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

== CCP2.3 Adaptation in Cities and Settlements by the Sea ==

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=== CCP2.3.1 Introduction ===

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This section extends SROCC [[IPCC:Wg2:Chapter:Chapter-4|Chapter 4]] ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ), which focused on SLR, and draws from Chapters 6 and 9–15 to cover all C&amp;S archetypes. Adaptation interventions span psychosocial (e.g., awareness raising), economic (e.g., insurance), physical (e.g., retreat), technical (e.g., sea walls) and natural dimensions (e.g., wetland restoration; [[#Nicholls--2015|Nicholls et al., 2015]] ). Adaptation strategies for coastal C&amp;S are typically classified in terms of protect, accommodate, advance and retreat, which are used below.

Some coastal cities have adapted to meters of SLR in the past, indicating that adaptation is feasible ( [[#Esteban--2020a|Esteban et al., 2020a]] ), but future adaptation options are influenced by variations in projected socioeconomic conditions and rates of SLR (Cross-Chapter Box SLR in Chapter 3). To date, interventions are typically implemented reactively in response to extreme events ( ''high confidence'' ), but leading adaptors are increasingly proactive ( ''medium confidence'' ; [[#Araos--2016|Araos et al., 2016]] ; [[#Dulal--2019|Dulal, 2019]] ; [[#Dedekorkut-Howes--2020|Dedekorkut-Howes et al., 2020]] ) and those that move from previously rigid to more adaptive and flexible solutions, using an adaptation-pathways approach that keeps options open in the face of uncertainty, have improved climate risk management ( ''high confidence'' ; Sections 9.9.4; 10.5; 11.7; 12.5.5; 13.2; 14.7; 15.5; Cross-Chapter Box DEEP in Chapter 17; [[#Walker--2013|Walker et al., 2013]] ; [[#Marchau--2019|Marchau et al., 2019]] ).

The effectiveness of different strategies and interventions is mediated by physical coastal features for hard adaptation measures and by the scope and depth of soft adaptation measures, for example by the coverage and extent of social safety nets for the urban poor ( [[IPCC:Wg2:Chapter:Chapter-6#6.3|Section 6.3]] ). Their feasibility is also shaped by socioeconomic, cultural, political and institutional factors, for example social acceptance of measures (Section CCP2.2, SMCCP2.2.4). Together, response effectiveness and feasibility shape the solution space for mediating risks ( [[IPCC:Wg2:Chapter:Chapter-1#1.3.1.2|Section 1.3.1.2]] ; Figure CCP2.3; [[#Simpson--2021|Simpson et al., 2021]] ), which is achieved chiefly through governance interventions, for example laws and regulations ( [[#Haasnoot--2020|Haasnoot et al., 2020]] ). Access to financial resources expands the solution space, most notably for some resource-rich coastal archetypes (Section CCP2.4.2; Table SMCCP2.1; Sections 3.6; 14.7), but rapid population growth and unfolding climate-driven impacts can increase risks ( [[#Haasnoot--2021a|Haasnoot et al., 2021a]] ), especially for small island and poorer C&amp;S ( ''high confidence'' ; [[IPCC:Wg2:Chapter:Chapter-15#15.3|Section 15.3]] ; [[#Magnan--2020|Magnan and Duvat, 2020]] ).

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=== CCP2.3.2 Protection of Coastal Cities and Settlements ===

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==== CCP2.3.2.1 Hard Engineering Measures ====

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Hard engineering protective measures are commonly used to reduce coastal flooding and to drain or store excess water from intense precipitation. Many coastal cities, in particular densely populated and high-resource archetypes, have planned and are planning to continue a protection-based strategy, comprising, for example, breakwaters, sea walls and/or dikes, which could be raised or complemented with large barriers, or with ‘super-levees’ enabling construction on top of them ( ''high confidence'' ; Table SMCCP2.1; [[#Takagi--2016|Takagi et al., 2016]] ; [[#Haasnoot--2019|Haasnoot et al., 2019]] ; [[#Hall--2019|Hall et al., 2019]] ; [[#Esteban--2020b|Esteban et al., 2020b]] ).

