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

=== 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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'''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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