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

=== 1.3.1 Definitions ===

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Consistent with the AR5 (IPCC, 2014a) <sup>[[#fn:r205|205]]</sup> , ‘impact’ in this report refers to the effects of climate change on human and natural systems. Impacts may include the effects of changing hazards, such as the frequency and intensity of heat waves. ‘Risk’ refers to potential negative impacts of climate change where something of value is at stake, recognizing the diversity of values. Risks depend on hazards, exposure, vulnerability (including sensitivity and capacity to respond) and likelihood. Climate change risks can be managed through efforts to mitigate climate change forcers, adaptation of impacted systems, and remedial measures (Section 1.4.1).

In the context of this report, ''regional'' impacts of ''global'' warming at 1.5°C and 2°C are assessed in Chapter 3. The ‘ ''warming experience at 1.5°C'' ’ is that of regional climate change (temperature, rainfall, and other changes) at the time when global average temperatures, as defined in Section 1.2.1, reach 1.5°C above pre-industrial (the same principle applies to impacts at any other global mean temperature). Over the decade 2006–2015, many regions have experienced higher than average levels of warming and some are already now 1.5°C or more warmer with respect to the pre-industrial period (Figure 1.3). At a global warming of 1.5°C, some seasons will be substantially warmer than 1.5°C above pre-industrial (Seneviratne et al., 2016) <sup>[[#fn:r206|206]]</sup> . Therefore, most regional impacts of a global mean warming of 1.5°C will be different from those of a regional warming by 1.5°C.

The impacts of 1.5°C global warming will vary in both space and time (Ebi et al., 2016) <sup>[[#fn:r207|207]]</sup> . For many regions, an increase in global mean temperature by 1.5°C or 2°C implies substantial increases in the occurrence and/or intensity of some extreme events (Fischer and Knutti, 2015; Karmalkar and Bradley, 2017; King et al., 2017; Chevuturi et al., 2018) <sup>[[#fn:r208|208]]</sup> , resulting in different impacts (see Chapter 3). By comparing impacts at 1.5°C versus those at 2°C, this report discusses the ‘avoided impacts’ by maintaining global temperature increase at or below 1.5°C as compared to 2°C, noting that these also depend on the pathway taken to 1.5°C (see Section 1.2.3 and Cross-Chapter Box 8 in Chapter 3 on 1.5°C warmer worlds). Many impacts take time to observe, and because of the warming trend, impacts over the past 20 years were associated with a level of human-induced warming that was, on average, 0.1°C–0.23°C colder than its present level, based on the AR5 estimate of the warming trend over this period (Section 1.2.1 and Kirtman et al., 2013) <sup>[[#fn:r209|209]]</sup> . Attribution studies (e.g., van Oldenborgh et al., 2017) <sup>[[#fn:r210|210]]</sup> can address this bias, but informal estimates of ‘recent impact experience’ in a rapidly warming world necessarily understate the temperature-related impacts of the current level of warming.

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