Protection is effective in the short- to medium-term for many coastal cities, and can be cost effective in the 21st century (Section CCP2.4.2), but residual risk remains because protection can fail. Even under RCP8.5, technical limits to hard protection may only be reached after 2100 in many regions, but socioeconomic and institutional barriers may be reached before then ( [[#Hinkel--2018|Hinkel et al., 2018]] ). With progressive SLR, protection eventually becomes unaffordable and impractical ( [[#Strauss--2021|Strauss et al., 2021]] ). Combining hard engineering measures with nature-based solutions, spatial planning and early warning systems can help to contain residual risk ( [[#Du--2020|Du et al., 2020]] ). Protective works do not prevent salinisation and higher groundwater levels ( [[#Alves--2020|Alves et al., 2020]] ), and can lead to loss of coastal habitat (Cross-Chapter Box SLR in Chapter 3; [[#Achete--2017|Achete et al., 2017]] ; [[#Cooper--2020|Cooper et al., 2020]] ). Hard protective measures also create long-term path dependency as they last for decades and attract new development, locking in impact and exposure as C&amp;S grow, with the expectation of ongoing protection (Chapter 3; [[#Di%20Baldassarre--2015|Di Baldassarre et al., 2015]] ; [[#Gibbs--2016|Gibbs, 2016]] ; [[#Griggs--2019|Griggs and Patsch, 2019]] ; [[#Siders--2019a|Siders, 2019a]] ).

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==== CCP2.3.2.2 Soft Engineering and Sediment-Based Measures ====

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Sediment-based interventions, for example beach nourishment, aim to limit coastal erosion and flood risk and have become a widely applied strategy, especially in open-coast archetypal C&amp;S. This is in part because there is less impact on adjacent beaches and coastal ecology and also lower construction and maintenance costs compared to hard protection ( ''high confidence'' ; [[#Parkinson--2018|Parkinson and Ogurcak, 2018]] ). In addition, it is considered a flexible strategy under more rapid SLR conditions ( [[#Kabat--2009|Kabat et al., 2009]] ; [[#Stive--2013|Stive et al., 2013]] ) and can be applied in the form of a mega-nourishment strategy, wherein natural currents distribute sand along the coast ( [[#Stive--2013|Stive et al., 2013]] ; [[#de%20Schipper--2021|de Schipper et al., 2021]] ). However, there are limits to this strategy due to environmental impacts, costs and the availability of potential and permitted sand reserves, which may be unable to keep up with higher rates of SLR ( [[#Parkinson--2018|Parkinson and Ogurcak, 2018]] ; [[#Haasnoot--2019|Haasnoot et al., 2019]] ; [[#Harris--2021|Harris et al., 2021]] ; [[#Staudt--2021|Staudt et al., 2021]] ). Simultaneously, other socioeconomic needs (e.g., damming rivers or for building and transport infrastructure) may compete for sand as a limited resource ( [[#Torres--2017|Torres et al., 2017]] ; [[#Bendixen--2019|Bendixen et al., 2019]] ). Regional and global governance provisions (e.g., spatial reservations for sand mining, international frameworks for distribution) could improve long-term feasibility ( [[#Torres--2017|Torres et al., 2017]] ; [[#Parkinson--2018|Parkinson and Ogurcak, 2018]] ; [[#Bendixen--2019|Bendixen et al., 2019]] ; [[#Haasnoot--2019|Haasnoot et al., 2019]] ).

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==== CCP2.3.2.3 Nature-Based Measures ====

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Nature-based measures, such as retaining mangroves and marshes, have been successful in reducing deaths and damage due to storm surges ( ''medium evidence, high agreement'' ; [[#Das--2009|Das and Vincent, 2009]] ; [[#Saleh--2016|Saleh and Weinstein, 2016]] ; [[#Narayan--2017|Narayan et al., 2017]] ; [[#Triyanti--2017|Triyanti et al., 2017]] ; [[#Hochard--2019|Hochard et al., 2019]] ; [[#del%20Valle--2020|del Valle et al., 2020]] ), and reportedly provide USD 23.2 billion yr–1 in storm protection services across the USA ( [[#Saleh--2016|Saleh and Weinstein, 2016]] ). They are also a cost-effective strategy ( ''medium confidence'' ) that provides C&amp;S with additional co-benefits through ecosystem services ( ''high confidence'' ; Cross-Chapter Box NATURAL in Chapter 2; [[IPCC:Wg2:Chapter:Chapter-2#2.2|Section 2.2.4]] ; [[#Narayan--2016|Narayan et al., 2016]] ; [[#Depietri--2017|Depietri and McPhearson, 2017]] ; [[#Morris--2018|Morris et al., 2018]] ; [[#Reguero--2018|Reguero et al., 2018]] ; [[#Chausson--2020|Chausson et al., 2020]] ; [[#Du--2020|Du et al., 2020]] ; NIES and ISME, 2020; [[#Reguero--2020|Reguero et al., 2020]] ; [[#Sudmeier-Rieux--2021|Sudmeier-Rieux et al., 2021]] ).

