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

== 1.3 Impacts at 1.5°C and Beyond ==

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=== 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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=== 1.3.2 Drivers of Impacts ===

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Impacts of climate change are due to multiple environmental drivers besides rising temperatures, such as rising atmospheric CO <sub>2</sub> , shifting rainfall patterns (Lee et al., 2018) <sup>[[#fn:r211|211]]</sup> , rising sea levels, increasing ocean acidification, and extreme events, such as floods, droughts, and heat waves (IPCC, 2014a) <sup>[[#fn:r212|212]]</sup> . Changes in rainfall affect the hydrological cycle and water availability (Schewe et al., 2014; Döll et al., 2018; Saeed et al., 2018) <sup>[[#fn:r213|213]]</sup> . Several impacts depend on atmospheric composition, increasing atmospheric carbon dioxide levels leading to changes in plant productivity (Forkel et al., 2016) <sup>[[#fn:r214|214]]</sup> , but also to ocean acidification (Hoegh-Guldberg et al., 2007) <sup>[[#fn:r215|215]]</sup> . Other impacts are driven by changes in ocean heat content such as the destabilization of coastal ice sheets and sea level rise (Bindoff et al., 2007; Chen et al., 2017) <sup>[[#fn:r216|216]]</sup> , whereas impacts due to heat waves depend directly on ambient air or ocean temperature (Matthews et al., 2017) <sup>[[#fn:r217|217]]</sup> . Impacts can be direct, such as coral bleaching due to ocean warming, and indirect, such as reduced tourism due to coral bleaching. Indirect impacts can also arise from mitigation efforts such as changed agricultural management (Section 3.6.2) or remedial measures such as solar radiation modification (Section 4.3.8, Cross-Chapter Box 10 in Chapter 4).

Impacts may also be triggered by combinations of factors, including ‘impact cascades’ (Cramer et al., 2014) <sup>[[#fn:r218|218]]</sup> through secondary consequences of changed systems. Changes in agricultural water availability caused by upstream changes in glacier volume are a typical example. Recent studies also identify compound events (e.g., droughts and heat waves), that is, when impacts are induced by the combination of several climate events (AghaKouchak et al., 2014; Leonard et al., 2014; Martius et al., 2016; Zscheischler and Seneviratne, 2017) <sup>[[#fn:r219|219]]</sup> .

There are now techniques to attribute impacts formally to anthropogenic global warming and associated rainfall changes (Rosenzweig et al., 2008; Cramer et al., 2014; Hansen et al., 2016) <sup>[[#fn:r220|220]]</sup> , taking into account other drivers such as land-use change (Oliver and Morecroft, 2014) <sup>[[#fn:r221|221]]</sup> and pollution (e.g., tropospheric ozone; Sitch et al., 2007) <sup>[[#fn:r222|222]]</sup> . There are multiple lines of evidence that climate change has observable and often severely negative effects on people, especially where climate-sensitive biophysical conditions and socio-economic and political constraints on adaptive capacities combine to create high vulnerabilities (IPCC, 2012a; 2014a; World Bank, 2013) <sup>[[#fn:r223|223]]</sup> . The character and severity of impacts depend not only on the hazards (e.g., changed climate averages and extremes) but also on the vulnerability (including sensitivities and adaptive capacities) of different communities and their exposure to climate threats. These impacts also affect a range of natural and human systems, such as terrestrial, coastal and marine ecosystems and their services; agricultural production; infrastructure; the built environment; human health; and other socio-economic systems (Rosenzweig et al., 2017) <sup>[[#fn:r224|224]]</sup> .

Sensitivity to changing drivers varies markedly across systems and regions. Impacts of climate change on natural and managed ecosystems can imply loss or increase in growth, biomass or diversity at the level of species populations, interspecific relationships such as pollination, landscapes or entire biomes. Impacts occur in addition to the natural variation in growth, ecosystem dynamics, disturbance, succession and other processes, rendering attribution of impacts at lower levels of warming difficult in certain situations. The same magnitude of warming can be lethal during one phase of the life of an organism and irrelevant during another. Many ecosystems (notably forests, coral reefs and others) undergo long-term successional processes characterised by varying levels of resilience to environmental change over time. Organisms and ecosystems may adapt to environmental change to a certain degree, through changes in physiology, ecosystem structure, species composition or evolution. Large-scale shifts in ecosystems may cause important feedbacks, in terms of changing water and carbon fluxes through impacted ecosystems – these can amplify or dampen atmospheric change at regional to continental scale. Of particular concern is the response of most of the world’s forests and seagrass ecosystems, which play key roles as carbon sinks (Settele et al., 2014; Marbà et al., 2015) <sup>[[#fn:r225|225]]</sup> .

Some ambitious efforts to constrain atmospheric greenhouse gas concentrations may themselves impact ecosystems. In particular, changes in land use, potentially required for massively enhanced production of biofuels (either as simple replacement of fossil fuels, or as part of bioenergy with carbon capture and storage, BECCS) impact all other land ecosystems through competition for land (e.g., Creutzig, 2016) <sup>[[#fn:r226|226]]</sup> (see Cross-Chapter Box 7 in Chapter 3, Section 3.6.2.1).

Human adaptive capacity to a 1.5°C warmer world varies markedly for individual sectors and across sectors such as water supply, public health, infrastructure, ecosystems and food supply. For example, density and risk exposure, infrastructure vulnerability and resilience, governance, and institutional capacity all drive different impacts across a range of human settlement types (Dasgupta et al., 2014; Revi et al., 2014; Rosenzweig et al., 2018) <sup>[[#fn:r227|227]]</sup> . Additionally, the adaptive capacity of communities and human settlements in both rural and urban areas, especially in highly populated regions, raises equity, social justice and sustainable development issues. Vulnerabilities due to gender, age, level of education and culture act as compounding factors (Arora-Jonsson, 2011; Cardona et al., 2012; Resurrección, 2013; Olsson et al., 2014; Vincent et al., 2014) <sup>[[#fn:r228|228]]</sup> .

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=== 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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