Editing IPCC:AR6/SR15/Chapter-3 (section)

=== 3.4.8 Urban Areas ===

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There is new literature on urban climate change and its differential impacts on and risks for infrastructure sectors – energy, water, transport and buildings – and vulnerable populations, including those living in informal settlements (UCCRN, 2018) <sup>[[#fn:r1017|1017]]</sup> . However, there is limited literature on the risks of warming of 1.5°C and 2°C in urban areas. Heat-related extreme events (Matthews et al., 2017) <sup>[[#fn:r1018|1018]]</sup> , variability in precipitation (Yu et al., 2018) <sup>[[#fn:r1019|1019]]</sup> and sea level rise can directly affect urban areas (Section 3.4.5, Bader et al., 2018; Dawson et al., 2018) <sup>[[#fn:r1020|1020]]</sup> . Indirect risks may arise from interactions between urban and natural systems.

Future warming and urban expansion could lead to more extreme heat stress (Argüeso et al., 2015; Suzuki-Parker et al., 2015) <sup>[[#fn:r1021|1021]]</sup> . At 1.5°C of warming, twice as many megacities (such as Lagos, Nigeria and Shanghai, China) could become heat stressed, exposing more than 350 million more people to deadly heat by 2050 under midrange population growth. Without considering adaptation options, such as cooling from more reflective roofs, and overall characteristics of urban agglomerations in terms of land use, zoning and building codes (UCCRN, 2018) <sup>[[#fn:r1022|1022]]</sup> , Karachi (Pakistan) and Kolkata (India) could experience conditions equivalent to the deadly 2015 heatwaves on an annual basis under 2°C of warming (Akbari et al., 2009; Oleson et al., 2010; Matthews et al., 2017) <sup>[[#fn:r1023|1023]]</sup> . Warming of 2°C is expected to increase the risks of heatwaves in China’s urban agglomerations (Yu et al., 2018) <sup>[[#fn:r1024|1024]]</sup> . Stabilizing at 1.5°C of warming instead of 2°C could decrease mortality related to extreme temperatures in key European cities, assuming no adaptation and constant vulnerability (Jacob et al., 2018; Mitchell et al., 2018a) <sup>[[#fn:r1025|1025]]</sup> . Holding temperature change to below 2°C but taking urban heat islands (UHI) into consideration, projections indicate that there could be a substantial increase in the occurrence of deadly heatwaves in cities. The urban impacts of these heatwaves are expected to be similar at 1.5°C and 2°C and substantially larger than under the present climate (Matthews et al., 2017; Yu et al., 2018) <sup>[[#fn:r1026|1026]]</sup> . Increases in the intensity of UHI could exacerbate warming of urban areas, with projections ranging from a 6% decrease to a 30% increase for a doubling of CO <sub>2</sub> (McCarthy et al., 2010) <sup>[[#fn:r1027|1027]]</sup> . Increases in population and city size, in the context of a warmer climate, are projected to increase UHI (Georgescu et al., 2012; Argüeso et al., 2014; Conlon et al., 2016; Kusaka et al., 2016; Grossman-Clarke et al., 2017) <sup>[[#fn:r1028|1028]]</sup> .

For extreme heat events, an additional 0.5°C of warming implies a shift from the upper bounds of observed natural variability to a new global climate regime (Schleussner et al., 2016b) <sup>[[#fn:r1029|1029]]</sup> , with distinct implications for the urban poor (Revi et al., 2014; Jean-Baptiste et al., 2018; UCCRN, 2018) <sup>[[#fn:r1030|1030]]</sup> . Adverse impacts of extreme events could arise in tropical coastal areas of Africa, South America and Southeast Asia (Schleussner et al., 2016b) <sup>[[#fn:r1031|1031]]</sup> . These urban coastal areas in the tropics are particularly at risk given their large informal settlements and other vulnerable urban populations, as well as vulnerable assets, including businesses and critical urban infrastructure (energy, water, transport and buildings) (McGranahan et al., 2007; Hallegatte et al., 2013; Revi et al., 2014; UCCRN, 2018) <sup>[[#fn:r1032|1032]]</sup> . Mediterranean water stress is projected to increase from 9% at 1.5°C to 17% at 2°C compared to values in 1986–2005 period. Regional dry spells are projected to expand from 7% at 1.5°C to 11% at 2°C for the same reference period. Sea level rise is expected to be lower at 1.5°C than 2°C, lowering risks for coastal metropolitan agglomerations (Schleussner et al., 2016b) <sup>[[#fn:r1033|1033]]</sup> .

Climate models are better at projecting implications of greenhouse gas forcing on physical systems than at assessing differential risks associated with achieving a specific temperature target (James et al., 2017) <sup>[[#fn:r1034|1034]]</sup> . These challenges in managing risks are amplified when combined with the scale of urban areas and assumptions about socio-economic pathways (Krey et al., 2012; Kamei et al., 2016; Yu et al., 2016; Jiang and Neill, 2017) <sup>[[#fn:r1035|1035]]</sup> .

In summary, in the absence of adaptation, in most cases, warming of 2°C poses greater risks to urban areas than warming of 1.5°C, depending on the vulnerability of the location (coastal or non-coastal) ( ''high confidence'' ), businesses, infrastructure sectors (energy, water and transport), levels of poverty, and the mix of formal and informal settlements.

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