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

== CCP2.1 Context of Cities and Settlements by the Sea ==

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=== CCP2.1.1 Introduction and Context ===

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This cross-chapter paper examines the distinctive roles played by C&amp;S by the sea in vulnerability and coastal hazard risk reduction, adaptation, resilience and sustainability in a changing climate. The paper builds upon evidence from AR5 ( [[#Wong--2014|Wong et al., 2014]] ), the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC; [[#Magnan--2019|Magnan et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) and draws material from across WGII AR6 (especially Chapters 3, 6, 9–15). It differs from the SLR-focused analysis of urban areas in SROCC ( [[IPCC:Wg2:Chapter:Chapter-4#4.3|Section 4.3]] ) through a more integrated assessment that distinguishes between archetypal coastal C&amp;S (Section CCP2.1.2), sectoral risks to C&amp;S by the sea (Section CCP2.2), responses to address these risks (Section CCP2.3) and enabling conditions and lessons learned (Section CCP2.4).

We define C&amp;S as concentrated human habitation centres, whether small or large, rural or urban ( [[IPCC:Wg2:Chapter:Chapter-6#6.1.3|Section 6.1.3]] ). We highlight the unique exposure and vulnerability of coastal C&amp;S resulting from rapid urbanisation at the narrow land–sea interface, and a high concentration of economic activity and at-risk people, many with long-standing cultural ties to the coast and dependence on coastal ecosystems that are prone to climate change impacts ( ''high confidence'' ; [[#He--2019|He and Silliman, 2019]] ; [[#Lau--2019|Lau et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Sterzel--2020|Sterzel et al., 2020]] ) ''.''

Presently, the coastal C&amp;S population exposure to ocean-driven impacts from SLR and other climate-driven impacts is considerable by any measure ( [[#Buddemeier--2008|Buddemeier et al., 2008]] ; [[#Barragán--2015|Barragán and de Andrés, 2015]] ; [[#Kay--2017|Kay and Alder, 2017]] ; [[#Haasnoot--2019|Haasnoot et al., 2019]] ; [[#McMichael--2020|McMichael et al., 2020]] ; [[#Sterzel--2020|Sterzel et al., 2020]] ). In 2020, almost 11% of the global population—896 million people—resided in C&amp;S within the low-elevation coastal zone (LECZ; coastal areas below 10 m of elevation above sea level that are hydrologically connected to the sea; [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ), a figure which will potentially increase beyond 1 billion by 2050 ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). Infrastructural and economic assets worth USD 6,500–11,000 billion are also exposed in the 1-in-100-year floodplain for C&amp;S of all sizes ( [[#Neumann--2015|Neumann et al., 2015]] ; [[#Muis--2016|Muis et al., 2016]] ; [[#Brown--2018|Brown et al., 2018]] ; [[#Andrew--2019|Andrew et al., 2019]] ; [[#Kulp--2019|Kulp and Strauss, 2019]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ; [[#Thomas--2020|Thomas et al., 2020]] ; [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ; [[#Hooijer--2021|Hooijer and Vernimmen, 2021]] ).

Further, coastal cities located at higher elevations (e.g., São Paulo, Brazil) or distantly located inland along tidally influenced rivers (e.g., the Recife Metropolitan Region, Brazil) also have populations and infrastructure exposed to climate impacts. As such, the inclusion of C&amp;S beyond the LECZ is warranted when assessing climate impacts and associated exposure, vulnerabilities and risks. The coastal zone includes some of the world’s largest, most densely populated megacities, as well as the fastest-growing urban areas. However, vast coastal areas are sparsely populated, with populations in these regions concentrated in smaller C&amp;S, including along subsiding shorelines and in deltas ( [[#Nicholls--2002|Nicholls and Small, 2002]] ; [[#McGranahan--2007|McGranahan et al., 2007]] ; [[#Merkens--2018|Merkens et al., 2018]] ; [[#Edmonds--2020|Edmonds et al., 2020]] ; [[#Nicholls--2021|Nicholls et al., 2021]] ). From this wider perspective, climate change impacts on the coast directly or indirectly affect a large portion of the global population, economic activity and associated critical infrastructure. Some estimates suggest that 23–37% of the global population lives within 100 km of the shoreline ( [[#Nicholls--2002|Nicholls and Small, 2002]] ; [[#Shi--2003|Shi and Singh, 2003]] ; Christopher Small and Joel E. Cohen, 2004; [[#McMichael--2020|McMichael et al., 2020]] ).

C&amp;S by the sea are thus on the frontline of action to adapt to climate change, mitigate greenhouse gas emissions and chart CRD pathways for several distinct reasons. First, home to a concentrated (and growing) portion of the world’s population, many coastal C&amp;S are simultaneously exposed and vulnerable to climate-compounded hazards as well as being centres of creativity and innovation ( [[#Glavovic--2013|Glavovic, 2013]] ; [[#Crescenzi--2017|Crescenzi and Rodríguez-Pose, 2017]] ; [[#Druzhinin--2021|Druzhinin et al., 2021]] ; [[#Mariano--2021|Mariano et al., 2021]] ; [[#Storbjörk--2021|Storbjörk and Hjerpe, 2021]] ). Second, people in C&amp;S by the sea rely on coastal ecosystems, many of which are highly sensitive to climate change impacts that compound non-climatic risks and increase the precarity of coastal livelihoods ( [[#Lu--2018|Lu et al., 2018]] ; [[#He--2019|He and Silliman, 2019]] ; [[#Thrush--2021|Thrush et al., 2021]] ). Third, coastal C&amp;S are linked together through a network of ports and harbours that underpin global trade and exchange, but which are prone to climate change impacts, especially SLR, with significant implications for global CRD prospects ( [[#Becker--2018|Becker et al., 2018]] ; [[#Christodoulou--2019|Christodoulou et al., 2019]] ; [[#Walsh--2019|Walsh et al., 2019]] ; [[#Hanson--2020|Hanson and Nicholls, 2020]] ). For these reasons, this paper assesses responses, enabling conditions and lessons learned for addressing climate change in C&amp;S by the sea.

