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

== 1.3.3 Uncertainty and Non-Linearity of Impacts ==

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Uncertainties in projections of future climate change and impacts come from a variety of different sources, including the assumptions made regarding future emission pathways (Moss et al., 2010) <sup>[[#fn:r229|229]]</sup> , the inherent limitations and assumptions of the climate models used for the projections, including limitations in simulating regional climate variability (James et al., 2017) <sup>[[#fn:r230|230]]</sup> , downscaling and bias-correction methods (Ekström et al., 2015) <sup>[[#fn:r231|231]]</sup> , the assumption of a linear scaling of impacts with GMST used in many studies (Lewis et al., 2017; King et al., 2018b) <sup>[[#fn:r232|232]]</sup> , and in impact models (e.g., Asseng et al., 2013) <sup>[[#fn:r233|233]]</sup> . The evolution of climate change also affects uncertainty with respect to impacts. For example, the impacts of overshooting 1.5°C and stabilization at a later stage compared to stabilization at 1.5°C without overshoot may differ in magnitude (Schleussner et al., 2016) <sup>[[#fn:r234|234]]</sup> .

AR5 (IPCC, 2013b) <sup>[[#fn:r235|235]]</sup> and World Bank (2013) <sup>[[#fn:r236|236]]</sup> underscored the non-linearity of risks and impacts as temperature rises from 2°C to 4°C of warming, particularly in relation to water availability, heat extremes, bleaching of coral reefs, and more. Recent studies (Schleussner et al., 2016; James et al., 2017; Barcikowska et al., 2018; King et al., 2018a) <sup>[[#fn:r237|237]]</sup> assess the impacts of 1.5°C versus 2°C warming, with the same message of non-linearity. The resilience of ecosystems, meaning their ability either to resist change or to recover after a disturbance, may change, and often decline, in a non-linear way. An example are reef ecosystems, with some studies suggesting that reefs will change, rather than disappear entirely, and with particular species showing greater tolerance to coral bleaching than others (Pörtner et al., 2014) <sup>[[#fn:r238|238]]</sup> . A key issue is therefore whether ecosystems such as coral reefs survive an overshoot scenario, and to what extent they would be able to recover after stabilization at 1.5°C or higher levels of warming (see Box 3.4).

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