Nature-based measures can reduce inland propagation of extreme sea levels (high tides, storm surges; ''high agreement'' ; [[#Godfroy--2019|Godfroy et al., 2019]] ; [[#James--2020|James et al., 2020]] ; [[#Zhu--2020b|Zhu et al., 2020b]] ), with vertical reduction in water levels ranging from 5 to 50 cm/km behind large mangroves and marshes ( [[#Stark--2015|Stark et al., 2015]] ; [[#Van%20Coppenolle--2020|Van Coppenolle and Temmerman, 2020]] ). They also attenuate wind-driven waves and reduce shoreline erosion ( ''high agreement'' ), and this can be by as much as 90% over stretches of 10–100 m for dense mangrove and marsh vegetation ( ''medium evidence'' ; [[#Li--2014|Li et al., 2014]] ; [[#Möller--2014|Möller et al., 2014]] ; [[#Vuik--2016|Vuik et al., 2016]] ; [[#Vuik--2018|Vuik et al., 2018]] ; [[#Godfroy--2019|Godfroy et al., 2019]] ; [[#Zhu--2020a|Zhu et al., 2020a]] ) and up to 40% for dunes ( [[#Feagin--2019|Feagin et al., 2019]] ). Coral reefs on average reduce wave energy by 97% ( [[#Ferrario--2014|Ferrario et al., 2014]] ). Seagrass meadows attenuate wind waves to a lesser extent, and are only effective in water &lt;0.2 m deep ( [[#Ondiviela--2014|Ondiviela et al., 2014]] ; [[#Narayan--2016|Narayan et al., 2016]] ; [[#Morris--2019|Morris et al., 2019]] ).

Within limits, coastal ecosystems can respond to RSL through sediment accretion and lateral inland movement ( [[#Kirwan--2016|Kirwan et al., 2016]] ; [[#Schuerch--2018|Schuerch et al., 2018]] ). Nature-based measures have the greatest potential in coastal deltas and estuaries, where human populations are exposed, but large ecosystems, like mangroves and marshes, can be conserved and restored ( [[#Menéndez--2020|Menéndez et al., 2020]] ; [[#Van%20Coppenolle--2020|Van Coppenolle and Temmerman, 2020]] ). Their feasibility depends on physical, ecological, institutional and socioeconomic conditions that are typically locality dependent ( [[#Temmerman--2015|Temmerman and Kirwan, 2015]] ; [[#Arkema--2017|Arkema et al., 2017]] ); space may not be available in certain places (e.g., intensive urbanization on the shoreline), or these measures may conflict with other human demands for scarce land ( [[#Tian--2016|Tian et al., 2016]] ). Successful nature-based measures require site-specific knowledge and science-based design, pilot monitoring and adaptive upscaling ( [[#Evans--2017|Evans et al., 2017]] ; [[#Nesshöver--2017|Nesshöver et al., 2017]] ), as well as a more rigorous understanding of long-term performance, maintenance and costs ( [[#Kumar--2021|Kumar et al., 2021]] ).

