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

==== 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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