2010; Matthews and Zickfeld, 2012; Mauritsen and Pincus, 2017) <sup>[[#fn:r164|164]]</sup> . Cooling SLCFs (those associated with negative radiative forcing) such as sulphate aerosols create a positive ZEC, as elimination of these forcers results in rapid increase in radiative forcing and warming (Figure 1.5, green lines) (Matthews and Zickfeld, 2012; Mauritsen and Pincus, 2017; Samset et al., 2018) <sup>[[#fn:r165|165]]</sup> . Estimates of the warming commitment from eliminating aerosol emissions are affected by large uncertainties in net aerosol radiative forcing (Myhre et al., 2013, 2017) <sup>[[#fn:r166|166]]</sup> and the impact of other measures affecting aerosol loading (e.g., Fernández et al., 2017) <sup>[[#fn:r167|167]]</sup> . If present-day emissions of all GHGs (short- and long-lived) and aerosols (including sulphate, nitrate and carbonaceous aerosols) are eliminated (Figure 1.5, yellow lines) GMST rises over the following decade, driven by the removal of negative aerosol radiative forcing. This initial warming is followed by a gradual cooling driven by the decline in radiative forcing of short-lived greenhouse gases (Matthews and Zickfeld, 2012; Collins et al., 2013) <sup>[[#fn:r168|168]]</sup> . Peak warming following elimination of all emissions was assessed at a few tenths of a degree in AR5, and century-scale warming was assessed to change only slightly relative to the time emissions are reduced to zero (Collins et al., 2013) <sup>[[#fn:r169|169]]</sup> . New evidence since AR5 suggests a larger methane forcing (Etminan et al., 2016) <sup>[[#fn:r170|170]]</sup> but no revision in the range of aerosol forcing (although this remains an active field of research, e.g., Myhre et al., 2017) <sup>[[#fn:r171|171]]</sup> . This revised methane forcing estimate results in a smaller peak warming and a faster temperature decline than assessed in AR5 (Figure 1.5, yellow line).

Expert judgement based on the available evidence (including model simulations, radiative forcing and climate sensitivity) suggests that if all anthropogenic emissions were reduced to zero immediately, any further warming beyond the 1°C already experienced would ''likely'' be less than 0.5°C over the next two to three decades, and also ''likely'' less than 0.5°C on a century time scale.

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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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