Nature-based measures are increasingly implemented in combination with hard protection measures ( [[#Hu--2019|Hu et al., 2019]] ; [[#Schoonees--2019|Schoonees et al., 2019]] ; [[#Morris--2020|Morris et al., 2020]] ; [[#Oanh--2020|Oanh et al., 2020]] ). They can reduce dike failure and increase design life where sediment accretion allows wetlands to respond to SLR ( [[#Jongman--2018|Jongman, 2018]] ; [[#Vuik--2019|Vuik et al., 2019]] ; [[#Zhu--2020a|Zhu et al., 2020a]] ). There is ''high agreement'' that a hybrid strategy combining hard and soft protection strategies is more effective and less costly under many circumstances, and there is ''limited evidence'' that technical limits will be encountered with such a strategy for low-lying C&amp;S built on soft or permeable soil or with high exposure to monsoons and river discharges ( [[#Spalding--2014|Spalding et al., 2014]] ; [[#Sutton-Grier--2015|Sutton-Grier et al., 2015]] ; [[#Pontee--2016|Pontee et al., 2016]] ; [[#Morris--2018|Morris et al., 2018]] ; [[#Reguero--2018|Reguero et al., 2018]] ; [[#Du--2020|Du et al., 2020]] ; [[#Morris--2020|Morris et al., 2020]] ; [[#Seddon--2020|Seddon et al., 2020]] ; [[#Waryszak--2021|Waryszak et al., 2021]] ).

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=== CCP2.3.3 Accommodation of the Built Environment ===

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The most effective solution for limiting the growth of climate risks in C&amp;S by the sea is to avoid new development in coastal locations prone to major flooding and/or SLR impacts ( ''very high confidence'' ; Cross-Chapter Box SLR in Chapter 3; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Doberstein--2019|Doberstein et al., 2019]] ). For existing C&amp;S, accommodation includes biophysical and institutional responses to reduce exposure and/or vulnerability of coastal residents, human activities, ecosystems and the built environment, enabling continued habitation of coastal C&amp;S ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). Next to hard protection, accommodation is the most widely used adaptation strategy across all archetypes to date ( ''high confidence'' ; [[#Sayers--2015|Sayers et al., 2015]] ; [[#Olazabal--2019|Olazabal et al., 2019]] ; [[#Le--2020|Le, 2020]] ). Measures include elevation or flood proofing of houses and other infrastructure ( [[#Garschagen--2015|Garschagen, 2015]] ; [[#Aerts--2018|Aerts et al., 2018]] ; [[#Buchori--2018|Buchori et al., 2018]] ; [[#Jamero--2018|Jamero et al., 2018]] ; [[#Tamura--2019|Tamura et al., 2019]] ), spatial planning (e.g., [[#Duy--2018|Duy et al., 2018]] ), amphibious building designs ( [[#Nilubon--2016|Nilubon et al., 2016]] ), increasing water storage and/or drainage capacity within C&amp;S ( [[#Chan--2018|Chan et al., 2018]] ), early warning systems and disaster responses ( [[#Hissel--2014|Hissel et al., 2014]] ) and slum upgrading ( [[#Jain--2017|Jain et al., 2017]] ; [[#Olthuis--2020|Olthuis et al., 2020]] ).

Raising land, or individual buildings, can avert flooding and be accomplished artificially or by nature-based interventions through river diversion and control in estuarine and deltaic archetypes ( [[#Nittrouer--2012|Nittrouer et al., 2012]] ; [[#Auerbach--2015|Auerbach et al., 2015]] ; [[#Day--2016|Day et al., 2016]] ; [[#Sánchez-Arcilla--2016|Sánchez-Arcilla et al., 2016]] ; [[#Hiatt--2019|Hiatt et al., 2019]] ; [[#Cornwall--2021|Cornwall, 2021]] ). Nature-based land elevation is limited by sediment supply and can address SLR rates of up to 10 mm yr–1 ( [[#Kleinhans--2010|Kleinhans et al., 2010]] ; [[#Kirwan--2016|Kirwan et al., 2016]] ; [[#IPCC--2019|IPCC, 2019]] ). It also assumes that existing land-use patterns permit land raising (e.g., in rural or newly developed areas; [[#Scussolini--2017|Scussolini et al., 2017]] ). Artificial land raising can achieve significant elevations and be implemented over a large spatial scale ( [[#Esteban--2015|Esteban et al., 2015]] ; [[#Esteban--2019|Esteban et al., 2019]] ). Raising land can be cost beneficial for small areas or where lower safety levels are satisfactory, but protection is usually more economical for larger areas, although both strategies are often combined ( [[#Lendering--2020|Lendering et al., 2020]] ).

