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

== 1.2.3 Definition of 1.5°C Pathways: Probability, Transience, Stabilization and Overshoot ==

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Pathways considered in this report, consistent with available literature on 1.5°C, primarily focus on the time scale up to 2100, recognising that the evolution of GMST after 2100 is also important. Two broad categories of 1.5°C pathways can be used to characterise mitigation options and impacts: pathways in which warming (defined as 30-year averaged GMST relative to pre-industrial levels, see Section 1.2.1) remains below 1.5°C throughout the 21st century, and pathways in which warming temporarily exceeds (‘overshoots’) 1.5°C and returns to 1.5°C either before or soon after 2100. Pathways in which warming exceeds 1.5°C before 2100, but might return to that level in some future century, are not considered 1.5°C pathways.

Because of uncertainty in the climate response, a ‘prospective’ mitigation pathway (see Cross-Chapter Box 1 in this chapter), in which emissions are prescribed, can only provide a level of probability of warming remaining below a temperature threshold. This probability cannot be quantified precisely since estimates depend on the method used (Rogelj et al., 2016b; Millar et al., 2017b; Goodwin et al., 2018; Tokarska and Gillett, 2018) <sup>[[#fn:r107|107]]</sup> . This report defines a ‘1.5°C pathway’ as a pathway of emissions and associated possible temperature responses in which the majority of approaches using presently available information assign a probability of approximately one-in-two to two-in-three to warming remaining below 1.5°C or, in the case of an overshoot pathway, to warming returning to 1.5°C by around 2100 or earlier. Recognizing the very different potential impacts and risks associated with high-overshoot pathways, this report singles out 1.5°C pathways with no or limited (&lt;0.1°C) overshoot in many instances and pursues efforts to ensure that when the term ‘1.5°C pathway’ is used, the associated overshoot is made explicit where relevant. In Chapter 2, the classification of pathways is based on one modelling approach to avoid ambiguity, but probabilities of exceeding 1.5°C are checked against other approaches to verify that they lie within this approximate range. All these absolute probabilities are imprecise, depend on the information used to constrain them, and hence are expected to evolve in the future. Imprecise probabilities can nevertheless be useful for decision-making, provided the imprecision is acknowledged (Hall et al., 2007; Kriegler et al., 2009; Simpson et al., 2016) <sup>[[#fn:r108|108]]</sup> . Relative and rank probabilities can be assessed much more consistently: approaches may differ on the absolute probability assigned to individual outcomes, but typically agree on which outcomes are more probable.

Importantly, 1.5°C pathways allow a substantial (up to one-in-two) chance of warming still exceeding 1.5°C. An ‘adaptive’ mitigation pathway in which emissions are continuously adjusted to achieve a specific temperature outcome (e.g., Millar et al., 2017b) <sup>[[#fn:r109|109]]</sup> reduces uncertainty in the temperature outcome while increasing uncertainty in the emissions required to achieve it. It has been argued (Otto et al., 2015; Xu and Ramanathan, 2017) <sup>[[#fn:r110|110]]</sup> that achieving very ambitious temperature goals will require such an adaptive approach to mitigation, but very few studies have been performed taking this approach (e.g., Jarvis et al., 2012) <sup>[[#fn:r111|111]]</sup> .

Figure 1.4 illustrates categories of (a) 1.5°C pathways and associated (b) annual and (c) cumulative emissions of CO <sub>2</sub> . It also shows (d) an example of a ‘time-integrated impact’ that continues to increase even after GMST has stabilised, such as sea level rise. This schematic assumes for the purposes of illustration that the fractional contribution of non-CO <sub>2</sub> climate forcers to total anthropogenic forcing (which is currently increasing, Myhre et al., 2017) <sup>[[#fn:r112|112]]</sup> is approximately constant from now on. Consequently, total human-induced warming is proportional to cumulative CO <sub>2</sub> emissions (solid line in c), and GMST stabilises when emissions reach zero. This is only the case in the most ambitious scenarios for non-CO <sub>2</sub> mitigation (Leach et al., 2018) <sup>[[#fn:r113|113]]</sup> . A simple way of accounting for varying non-CO <sub>2</sub> forcing in Figure 1.4 would be to note that every 1 W m <sup>−2</sup> increase in non-CO <sub>2</sub> forcing between now and the decade or two immediately prior to the time of peak warming reduces cumulative CO <sub>2</sub> emissions consistent with the same peak warming by approximately 1100 GtCO <sub>2</sub> , with a range of 900-1500 GtCO <sub>2</sub>  (using values from AR5: Myhre et al., 2013; Allen et al., 2018; Jenkins et al., 2018 <sup>[[#fn:r114|114]]</sup> ; Cross-Chapter Box 2 in this chapter).

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