Editing IPCC:AR6/WGII/Chapter-6 (section)

==== 6.2.2.1 Temperatures and the Urban Heat Island ====

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Higher temperatures associated with climate change, through warmer global average temperatures and regional heatwave episodes, will interact with urban systems in a variety of ways (Doblas-Reyes et al., 2021 Box 10.3). Future urbanisation will amplify projected local air temperature increase, particularly by strong influence on minimum temperatures, which is approximately comparable in magnitude to global warming ( ''high confidence'' ) (Arias et al. In Press Box TS14). Within cities, exposure to heat island effects is uneven, with some populations disproportionately exposed to risk including low income communities, children, the elderly, disabled, and ethnic minorities (Quintana-Talvac et al., 2021; Sabrin et al., 2020; [[#Chambers--2020|Chambers, 2020]] ; and see later in this section).

The risks to cities, settlements and infrastructure from heatwaves will worsen ( ''high confidence'' ) (Leal Filho et al., 2021; see also Sections 6.2.5; 6.3.3.1, Arias et al. In Press Box TS14). Depending on the RCP, between half (RCP2.6) to three-quarters (RCP8.5) of the human population could be exposed to periods of life-threatening climatic conditions arising from coupled impacts of extreme heat and humidity by 2100 (Figure 6.3; Mora et al., 2017; Zhao et al., 2021). Cities in mid-latitudes are potentially subject to twice the levels of heat stress compared with their rural surroundings under all RCP scenarios by 2050, for example Belgian cities (Wouters et al., 2017). A disproportionate level of exposure exists in subtropical cities subject to year-round warm temperatures and higher humidity, requiring less warming to exceed ‘dangerous’ thresholds, for example Nairobi (Scott et al., 2017) and São Paulo (Diniz, Gonçalves and Sheridan, 2020). It is expected that more than 90% of the 300 million people who will be exposed to super- and ultra-extreme heatwaves in the Middle East and North Africa will live in urban centres (Zittis et al., 2021), while the major driver for increased heat exposure is the combination of global warming and population growth in already-warm cities in regions including Africa, India and the Middle East ( [[#Klein--2021|Klein and Anderegg, 2021]] ).

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'''Figure 6.3 |''' '''Global distribution of population exposed to hyperthermia from extreme heat for (a) the present, and projections from selected Representative Concentration Pathways in (b) the mid-21st century and (c) the end of the 21st century.''' Shading indicates projected number of days in a year in which conditions of air temperature and humidity surpass a common threshold beyond which climate conditions turned deadly and pose a risk of death (Mora et al ''.'' , 2017). Named cities are the top 15 urban areas by population size during 2020, 2050 and 2100, respectively, as projected by [[#Hoornweg--2017|Hoornweg and Pope (2017)]]

Locally, the urban heat island also elevates temperatures within cities relative to their surroundings. It is caused by physical changes to the surface energy balance of the pre-urban site from urbanisation, resulting from the thermal characteristics and spatial arrangement of the built environment, and anthropogenic heat release (Oke et al., 2017; Chow et al., 2014; [[#Susca--2020|Susca and Pomponi, 2020]] ; Doblas-Reyes et al., 2021 FAQ10.1). A considerable body of evidence exists on how the multi-scale impacts and consequent risks arise when local elevated temperatures within settlements are enhanced by climate change, with specific elements of this affecting megacities (Darmanto et al., 2019). The urban heat island itself is amplified during heatwaves ( [[#Founda--2017|Founda and Santamouris, 2017]] ), but the extent to which varies regionally and by time of day (Ward et al., 2016a; Zhao et al., 2018b; Eunice Lo et al., 2020). When combined with warming induced by urban growth, extreme heat risks are expected to affect half of the future urban population, with a particular impact in the tropical Global South and in coastal cities and settlements (Huang et al., 2019; Section [https://www.ipcc.ch/chapter/6#CCP2.2 CCP2.2.2] ; Table CCP2.A.1).