Accommodation measures can be very effective for current conditions and small changes in SLR ( [[#Laurice%20Jamero--2017|Laurice Jamero et al., 2017]] ; [[#Scussolini--2017|Scussolini et al., 2017]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Du--2020|Du et al., 2020]] ; [[#Haasnoot--2021a|Haasnoot et al., 2021a]] ), and buy time to prepare for more significant changes in sea level and other climate-compounded coastal hazards. However, limits to this strategy occur comparatively soon in some locations, possibly requiring protection in the medium term and retreat in the long run and beyond 2100, particularly in scenarios of dramatic SLR ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). For the foreseeable future, accommodation can play an important role in combination with protective measures to form hybrid interventions, with higher effectiveness than either approach in isolation ( [[#Du--2020|Du et al., 2020]] ). Accommodation can play an increasingly important role where hard protection is neither technically nor financially viable, but detailed studies about expected trends of accommodation are lacking ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ).

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=== CCP2.3.4 Advance ===

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An advance strategy creates new land by building seaward, which can reduce risk for the hinterland and the newly elevated land, either by land reclamation through landfilling or polderisation through planting of vegetation to support natural land accretion ( [[#Wang--2014|Wang et al., 2014]] ; [[#Sengupta--2018|Sengupta et al., 2018]] ). Advance has occurred in all archetypes ( ''high confidence'' ), from open coasts (e.g., Singapore) and small atolls (e.g., Hulhumalé in the Maldives; [[#Hinkel--2018|Hinkel et al., 2018]] ; [[#Brown--2020|Brown et al., 2020]] ), to cities on estuaries (e.g., Rotterdam) and deltas (e.g., Shanghai [[#Sengupta--2020|Sengupta et al., 2020]] ), and mountainous coasts (e.g., Hong Kong SAR, China). Earth observations show that between 14,000 and 33,700 km 2 of land has been gained in coastal areas over the past 30 years, the dominant drivers being urban development and activities like fish farming ( [[#Donchyts--2016|Donchyts et al., 2016]] ; [[#Zhang--2017|Zhang et al., 2017]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ). Advancing seawards through large floating structures may be a viable option in the future ( [[#Wang--2019|Wang et al., 2019]] ; [[#Setiadi--2020|Setiadi et al., 2020]] ; [[#Wang--2020|Wang and Wang, 2020]] ) but is at an experimental stage, and, so far, only applied in calm water within a city as part of an accommodate strategy ( [[#Scussolini--2017|Scussolini et al., 2017]] ; [[#Penning-Rowsell--2020|Penning-Rowsell, 2020]] ; [[#Storbjörk--2021|Storbjörk and Hjerpe, 2021]] ).

Advance is seen as an attractive option to adapt to SLR in growing cities that are already densely populated and have limited available land for safe development, with a moderate to high adaptive capacity. But advance can have significant negative impacts on coastal ecosystems and livelihoods, requires substantial financial and material resources and time to build, and may be subject to land subsidence ( [[#Jeuken--2014|Jeuken et al., 2014]] ; [[#Garschagen--2018|Garschagen et al., 2018]] ; [[#Brown--2019|Brown et al., 2019]] ; [[#NYCEDC--2019|NYCEDC, 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Sengupta--2020|Sengupta et al., 2020]] ; [[#Bendixen--2021|Bendixen et al., 2021]] ).

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=== CCP2.3.5 Retreat ===

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Retreat is a strategy to reduce exposure and eventually risks facing coastal C&amp;S by moving people, assets and activities out of coastal hazard zones ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). This includes adaptive migration, involuntary displacement and planned relocation of population and assets from the coast ( [[IPCC:Wg2:Chapter:Chapter-7#7.2.6|Section 7.2.6]] ; Cross-Chapter Box CB-MIGRATE in Chapter 7).

