Editing IPCC:AR6/WGII/Chapter-3 (section)

===== 3.6.3.1.1 Coastal community development and settlement =====

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Coastal adaptation often addresses the risk of flooding and erosion from SLR, changes in storm activity and degradation of coastal ecosystems and their services ( ''high confidence'' ) (Sections 3.4.2, 3.5; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). Without coastal protection, people and property will be increasingly exposed to coastal flooding after 2050, especially under RCP8.5 (Cross-Chapter Box SLR in Chapter 3; [[#Bevacqua--2020|Bevacqua et al., 2020]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). This section assesses adaptation responses for coastal ecosystems, addressing loss of natural coastal protection (Sections 3.4.2.1, 3.4.2.4–3.4.2.6), and the need for relocation ( [[#3.6.2.1|Section 3.6.2.1.2]] ). Adaptation responses specific to SLR are assessed in detail in Cross-Chapter Box SLR in Chapter 3, while adaptation in coastal cities and settlements is assessed in Chapter 6.

Coastal conservation tends to involve cost-effective, low-impact actions that aim to support both adaptation and mitigation by conserving a wide array of ecosystem functions and services ( [[#Gattuso--2018|Gattuso et al., 2018]] ; [[#Gattuso--2021|Gattuso et al., 2021]] ), and that are achievable by nations with extensive coastlines or low-income status ( [[#Herr--2017|Herr et al., 2017]] ; [[#Taillardat--2018|Taillardat et al., 2018]] ). Where coastlines are undeveloped, the lowest-risk option is to avoid new development, but elsewhere, coastal conservation includes protection of key assets, accommodation of SLR, advancing defences seawards or upwards, or planned retreat from the coast (Cross-Chapter Box SLR in Chapter 3).

Hard-engineered structures like seawalls are generally more costly than nature-based adaptations ( ''high confidence'' ) ( [[#Hérivaux--2018|Hérivaux et al., 2018]] ; [[#Haasnoot--2019|Haasnoot et al., 2019]] ; [[#Nicholls--2019|Nicholls et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) and can lock communities into engineered responses in the future (Cross-Chapter Box SLR in Chapter 3), creating trade-offs with mitigation goals, which constitutes maladaptation ( [[#Nunn--2021|Nunn et al., 2021]] ) that carries ecological and cultural costs (Sections 3.4.2.4, 3.4.2.6, 3.5.6). As a result, there is ''high agreement'' on the importance of shifting from hard infrastructure to soft infrastructure for coastal defence ( [[#Toimil--2020|Toimil et al., 2020]] ; [[#Nunn--2021|Nunn et al., 2021]] ). The common remedy for beach erosion is beach nourishment ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Pinto--2020|Pinto et al., 2020]] ; [[#Elko--2021|Elko et al., 2021]] ), which provides rapid results but poorly quantified trade-offs between efficacy, long-term cost, utility to beach users and ecological damage ( [[#de%20Schipper--2021|de Schipper et al., 2021]] ).

