Editing IPCC:AR6/WGII/Cross-Chapter-Paper-2 (section)

== CCP2.2 Climate Change Risks to Cities and Settlements by the Sea ==

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Coastal C&amp;S are at the forefront of climate risk (FAQ CCP2.1). The dynamic interaction between ocean and climate drivers and varied coastal geographies influences the character of coastal risks, including many that are unique to C&amp;S by the sea. The interaction of coastal hazards with exposure and vulnerability is differentiated by coastal archetypes, leading to distinct climate change-compounded risks and associated responses (Figure CCP2.2; [[IPCC:Wg2:Chapter:Chapter-1#1.3.1.2|Section 1.3.1.2]] ; [[#Simpson--2021|Simpson et al., 2021]] ).

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'''Figure CCP2.2 |''' '''Schematic of how climate and ocean drivers (from WGI Chapter 12.''' '''4.10.2 (Ranasinghe et al., 2021)) and consequential physical impacts on coastal cities and settlements (C&amp;S) influence risks assessed in (Section CCP2.2; figure based on [[#Simpson--2021|Simpson et al., 2021]] , and [[IPCC:Wg2:Chapter:Chapter-1#1.3.1.2|Section 1.3.1.2]] ).''' These risks to C&amp;S by the sea are shaped and mediated by adaptation interventions aimed at reducing vulnerability and exposure to coastal hazards given settlement archetypes, as well as by expanding the space for responses to risk via the enabling conditions assessed in Section CCP2.4. Note that exposure to coastal hazards is controlled chiefly by underlying coastal C&amp;S geomorphology and changes in coastal hazards and urban growth, including population and infrastructure growth. Vulnerability is controlled, for example, by socioeconomic development and inequality, and responses that shape the risks assessed in Section CCP2.3 can be enhanced by enabling conditions, including behavioural change, conducive finance and prudent governance.

Overall, interactions between climatic and non-climatic drivers of coastal change are increasing the frequency and intensity of many coastal hazards, with settlement archetypes and the wider coastal zone subject to escalating risk ( ''high confidence'' ; Figure CCP2.2; Table SMCCP2.1 for examples of selected coastal C&amp;S). Risks can vary markedly between different archetypes: C&amp;S sited on deltaic and estuarine coasts face additional risks of pluvial flooding compared to open coasts, while greater vulnerabilities arise in coastal settlements with higher inequalities.

Risks to C&amp;S by the sea were extensively covered in SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) and also in Chapters 3, 6 and regional chapters; in this paper, specific risks to livelihoods, activities, the built environment and ecosystems are assessed in detail in Supplementary Material SMCCP2.1. The ocean and climate impact drivers influencing these risks are assessed in WGI (Ranasinghe et al., 2021 ), which include extreme heat, pluvial floods from increasing rainfall intensity, coastal erosion and coastal flood driven by increasing SLR, and tropical cyclone storm surges ( ''high confidence'' ). Further, Arctic coastal settlements are particularly exposed to climate change due to sea ice retreat as well as from permafrost melt ( ''high confidence'' ).

Without adaptation, risks to land and people in coastal C&amp;S from pluvial and coastal flooding will ''very likely'' [[#footnote-001|2]] increase substantially by 2100 and ''likely'' beyond as a result of SLR, with significant impacts even under RCP2.6 ( [[#Neumann--2015|Neumann et al., 2015]] ; [[#Muis--2016|Muis et al., 2016]] ; [[#Brown--2018|Brown et al., 2018]] ; [[#Nicholls--2018|Nicholls et al., 2018]] ; [[#Kulp--2019|Kulp and Strauss, 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ; [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ). Across these studies, by 2100, 158–510 million people and USD 7,919–12,739 billion assets under RCP4.5, and 176–880 million people and USD 8,813–14,178 billion assets under RCP8.5 will be within the 1-in-100-year floodplain ( ''very high confidence'' ). There is ''medium confidence'' that accelerated SLR will increase shoreline erosion globally, although biophysical feedbacks will allow many coastlines to maintain relatively stable morphology if room exists to accommodate mangroves in estuarine and deltaic coasts and beach movement along open coasts ( [[#Kench--2015|Kench et al., 2015]] ; [[#McLean--2015|McLean and Kench, 2015]] ; [[#Perkins--2015|Perkins et al., 2015]] ; [[#Richards--2016|Richards and Friess, 2016]] ; [[#CCC--2017|CCC, 2017]] ; [[#Duncan--2018|Duncan et al., 2018]] ; [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ; [[#Schuerch--2018|Schuerch et al., 2018]] ; [[#Ghosh--2019|Ghosh et al., 2019]] ; [[#Masselink--2020|Masselink et al., 2020]] ; [[#Toimil--2020|Toimil et al., 2020]] ; [[#Vousdoukas--2020b|Vousdoukas et al., 2020b]] ). Limiting emissions to RCP2.6 (corresponding to a mean post-industrial global temperature increase of 1.5–2°C) significantly reduces future SLR risks ( [[#Hinkel--2014|Hinkel et al., 2014]] ; [[#Brown--2018|Brown et al., 2018]] ; [[#Nicholls--2018|Nicholls et al., 2018]] ; [[#Schinko--2020|Schinko et al., 2020]] ). For example, by 2100, the population at risk of permanent submergence increases by 26% under RCP2.6 compared with 53% under RCP8.5 (median values from [[#Kulp--2019|Kulp and Strauss, 2019]] ).