Heat risk is associated with a range of health issues for urban residents, with the consequences of higher urban temperatures being unevenly distributed across urban populations ( ''high confidence'' ). Clear evidence exists of increased health risks to elderly populations in settlements, especially higher levels of mortality in elderly populations from urban heat islands during heatwave events ( [[#Fernandez%20Milan--2015|Fernandez Milan and Creutzig, 2015]] ; Taylor et al., 2015; Ward et al., 2016a; Heaviside, Macintyre and Vardoulakis, 2017; Gough et al., 2019; Xu et al., 2020a), while health and fitness variables are also major determinants of the effects of heat stress (Schuster et al., 2017) (see also Table 7.2). Heat stress and dehydration are also related to behavioural and learning concerns, with dehydration impairing concentration and cognition for both adults and children ( [[#Merhej--2019|Merhej, 2019]] ). Literature on paediatric heat exposure is associated with increases in emergency department visits for heat-related illnesses, electrolyte imbalances, fever, renal disease and respiratory disease in young children (Winquist et al., 2016), with less severe outcomes such as lethargy, headaches, rashes, cramps and exhaustion negatively affecting children in school and play environments ( [[#Vanos--2015|Vanos, 2015]] ; [[#Hyndman--2017|Hyndman, 2017]] ). Young children in cities are particularly sensitive to heatwaves, and may have little experience or capacity to cope with heat extremes ( [[#Norwegian%20Red%20Cross--2019|Norwegian Red Cross, 2019]] ). Such vulnerability of young children to heat is compounded with projected urbanisation rates and poor infrastructure, particularly in South Asian and in African cities ( [[#Smith--2019|Smith, 2019]] ). There is evidence that socioeconomically disadvantaged populations are more ''likely'' to live in hotter parts of cities associated with higher-density residential land use in dwellings with less effective insulation built with poorer or older construction materials (Inostroza, Palme and de la Barrera, 2016; Tomlinson et al., 2011). Specific emerging risks for occupational and related heat illnesses are found in urban tropical or subtropical low- and middle-income countries (Andrews et al., 2018; Green et al., 2019).

There is an emerging risk of diminished indoor thermal comfort due to climate change, evidenced by research into negatively affected thermal comfort indices and/or increased number of overheating hours under future emissions scenarios ( ''medium confidence'' ) (e.g., [[#Liu--2015|Liu and Coley, 2015]] ; van Hooff et al., 2014; Vardoulakis et al., 2015; [[#Dodoo--2016|Dodoo and Gustavsson, 2016]] ; [[#Invidiata--2016|Invidiata and Ghisi, 2016]] ; [[#Makantasi--2016|Makantasi and Mavrogianni, 2016]] ; [[#Mulville--2016|Mulville and Stravoravdis, 2016]] ; Taylor et al., 2016; Hamdy et al., 2017; Pérez-Andreu et al., 2018; Salthammer et al., 2018; Dino and Meral Akgül, 2019; [[#Osman--2019|Osman and Sevinc, 2019]] ; Roshan, Oji and Attia, 2019). Decreases in thermal comfort and increases in overheating risks depend on building characteristics, such as thermal resistance, presence of solar shading, thermal mass, ventilation, orientation and geographical location (e.g., [[#Liu--2015|Liu and Coley, 2015]] ; van Hooff et al., 2014; Vardoulakis et al., 2015; [[#Dodoo--2016|Dodoo and Gustavsson, 2016]] ; [[#Invidiata--2016|Invidiata and Ghisi, 2016]] ; [[#Makantasi--2016|Makantasi and Mavrogianni, 2016]] ; [[#Mulville--2016|Mulville and Stravoravdis, 2016]] ; Taylor et al., 2016; Hamdy et al., 2017; Pérez-Andreu et al., 2018; Salthammer et al., 2018; Dino and Meral Akgül, 2019; [[#Osman--2019|Osman and Sevinc, 2019]] ; Roshan, Oji and Attia, 2019; Alves, Gonçalves and Duarte, 2021). Most of these studies employed numerical simulations in which different climate scenarios were used to construct future climate data. In hot climates, energy-efficient buildings with high insulation values and high airtightness, which have insufficient protection from solar heat gains and/or limited ventilation capabilities, are generally more vulnerable to overheating than older buildings with lower insulation levels (e.g., van Hooff et al., 2014; Vardoulakis et al., 2015; [[#Makantasi--2016|Makantasi and Mavrogianni, 2016]] ; [[#Mulville--2016|Mulville and Stravoravdis, 2016]] ; Salthammer et al., 2018; [[#Fisk--2015|Fisk, 2015]] ; Hamdy et al., 2017; Fosas et al., 2018; [[#Ozarisoy--2019|Ozarisoy and Elsharkawy, 2019]] ; see also Fox-Kemper et al., 2021 9.7 for building heat mitigation/adaptation links).

