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

==== 1.2.3.3 Impacts at 1.5°C warming associated with different pathways: transience versus stabilisation ====

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Figure 1.4 also illustrates time scales associated with different impacts. While many impacts scale with the change in GMST itself, some (such as those associated with ocean acidification) scale with the change in atmospheric CO <sub>2</sub> concentration, indicated by the fraction of cumulative CO <sub>2</sub> emissions remaining in the atmosphere (dotted lines in Figure 1.4c). Others may depend on the rate of change of GMST, while ‘time-integrated impacts’, such as sea level rise, shown in Figure 1.4d continue to increase even after GMST has stabilised.

Hence impacts that occur when GMST reaches 1.5°C could be very different depending on the pathway to 1.5°C. CO <sub>2</sub> concentrations will be higher as GMST rises past 1.5°C (transient warming) than when GMST has stabilized at 1.5°C, while sea level and, potentially, global mean precipitation (Pendergrass et al., 2015) <sup>[[#fn:r119|119]]</sup> would both be lower (see Figure 1.4). These differences could lead to very different impacts on agriculture, on some forms of extreme weather (e.g., Baker et al., 2018) <sup>[[#fn:r120|120]]</sup> , and on marine and terrestrial ecosystems (e.g., Mitchell et al., 2017 <sup>[[#fn:r121|121]]</sup> and Boxes 3.1 and 3.2). Sea level would be higher still if GMST returns to 1.5°C after an overshoot (Figure 1.4 d), with potentially significantly different impacts in vulnerable regions. Temperature overshoot could also cause irreversible impacts (see Chapter 3).

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'''Figure 1.4'''

<span id="different-1.5c-pathways-schematic-1-illustration-of-the-relationship-between-a-global-mean-surface-temperature-gmst-change-b-annual-rates-of-co-2-emissions-assuming-constant-fractional-contribution-of-non-co-2-forcing-to-total-human-induced-warming-c-total-cumulative-co-2-emissions-solid-lines-and-the-fraction-thereof-remaining-in-the-atmosphere-dashed-lines-these-also-indicates-changes-in-atmospheric-co-2-concentrations-and-d-a-time-integrated-impact-such-as-sea-level-rise-that-continues-to-increase-even-after-gmst-has-stabilized."></span>
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'''Different 1.5°C pathways Schematic <sup>[[#fn:1|1]]</sup> illustration of the relationship between (a) global mean surface temperature (GMST) change; (b) annual rates of CO <sub>2</sub> emissions, assuming constant fractional contribution of non-CO <sub>2</sub> forcing to total human-induced warming; (c) total cumulative CO <sub>2</sub> emissions (solid lines) and the fraction thereof remaining in the atmosphere (dashed lines; these also indicates changes in atmospheric CO <sub>2</sub> concentrations); and (d) a time-integrated impact, such as sea level rise, that continues to increase even after GMST has stabilized.'''
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[[File:821be06d1277f0d233698c109dc6082d figure-1.4-1024x717.png]]

Different 1.5°C pathways Schematic <sup>[[#fn:1|1]]</sup> illustration of the relationship between (a) global mean surface temperature (GMST) change; (b) annual rates of CO <sub>2</sub> emissions, assuming constant fractional contribution of non-CO <sub>2</sub> forcing to total human-induced warming; (c) total cumulative CO <sub>2</sub> emissions (solid lines) and the fraction thereof remaining in the atmosphere (dashed lines; these also indicates changes in atmospheric CO <sub>2</sub> concentrations); and (d) a time-integrated impact, such as sea level rise, that continues to increase even after GMST has stabilized. Colours indicate different 1.5°C pathways. Brown: GMST remaining below and stabilizing at 1.5°C in 2100; Green: a delayed start but faster emission reductions pathway with GMST remaining below and reaching 1.5°C earlier; Blue: a pathway temporarily exceeding 1.5°C, with temperatures reduced to 1.5°C by net negative CO <sub>2</sub> emissions after temperatures peak; and Yellow: a pathway peaking at 1.5°C and subsequently declining. Temperatures are anchored to 1°C above pre-industrial in 2017; emissions–temperature relationships are computed using a simple climate model (Myhre et al., 2013; Millar et al., 2017a; Jenkins et al., 2018) <sup>[[#fn:r122|122]]</sup> with a lower value of the Transient Climate Response (TCR) than used in the quantitative pathway assessments in Chapter 2 to illustrate qualitative differences between pathways: this figure is not intended to provide quantitative information. The time-integrated impact is illustrated by the semi-empirical sea level rise model of Kopp et al. (2016) <sup>[[#fn:r123|123]]</sup> .

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