There is ''high confidence'' regarding regionally differentiated but considerable global sectoral impacts in coastal C&amp;S arising from exposure to hazards. Tangible impacts include damage, loss of life and loss of livelihoods, especially fisheries and tourism ( [[#Tessler--2015|Tessler et al., 2015]] ; [[#Avelino--2018|Avelino et al., 2018]] ; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#Seekamp--2019|Seekamp et al., 2019]] ; [[#Arabadzhyan--2020|Arabadzhyan et al., 2020]] ); negative impacts on health and wellbeing, especially under extreme events ( [[#McIver--2016|McIver et al., 2016]] ; [[#Bakkensen--2019|Bakkensen and Mendelsohn, 2019]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Pugatch--2019|Pugatch, 2019]] ); and involuntary displacement and migration ( [[#Hauer--2017|Hauer, 2017]] ; [[#Davis--2018|Davis et al., 2018]] ; [[#Neef--2018|Neef et al., 2018]] ; [[#Boas--2019|Boas et al., 2019]] ; [[#McLeman--2021|McLeman et al., 2021]] ). Intangible impacts include psychological impacts due to extreme events such as heatwaves, flooding, droughts and tropical cyclones; heightened inequality in coastal archetypes with systematic gender/ethnicity/structural vulnerabilities; and loss of things of personal or cultural value and sense of place or connection, including an existential risk of the demise of nations due to submergence ( [[#Allison--2015|Allison and Bassett, 2015]] ; [[#Barnett--2017|Barnett, 2017]] ; [[#Schmutter--2017|Schmutter et al., 2017]] ; [[#Weir--2017|Weir et al., 2017]] ; [[#Farbotko--2020|Farbotko et al., 2020]] ; [[#Hauer--2020|Hauer et al., 2020]] ; [[#Hoffmann--2020|Hoffmann et al., 2020]] ; [[#Bell--2021|Bell et al., 2021]] ). Impacts extend beyond the coastal zone, for example disruption to ports and supply chains, with major geopolitical and economic ramifications from the C&amp;S to the global scale ( ''very high confidence'' ; [[#Becker--2018|Becker et al., 2018]] ; [[#Camus--2019|Camus et al., 2019]] ; [[#Christodoulou--2019|Christodoulou et al., 2019]] ; [[#Walsh--2019|Walsh et al., 2019]] ; [[#Hanson--2020|Hanson and Nicholls, 2020]] ; [[#Yang--2020|Yang and Ge, 2020]] ; [[#Izaguirre--2021|Izaguirre et al., 2021]] ; [[#León-Mateos--2021|León-Mateos et al., 2021]] ; [[#Ribeiro--2021|Ribeiro et al., 2021]] ).

Many coastal C&amp;S have densely built physical infrastructure and assets that are exposed and vulnerable to climate change-compounded coastal hazards. There is ''high confidence'' that SLR, land subsidence, poorly regulated coastal development and the rise of asset values are major drivers of future risk in all coastal archetypes and, without adaptation, built environment risks—especially in archetypes with high exposure due to rapid growth—are expected to rise considerably in this century across all RCPs ( [[#Koks--2019|Koks et al., 2019]] ; [[#Magnan--2019|Magnan et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Abadie--2020|Abadie et al., 2020]] ; [[#Nicholls--2021|Nicholls et al., 2021]] ). Archetypes with more informal settlements are often disproportionally exposed to coastal risks ( [[#Roy--2016|Roy et al., 2016]] ; [[#Hallegatte--2017|Hallegatte et al., 2017]] ; [[#Bangalore--2019|Bangalore et al., 2019]] ).