Planned relocation in coastal C&amp;S with high hazard exposure and climate impacts is already occurring and has been increasing in frequency ( ''medium confidence'' ; [[#Hino--2017|Hino et al., 2017]] ; [[#Mortreux--2018|Mortreux et al., 2018]] ), with some small islands purchasing land in other countries to facilitate movement ( [[#Klepp--2018|Klepp, 2018]] ). In the Arctic, the pressure to relocate away from the coast is expected to rise given the interacting effects of permafrost thaw and coastal erosion. Native villages in Alaska are already relocating ( [[#Ristroph--2017|Ristroph, 2017]] ; [[#Ristroph--2019|Ristroph, 2019]] ). Involuntary resettlement may be a secondary effect of large-scale hard coastal protection projects, or inner-city river and canal regulation. In Jakarta, for example, a new giant seawall project involves resettling coastal households along large parts of the coastline ( [[#Garschagen--2018|Garschagen et al., 2018]] ).

Increased migration is to be expected across different climate scenarios, but there is ''limited evidence'' and ''medium agreement'' about the scale of climate-induced migration at the coast ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; Chapter 16, RKR on peace). Planned relocation is expected to rise in C&amp;S in response to SLR and other coastal hazards ( ''high agreement'' , ''medium evidence'' ; [[#Siders--2019|Siders et al., 2019]] ). Relocation has predominantly been reactive to date, but increased attention is being given to pre-emptive resettlement and the potential pathways and necessary governance, finance and institutional arrangements to support this strategy ( [[#Ramm--2018|Ramm et al., 2018]] ; [[#Lawrence--2020|Lawrence et al., 2020]] ; [[#Haasnoot--2021a|Haasnoot et al., 2021a]] ). There is ''limited evidence'' about the costs of planned relocation and retreat more generally ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ).

Retreat can effectively reduce exposure of urban residents to coastal hazards and provide opportunity for re-establishment of ecosystem services ( ''very high confidence'' ; [[#Song--2018|Song et al., 2018]] ; [[#Carey--2020|Carey, 2020]] ; [[#Hindsley--2020|Hindsley and Yoskowitz, 2020]] ; [[#Lincke--2020|Lincke et al., 2020]] ; [[#Lincke--2021|Lincke and Hinkel, 2021]] ). But there is ''high confidence'' that it can sever cultural ties to the coast ( [[#Reimann--2018|Reimann et al., 2018]] ) and lead to negative and inequitable socioeconomic effects for resettled communities if not planned and implemented in ways that are inclusive and just and address cultural, place-attachment and livelihood considerations ( [[#Ajibade--2019|Ajibade, 2019]] ; [[#Adger--2020|Adger et al., 2020]] ; [[#Carey--2020|Carey, 2020]] ; [[#Jain--2021|Jain et al., 2021]] ; [[#Johnson--2021|Johnson et al., 2021]] ), as well as the rights and practices of Indigenous People ( [[#Nakashima--2018|Nakashima et al., 2018]] ; [[#Ristroph--2019|Ristroph, 2019]] ; [[#Mohamed%20Shaffril--2020|Mohamed Shaffril et al., 2020]] ). If planned well ahead and aligned with social goals, pathways to managed retreat can achieve positive outcomes and provide opportunities for transformation of coastal C&amp;S ( [[#Haasnoot--2021a|Haasnoot et al., 2021a]] ; [[#Mach--2021|Mach and Siders, 2021]] ). There is ''medium confidence'' that the availability of suitable and affordable land as well as appropriate financing is a major bottleneck for planned relocation ( [[#Alexander--2012|Alexander et al., 2012]] ; [[#Ong--2016|Ong et al., 2016]] ; [[#Hino--2017|Hino et al., 2017]] ; [[#Fisher--2019|Fisher and Goodliffe, 2019]] ; [[#Hanna--2019|Hanna et al., 2019]] ; [[#Buser--2020|Buser, 2020]] ; [[#Doberstein--2020|Doberstein et al., 2020]] ), particularly in very dense mega-urban areas ( [[#Ajibade--2019|Ajibade, 2019]] ) and crowded small islands ( [[#Neise--2019|Neise and Revilla Diez, 2019]] ; [[#Weber--2019|Weber et al., 2019]] ; [[#Kool--2020|Kool et al., 2020]] ; [[#Lincke--2020|Lincke et al., 2020]] ).

