Editing IPCC:AR6/WGII/Chapter-13 (section)

===== 13.6.1.5.2 Risks from heatwaves, cold waves and drought =====

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Heatwave days and number of long heatwaves increased in most capitals from 1998–2015 compared with 1980–1997 ( [[#Morabito--2017|Morabito et al., 2017]] ; [[#Seneviratne--2021|Seneviratne et al., 2021]] ). In the summer of 2018, many cities suffered from heatwaves attributed to climate change ( [[#Vogel--2019|Vogel et al., 2019]] ; [[#Undorf--2020|Undorf et al., 2020]] ). As a result, indoor overheating and reduced outdoor thermal comfort, often coupled with urban heat island (UHI) effect, have already impacted European cities (see also [[#13.7.1|Section 13.7.1]] ; [[#Di%20Napoli--2018|Di Napoli et al., 2018]] ; [[#EEA--2020b|EEA, 2020b]] ).

Heatwaves are ''likely'' to become a major threat, not only for SEU but also for WCE and EEU cities ( [[#Russo--2015|Russo et al., 2015]] ; [[#Guerreiro--2018|Guerreiro et al., 2018]] ; [[#Lorencova--2018|Lorencova et al., 2018]] ; [[#Smid--2019|Smid et al., 2019]] ). At 2°C GWL and SSP3, half of the European population will be under very high risk of heat stress in summer ( [[#Rohat--2019|Rohat et al., 2019]] ). The UHI effect will further increase urban temperatures ( [[#Estrada--2017|Estrada et al., 2017]] ). In many cities, hospitals and social housing tend to be located within the intense UHI, thus increasing exposure to vulnerable groups ( [[#EEA--2020b|EEA, 2020b]] ). There is ''high confidence'' that overheating during summer in buildings with insufficient ventilation and/or solar protection will increase strongly, with thermal comfort hours potentially decreasing by 74% in SEU at 3°C GWL ( [[#Jenkins--2014a|Jenkins et al., 2014a]] ; [[#Hamdy--2017|Hamdy et al., 2017]] ; [[#Heracleous--2018|Heracleous and Michael, 2018]] ; [[#Dino--2019|Dino and Meral Akgül, 2019]] ; [[#Shen--2020|Shen et al., 2020]] ). Highly insulated buildings, following present building standards, will be vulnerable to overheating, particularly under high GWL levels, unless adequate adaptation measures are applied ( [[#Williams--2013|Williams et al., 2013]] ; [[#Virk--2014|Virk et al., 2014]] ; [[#Mulville--2016|Mulville and Stravoravdis, 2016]] ; [[#Fosas--2018|Fosas et al., 2018]] ; [[#Ibrahim--2018|Ibrahim and Pelsmakers, 2018]] ; [[#Salem--2019|Salem et al., 2019]] ; [[#Tian--2020|Tian et al., 2020]] ). Cities in NEU and WCE are more vulnerable due to limited solar shading and fewer air conditioning installations ( [[#Ward--2016|Ward et al., 2016]] ; [[#Thomson--2019|Thomson et al., 2019]] ). Cooling energy demand in SEU buildings has been projected to increase by 81–104% by 2035 and 91–244% after 2065 compared with 1961–1990 depending on GWL ( [[#Cellura--2018|Cellura et al., 2018]] ). Increases of 31–73% by 2050 and 165–323% by 2100 compared with 1996–2005 were estimated for buildings in NEU ( [[#Dodoo--2016|Dodoo and Gustavsson, 2016]] ) with risks modified by adaptation ( [[#13.6.2|Section 13.6.2]] ; [[#Viguié--2020|Viguié et al., 2020]] ). Cold waves beyond 3°C GWL will not represent an effective threat for European cities at the end of the century, and only a marginal hazard under 2°C GWL ( [[#Smid--2019|Smid et al., 2019]] ).

At 2°C GWL and beyond, cities in SEU and large parts of WCE would exceed the historical maximum 12-month Drought Severity index of the past 50 years (see [[#13.2|Section 13.2]] on drought risks) and 30% will have at least 30% probability of exceeding this maximum every month ( [[#Guerreiro--2018|Guerreiro et al., 2018]] ). This could adversely affect the operation of municipal water services ( [[#Kingsborough--2016|Kingsborough et al., 2016]] ). For example, under 2°C GWL, the reservoir storage volume is predicted to decrease for all of England and Wales catchments, resulting in a probability of years with water-use restrictions doubling by 2050 and quadrupling by 2100 compared with 1975–2004 ( [[#Dobson--2020|Dobson et al., 2020]] ). The combination of high temperatures, drought and extreme winds, potentially coupled with insufficient preparedness and adaptation, may amplify the damage of wildfires in peri-urban environments ( [[#13.3.1.3|Section 13.3.1.3]] ). High fuel load combined with proximity of the built environment to wildland highly increases fire risks ( [[#EEA--2020b|EEA, 2020b]] ).

Extreme heat and drought causes shrinking and swelling of clays, threatening the stability of small houses in peri-urban environments ( [[#Pritchard--2015|Pritchard et al., 2015]] ), with damage costs of 0.9–1 billion EUR during the 2003 heatwave ( [[#Corti--2011|Corti et al., 2011]] ). In WCE and SEU, mean annual damage costs could increase by 50% for 2°C GWL, and by a factor of 2 for 3°C GWL ( [[#Naumann--2021|Naumann et al., 2021]] ).

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