There is ''high confidence'' that loss of coastal ecosystem services will increase risks to all coastal C&amp;S archetypes that include reduced provisioning of materials and food (e.g., wood, fishery habitat; [[#Kok--2021|Kok et al., 2021]] ), amelioration of coastal hazards (e.g., attenuation of storm surges, waves and containing erosion) ( [[IPCC:Wg2:Chapter:Chapter-2#2.3|Section 2.3.2.3]] ; [[#Godfroy--2019|Godfroy et al., 2019]] ; [[#Schoutens--2019|Schoutens et al., 2019]] ; [[#Zhu--2020b|Zhu et al., 2020b]] ), climate change mitigation (through carbon sequestration; [[#Macreadie--2017|Macreadie et al., 2017]] ; [[#Rovai--2018|Rovai et al., 2018]] ; [[#Ward--2020|Ward, 2020]] ), water quality regulation (nutrient, pollutant and sediment retention and cycling; [[#Wilson--2018|Wilson et al., 2018]] ; [[#Zhao--2018|Zhao et al., 2018]] ) and recreation and tourism ( [[#Pueyo-Ros--2018|Pueyo-Ros et al., 2018]] ).

Most studies of coastal C&amp;S focus on adaptation to a single or limited set of risks, but there is ''high confidence'' that compound and cascading risks significantly alter C&amp;S risk profiles ( [[#Nicholls--2015|Nicholls et al., 2015]] ; [[#Estrada--2017|Estrada et al., 2017]] ; [[#Edmonds--2020|Edmonds et al., 2020]] ; [[#Eilander--2020|Eilander et al., 2020]] ; [[#Yin--2020|Yin et al., 2020]] ; [[#Ghanbari--2021|Ghanbari et al., 2021]] ). Extreme events can lead to cascading infrastructure failures that cause damage and economic losses well beyond the coastal zone ( [[#Haraguchi--2016|Haraguchi and Kim, 2016]] ; [[#Kishore--2018|Kishore et al., 2018]] ; [[#Rey--2019|Rey et al., 2019]] ; [[#So--2019|So et al., 2019]] ), and have forced evacuation of C&amp;S and small islands ( [[#Look--2019|Look et al., 2019]] ; [[#Thomas--2020|Thomas and Benjamin, 2020]] ). These risks are exacerbated by non-climatic drivers, for example compound and cascading impacts arising from exposure to tropical cyclones and COVID-19 that threaten population health and hamper pandemic responses ( [[#Salas--2020|Salas et al., 2020]] ; [[#Shultz--2020a|Shultz et al., 2020a]] ; [[#Shultz--2020b|Shultz et al., 2020b]] ). There is emerging evidence ( ''low confidence'' ) from individual coastal C&amp;S and regional case studies (e.g., in Europe, Australia and the US) to illustrate the increasing influence of compound risks on vulnerability due to accelerating climate change ( [[#Wahl--2015|Wahl et al., 2015]] ; [[#Xu--2019|Xu et al., 2019]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ).

Figure CCP2.3 shows that ocean-driven coastal risks to people, land and infrastructure in East and Southeast Asia are highest compared to other regions, even for low levels of projected SLR. However, risks facing coastal C&amp;S are high across the globe, especially under higher SLR projections ( ''high confidence'' ). Without adaptation, the population at risk of a 100-year coastal flood increases by ~20% if the global mean sea level rises by 0.15 m relative to current levels; this at-risk population doubles at a rise of 0.75 m in mean sea level and triples at 1.4 m. Simultaneously, coastal C&amp;S are projected to experience shoreline retreat, with coastlines having more than 100 m of retreat increasing by ~165% if current mean sea levels rise between 0.23 and 0.53 m. Ocean-driven flooding in coastal C&amp;S is also projected to disrupt flights by up to three orders of magnitude per year in selected coastal C&amp;S as mean sea level increases. Typically, larger risks correspond to archetypes associated with higher inequality and high growth rates, especially in deltas, leading to larger vulnerability and exposure, respectively, under higher warming levels.

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'''Figure CCP2.3 |''' '''Map of coastal cities and settlements’ risks according to IPCC regions''' '', showing risks to people (number of people at risk from a 100-year coastal flood event; Haasnoot et al.'' '', 2021b), risks of loss of coastal land (length of coast with more than 100 m retreat; [[#Vousdoukas--2020b|Vousdoukas et al., 2020b]] ), risks to the built'' en vironment (airports at risk indicated by expected annual number of flights disrupted by coastal flooding ( [[#Yesudian--2021|Yesudian and Dawson, 2021]] ) and risk to wetlands (± indicates positive or negative area change; [[#Schuerch--2018|Schuerch et al., 2018]] ). Risks are reported against global mean sea level rise relative to 2020, depending on data availability.

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