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=== CCP2.3.6 Adaptation Pathways ===

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No single adaptation intervention comprehensively addresses coastal risks and enables CRD. An adaptation-pathways approach can facilitate long-term thinking, foresee maladaptive consequences and lock-ins, and address dynamic risk in the face of relentless and potentially high SLR; it can also frame adaptation as a series of manageable steps over time (Cross-Chapter Box DEEP in Chapter 17; Figure CCP2.4; [[#Haasnoot--2019|Haasnoot et al., 2019]] ). A portfolio of hard, soft and nature-based interventions can be used to implement strategies to protect, accommodate, retreat and advance, individually or in combination.

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[[File:602fc39cdd9891dc633e3e2f74084e03 IPCC_AR6_WGII_Figure_CCP2_004.png]]

'''Figure CCP2.4 |''' '''Generic adaptation pathways for coastal cities and settlements (a) and the typical solution space, with illustrative pathways for three coastal archetypes (b).''' As risk increases under rising sea levels, solutions need to be combined or sequenced in order to contain risk. Pathways involve different trade-offs. Based on Tables SMCCP2.1–2.3; Chapters 11 and 13; [[#Magnan--2020|Magnan and Duvat (2020)]] ; [[#Lawrence--2020|Lawrence et al. (2020)]] ; [[#Haasnoot--2019|Haasnoot et al. (2019)]] . Depending on local conditions, archetype and risk tolerance, alternative pathways are needed and possible to contain risk. Dashed lines indicate uncertainty in the pathway (a); dashed and plain borders are used for illustrating various local situations within each archetype (b).

The strategy and the portfolio of interventions can be adjusted in response to new information about SLR and other climate risks according to economic, environmental, social, institutional, technical or other objectives. In cases of rapid SLR, it may be necessary to implement a short-term protection strategy to buy time to implement more transformative and enduring strategies ( ''high confidence'' ; [[#Du--2020|Du et al., 2020]] ; [[#Lawrence--2020|Lawrence et al., 2020]] ; [[#Morris--2020|Morris et al., 2020]] ; [[#Haasnoot--2021a|Haasnoot et al., 2021a]] ). There is ''high agreement'' that combining and sequencing adaptation interventions can reduce risk over time ( [[#Du--2020|Du et al., 2020]] ; [[#Morris--2020|Morris et al., 2020]] ). Phasing interventions can help to spread costs and minimise regret ( [[#de%20Ruig--2019|de Ruig et al., 2019]] ), provided options are kept open to adjust to changing conditions ( [[#Buurman--2016|Buurman and Babovic, 2016]] ; [[#Haasnoot--2019|Haasnoot et al., 2019]] ; [[#Hall--2019|Hall et al., 2019]] ).

Many megacities plan to continue a protection strategy (Table SMCCP2.1). This becomes increasingly costly, institutionally challenging and requires space, possibly facilitated through local relocation. There is ''high agreement'' that many C&amp;S are locked-in to a self-reinforcing pathway: coastal defences have a long lifetime and attract people and assets that require further protection ( [[#Gralepois--2016|Gralepois et al., 2016]] ; [[#Bubeck--2017|Bubeck et al., 2017]] ; [[#Welch--2017|Welch et al., 2017]] ; [[#Di%20Baldassarre--2018|Di Baldassarre et al., 2018]] ; [[#Jongman--2018|Jongman, 2018]] ). Transitioning to alternative pathways may involve major transfer and sunk costs (e.g., [[#Gralepois--2016|Gralepois et al., 2016]] ), but these may prove to be less costly in the long term. Because of considerable inertia in the built form of cities, such transitions have a greater chance of success and alignment with societal goals if embedded early into C&amp;S planning and development processes that enable transformational change and CRD (Sections 6.4.8; 11.7; 13.11; Box 18.1; [[#Ürge-Vorsatz--2018|Ürge-Vorsatz et al., 2018]] ; Siders 2019b).

In islands, hybrid options of nature-based (where space and environmental conditions allow) and protection measures (on wealthy, already densely populated islands) could reduce risk for low SLR in the next few decades ( [[IPCC:Wg2:Chapter:Chapter-15#15.5|Section 15.5]] ). Where feasible, retreat is a compelling option to reduce risk (Figure CCP2.4). With higher rates and levels of SLR in the medium to long term, financial, governance and material barriers may differentiate resource-rich and more rural islands, leading to a dichotomy between which islands retreat or can rely on protection for a period of time.

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