Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.10.2 Cities and Settlements by the Sea ====

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Cities and settlements by the sea are exposed to specific climate and climate change patterns and to compound coastal hazard risks (AR6 WGII Cross-Chapter Paper 2). The AR5 WGII found that, in general, ‘urban climate change-related risks are increasing (including rising sea levels and storm surges, heat stress, extreme precipitation, inland and coastal flooding, landslides, drought, increased aridity, water scarcity, and air pollution)’ ( [[#Revi--2014|Revi et al., 2014]] ). Since AR5 a number of studies have been carried out to understand urban climate and its change. Box 10.3 identified a continuing strong role of the urban heat island in amplifying heat extremes in cities, although changes in the urban heat island are an order of magnitude smaller than projected localized warming trends ( ''very high confidence'' ).

Coastal cities’ proximity to the sea somewhat mitigates the effect of urban heat islands ( ''high confidence'' ) ( [[#Salvati--2017|Salvati et al., 2017]] ; [[#Santamouris--2017|Santamouris et al., 2017]] ; Y. [[#Wang--2018|]] [[#Wang--2018|Wang et al., 2018]] ; [[#Martinelli--2020|Martinelli et al., 2020]] ). Cities and settlements by the sea typically experience higher humidity levels than inland regions, combining with heat to enhance heat stress and induce exceedance of critical heat stress thresholds for outdoor activities, with potential enhanced exposure to heat for informal settlements (J. [[#Wang--2019|]] [[#Wang--2019|]] [[#Wang--2019|]] [[#Wang--2019|Wang et al., 2019]] ). Such threshold exceedances are projected to increase for many coastal areas ( ''high confidence'' ), including the Persian Gulf where heat stress is projected to be extreme ( [[#Pal--2016|Pal and Eltahir, 2016]] ; [[#Ahmadalipour--2018|Ahmadalipour and Moradkhani, 2018]] ), and some low-lying areas in Europe such as the Po Valley and coastal Mediterranean areas ( [[#Coppola--2021a|Coppola et al., 2021a]] ; [[#Schwingshackl--2021|Schwingshackl et al., 2021]] ; see also the heat stress index shown in Figure 12.4d–f).

Climate change-related variations in oceanic drivers (e.g., relative sea level, storm surge, ocean waves), combined with tropical cyclones, extreme precipitation and river flooding, are expected to lead to more frequent and more intense coastal flooding and erosion ( ''very high confidence'' ) impacting cities and settlements located especially in low-elevation coastal zones and mega-deltas ( [[#Chan--2012|Chan et al., 2012]] , 2018; [[#Karymbalis--2012|Karymbalis et al., 2012]] ; [[#Hemer--2013|Hemer et al., 2013]] ; [[#Aerts--2014|Aerts et al., 2014]] ; [[#Neumann--2015|]] [[#Neumann--2015|B. Neumann et al., 2015]] ; [[#Hauer--2016|Hauer et al., 2016]] ; [[#Ranasinghe--2016|Ranasinghe, 2016]] ; [[#Hinkel--2018|Hinkel et al., 2018]] ; [[#Mavromatidi--2018|Mavromatidi et al., 2018]] ; [[#Marcos--2019|Marcos et al., 2019]] ; see also Sections 12.3, 12.4.1–12.4.7 and 12.4.9). Coastal erosion and flooding also pose challenges to critical infrastructure such as roads, subway tunnels, electricity and phone networks, wastewater management plants and buildings ( [[#Grahn--2017|Grahn and Nyberg, 2017]] ; [[#Pregnolato--2017|Pregnolato et al., 2017]] ). Compound flooding due to simultaneous storm surges and high river flows have been found to be increasingly frequent in several cities and/or low-lying areas in Europe and the USA ( ''high confidence'' ) ( [[#Wahl--2015|Wahl et al., 2015]] ; [[#Bevacqua--2019|Bevacqua et al., 2019]] ; [[#Ganguli--2019|Ganguli and Merz, 2019]] ). [[IPCC:Wg1:Chapter:Chapter-11|Chapter 11]] found that the frequency of such compound flood events is projected to increase ( ''high confidence'' ). In addition to changes induced by sea level change, many cities and settlements by the sea are in regions where tropical cyclones are projected to become more intense and severe tropical cyclones more frequent ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-11#11.7|Section 11.7]] ).

The SROCC highlighted coastal settlements in the Arctic as being particularly exposed to several CID changes ( [[#Magnan--2019|Magnan et al., 2019]] ). Enhanced waves, due to extended season of sea ice retreat, are projected to foster coastal flooding and erosion ( [[#12.4.9|Section 12.4.9]] ; [[#Gudmestad--2018|Gudmestad, 2018]] ; [[#Casas-Prat--2020|Casas-Prat and Wang, 2020]] ). Climate change is also affecting sea ice quality and season length along coasts of the Arctic Ocean where populations depend on sea ice for hunting or transportation ( [[#12.4.9|Section 12.4.9]] ; [[#Pearce--2015|Pearce et al., 2015]] ).

'''In summary, coastal cities and settlements are particularly affected by a number of climatic impact-drivers that have already changed and will continue to change whatever the emissions scenario. These include increases in extreme heat, pluvial floods, coastal erosion and coastal flood''' ( high confidence '''). Increasing relative sea level, compounding with increasing tropical cyclone storm surge and rainfall intensity, will increase the probability of coastal city flooding''' ( high confidence '''). Arctic coastal settlements are particularly exposed to climate change due to sea ice retreat''' ( high confidence ''').'''

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