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=== CCP2.1.2 Urbanisation in Coastal Systems: Coastal City and Settlement Archetypes ===

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This assessment uses an archetype framework categorizing coastal C&amp;S according to geomorphological characteristics, urban growth, economic resources and inequalities (Figure CCP2.1). Three broadly defined coastal settlement geomorphologies are used in each row: open coasts (a coast with sediment without river mouths) and two transitional coastal zones with river mouths, distinguishing between estuaries (a wetland receiving sediment from both fluvial and marine sources, which is affected by tide, wave and river processes) and deltas (a wetland where fluvial sediment is supplied and deposited more rapidly than it can be redistributed by basin processes such as waves and tides; [[#Bhattacharya--1978|Bhattacharya, 1978]] ; [[#Barragán--2015|Barragán and de Andrés, 2015]] ; [[#Kay--2017|Kay and Alder, 2017]] ; [[#Haasnoot--2019|Haasnoot et al., 2019]] ; [[#Sterzel--2020|Sterzel et al., 2020]] ). Small island C&amp;S are not singled out in this typology, because their coastlines often include the geomorphic features listed above, or require a different adaptation approach at larger spatial scales ( [[#Haasnoot--2019|Haasnoot et al., 2019]] ). Several coastal C&amp;S have a combination of two typologies, for example, Maputo-Matola, Mozambique and Mumbai, India, having both open and transitional riverine coasts, and can be classed as mixed. We also acknowledge that several coastal C&amp;S may have areas sited in mountainous topography that abruptly rises from the coast (e.g., along the Mediterranean), but generally these cities have narrow, densely populated coastlines exhibiting these three archetypal categories ( [[#Blackburn--2019|Blackburn et al., 2019]] ). Arctic settlements are addressed separately in this cross-chapter paper.

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[[File:5afa5447685670cfb558723380a36564 IPCC_AR6_WGII_Figure_CCP2_001.png]]

'''Figure CCP2.1 |''' '''Archetypal cities and settlements (C&amp;S) affected by ocean, terrestrial, geological, atmospheric and hydrological hazards driven by climate change.''' Coastal C&amp;S are grouped by physical geomorphology along estuarine, deltaic or open coasts ( [[#Barragán--2015|Barragán and de Andrés, 2015]] ; [[#Kay--2017|Kay and Alder, 2017]] ; [[#Haasnoot--2019|Haasnoot et al., 2019]] ). C&amp;S are also classified according to relative inequality (e.g., urban Gini coefficient or poverty rates) and growth rates (e.g., recent population growth and increasing density of urban form or built-up areas over the past decade; [[#OECD--2018|OECD, 2018]] ; ''[[#CEIC--2021|CEIC, 2021]]'' ; [[#OECD--2020|OECD, 2020]] ). Settlement types (e.g., informal, low-density or high-density developments) and economic resources (e.g., urban per capita gross domestic product) are also reflected in their respective categories. The bottom map shows location, 2020 population size and geomorphological types.

Coastal C&amp;S within these geomorphological categories are further distinguished according to higher or lower rates of urban growth and inequality, which can be estimated through population growth from national census data or areal extent of urban development ( [[#CEIC--2021|CEIC, 2021]] ), as well as by relative urban inequalities estimated by Gini coefficient data and urban–rural poverty rates ( [[#OECD--2018|OECD, 2018]] ; [[#OECD--2020|OECD, 2020]] ). Combining geomorphological and socioeconomic data accounts for urban–rural interconnections and differences, with levels of capital generation, diversity of economic functions and human development indices having previously been used to discern cultural, economic, administrative and political differences between cities and their hinterland ( [[#Blackburn--2019|Blackburn et al., 2019]] ; [[#Rocle--2020|Rocle et al., 2020]] ). For instance, the ecological, cultural and economic footprint of tertiary sectors, for example, coastal tourism associated with the Australian Great Barrier Reef, stretches far beyond the nearest onshore settlement of Cairns ( [[#Bohnet--2010|Bohnet and Pert, 2010]] ; [[#Brodie--2016|Brodie and Pearson, 2016]] ).

Some caveats are warranted. First, locating a specific city or settlement in a particular archetype does not account for future reclassification due to growth or shifts in development trajectories. Second, significant socioeconomic, political and governance variations exist within many C&amp;S, such as impoverished informal settlements alongside wealthy neighbourhoods in cities like Cape Town and São Paulo (also see Table SMCCP2.1). Third, this archetype framework does not explicitly reveal important interconnections between coastal C&amp;S and their hinterlands, or between particular C&amp;S through maritime trade or other economic, sociocultural and geopolitical interdependencies. Notwithstanding these caveats, these archetypes reveal differentiated physical impacts and socioeconomic conditions, as well as the variable challenges and opportunities arising when addressing climate change impacts and projected risk, which, depending on coastal type, C&amp;S size and resource availability, help to inform efforts to adapt and chart CRD for each archetype ( [[#Sánchez-Arcilla--2016|Sánchez-Arcilla et al., 2016]] ; [[#Rocle--2020|Rocle et al., 2020]] ; [[#Sterzel--2020|Sterzel et al., 2020]] ).

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