Higher urban temperatures result in lower labour productivity levels and economic outputs ( ''medium confidence'' ) ( [[#Graff%20Zivin--2014|Graff Zivin and Neidell, 2014]] ; [[#Yi--2017|Yi and Chan, 2017]] ; Houser et al., 2015; [[#Stevens--2017|Stevens, 2017]] ; see [[IPCC:Wg2:Chapter:Chapter-8#8.2.1|Section 8.2.1]] ). Globally, urban heat stress is projected to reduce labour capacity by 20% in hot months by 2050 compared with a current 10% reduction (Dunne, Stouffer and John, 2013). Burke et al. (2015) demonstrate a nonlinear relationship between temperature and global economic productivity, with potential global losses of 23% by 2100 due to climate change alone. In specific cases, [[#Zander--2015|Zander et al. (2015)]] estimate heat-related reductions in urban labour productivity in Australia to cost USD 3.6–5.1 billion yr −1 , based on self-reported performance reduction and absenteeism among 1726 workers in 2013–14 [[#footnote-002|2]] ; while the high-temperature subsidies given in China at outdoor air temperatures above 35°C are projected to increase to USD 35.7 billion yr −1 after 2030 (compared with USD 5.5 billion yr −1 for 1979–2005) (Zhao et al., 2016) [[#footnote-001|3]] .

Higher urban temperatures place unequal economic stresses on residents and households through higher utilities demand during warm periods, for example, electricity in regions where air conditioning is predicted to become more prevalent, and due to medical costs associated with care for heat illnesses and related health effects, missed work and other related impacts ( ''medium confidence'' ) (Jovanović et al., 2015; Liu et al., 2019; Schmeltz, Petkova and Gamble, 2016; [[#Soebarto--2014|Soebarto and Bennetts, 2014]] ; [[#Zander--2019|Zander and Mathew, 2019]] ; Zander et al., 2015). Such stresses are projected to increase in many regions associated with continuing global-scale climate change and urbanisation (e.g., Véliz et al., 2017; Ang, Wang and Ma, 2017; Bezerra et al., 2021), although some of these effects in cold-climate cities are offset by reduced stresses in winter associated with urban heat island or rising temperatures more generally (see [[#6.2.2.4|Section 6.2.2.4]] ).

Thermal inequity can also be seen as a distributive justice risk ( [[#Mitchell--2018|Mitchell and Chakraborty, 2018]] ). There are often disproportionate increases of risk for individuals of lower socioeconomic status, especially migrants, from exposure to urban heat. These arise from inadequate housing, less access to air-conditioning, and occupations, such as manual labour and waste picking, that exacerbate heat exposure ( [[#Chu--2018|Chu and Michael, 2018]] ; Santha et al., 2016). Research from South Africa has shown that housing occupied by poor communities regularly experience indoor temperature fluctuations that are between 4°C and 5°C warmer compared with outdoor temperatures (Naicker et al., 2017); while evidence from the USA indicates that historical housing policies, particularly the ‘redlining’ of neighbourhoods based on racially motivated perceptions, are associated with areas that are exposed to elevated land surface temperatures (Hoffman, Shandas and Pendleton, 2020).

Social surveys from temperate and tropical cities highlight the risk of reduced quality of life during heat events, including increased incidence of personal discomfort in indoor and outdoor settings, elevated anxiety, depression and other indicators of adverse psychological health, and reductions in physical activity, social interactions, work attendance, tourism and recreation ( ''high confidence'' ) (Chow et al., 2016; Elnabawi, Hamza and Dudek, 2016; [[#Obradovich--2017|Obradovich and Fowler, 2017]] ; Wang et al., 2017; Wong et al., 2017; Lam, Loughnan and Tapper, 2018; Alves, Duarte and Gonçalves, 2016). Extreme heat may also have a cultural impact, for example affecting major sporting events, with negative impacts on the athletic performance (Brocherie, Girard and Millet, 2015; Casa et al., 2015) and the experience and health of spectators (Hosokawa, Grundstein and Casa, 2018; Kosaka et al., 2018; Matzarakis et al., 2018; Vanos et al., 2019).

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