Since SROCC, coastal adaptation using NbS, like restoration of coastal vegetation, has advanced substantially ( [[#Cohen-Shacham--2019|Cohen-Shacham et al., 2019]] ; [[#Kuhl--2020|Kuhl et al., 2020]] ; [[#Kumar--2020|Kumar et al., 2020]] ; [[#Morris--2020a|Morris et al., 2020a]] ). Field and modelling studies confirm that wetland restoration and preservation are key actions to restore coastal protection and reduce community vulnerability to flooding ( ''very high confidenc'' e) (see also [[#3.6|Section 3.6]] ; Chapter 15; Cross-Chapter Box SLR in Chapter 3; [[#Jones--2020|Jones et al., 2020]] ; [[#Menéndez--2020|Menéndez et al., 2020]] ; [[#Van%20Coppenolle--2020|Van Coppenolle and Temmerman, 2020]] ), while maintaining coastal ecosystem services ( [[#3.5|Section 3.5]] ). Restoring coral reefs, oyster reefs and mangroves ( [[#3.6.2.1|Section 3.6.2.1]] ) and protecting macrophyte meadows dissipates wave energy ( [[#3.4.2.1|Section 3.4.2.1]] ; [[#Yates--2017|Yates et al., 2017]] ; [[#Beck--2018|Beck et al., 2018]] ; [[#Wiberg--2019|Wiberg et al., 2019]] ; [[#Menéndez--2020|Menéndez et al., 2020]] ), accretes sediment and elevate shorelines, which reduces exposure to waves and storm surges, and offsets erosional losses ( ''medium confidence'' ) ( [[#Kench--2017|Kench and Mann, 2017]] ; [[#Pomeroy--2018|Pomeroy et al., 2018]] ; [[#Dasgupta--2019|Dasgupta et al., 2019]] ; [[#James--2019|James et al., 2019]] ; [[#Morris--2019|Morris et al., 2019]] ; [[#David--2020|David and Schlurmann, 2020]] ; [[#Masselink--2020|Masselink et al., 2020]] ). However, irreversible regime shifts in ocean ecosystems due to SLR and extreme events, such as MHWs, can limit or compromise restoration in the long term ( ''high confidence'' ) ( [[#3.4.3.3.3|Section 3.4.3.3.3]] ; Cross-Chapter Box SLR in Chapter 3; [[#Marzloff--2016|Marzloff et al., 2016]] ; [[#Johnson--2017a|Johnson et al., 2017a]] ). Under all warming scenarios, coastal wetlands will be impacted by warming and MHWs (Sections 3.2.2.1, 3.2.4.5; Cross-Chapter Box 9.1 in WGI Chapter 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), while also being pressed inland by RSLR ( [[#3.4.2.5|Section 3.4.2.5]] ; Cross-Chapter Box SLR in Chapter 3). Therefore, restoration and conservation are more successful when non-climate drivers are also minimised ( ''high confidence'' ) ( [[#Brodie--2020|Brodie et al., 2020]] ; [[#Duarte--2020|Duarte et al., 2020]] ; [[#Liu--2021|Liu et al., 2021]] ).

For highly exposed human settlements, migration is an adaptation option (e.g., for some island populations under extreme circumstances), but there are important uncertainties ( [[IPCC:Wg2:Chapter:Chapter-15#15.3.4.6|Section 15.3.4.6]] ), as international regimes develop around human rights, migration ( [[#Scobie--2019a|Scobie, 2019a]] ), displacement (George [[#Puthucherril--2012|Puthucherril, 2012]] ) and the implications for national sovereignty ( [[#Yamamoto--2014|Yamamoto and Esteban, 2014]] ) of disappearing land spaces caused by climate change. Colonial power dynamics can influence climate-change responses (Chapter 18), for example, when external funders favour migration over local desires to adapt in place to preserve national identity and sovereignty ( [[#Bordner--2020|Bordner et al., 2020]] ). Examples of relocation within livelihoods’ customary land show some successes ( [[IPCC:Wg2:Chapter:Chapter-15#15.3.4.6|Section 15.3.4.6]] ).

Evidence since SROCC (Section, 5.5.2.3.3; [[#Bindoff--2019a|Bindoff et al., 2019a]] ) continues to show that built infrastructure cannot address all of the adaptation challenges that coastal communities face. Coastal squeeze creates tensions between coastal development, armouring and habitat management (Sections 3.4.2.4–3.4.2.6). Managed realignment is the best option to reduce risks from SLR ( ''high confidence'' ) (Cross-Chapter Box SLR in Chapter 3) but requires transformative changes in coastal development and settlement (Felipe [[#Pérez--2021|Pérez and Tomaselli, 2021]] ; [[#Fitton--2021|Fitton et al., 2021]] ; [[#Mach--2021|Mach and Siders, 2021]] ; [[#Siders--2021|Siders et al., 2021]] ). Implementation of protective measures varies among nations and lack of financial resources limits the options available ( ''very high confidence'' ) (Cross-Chapter Box SLR in Chapter 3; [[#Hinkel--2018|Hinkel et al., 2018]] ; [[#Klöck--2019|Klöck and Nunn, 2019]] ).

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