Editing IPCC:AR6/SROCC/Chapter-4 (section)

===== 4.4.2.3.4 Effectiveness of ecosystem-based adaptation =====

While EbA has been able to reduce the impacts of sea level related hazards, there is still ''little agreement'' on the size of the effect (Gedan et al., 2011 <sup>[[#fn:r1674|1674]]</sup> ; Doswald et al., 2012 <sup>[[#fn:r1675|1675]]</sup> ; Lo, 2016; Renaud et al., 2016 <sup>[[#fn:r1676|1676]]</sup> ). Dozens of independent field, experimental and numerical studies have observed and measured the wave attenuation and flood reduction benefits provided by natural habitats, such as marsh and mangrove wetlands (Barbier and Enchelmeyer, 2014 <sup>[[#fn:r1677|1677]]</sup> ; Möller et al., 2014 <sup>[[#fn:r1678|1678]]</sup> ; Rupprecht et al., 2017 <sup>[[#fn:r1679|1679]]</sup> ), coral reefs (Ferrario et al., 2014 <sup>[[#fn:r1680|1680]]</sup> ; Storlazzi et al., 2017 <sup>[[#fn:r1681|1681]]</sup> ), oyster reefs (Scyphers et al., 2011 <sup>[[#fn:r1682|1682]]</sup> ) and submerged seagrass beds (Infantes et al., 2012 <sup>[[#fn:r1683|1683]]</sup> ). Local and global numerical studies indicate that marshes and mangroves can reduce present-day surge-related flood damages by &gt;15% annually, and the loss of a metre of living coral reef can double annual wave-related flood damages (Narayan et al., 2017 <sup>[[#fn:r1684|1684]]</sup> ; Beck et al., 2018 <sup>[[#fn:r1685|1685]]</sup> ). Artificial reef restoration along tens of metres of coastline using Reef Ball™ and other structures has been shown to reduce wave heights and stabilise beach widths (Reguero et al., 2018a <sup>[[#fn:r1686|1686]]</sup> ; Torres-Freyermuth et al., 2018 <sup>[[#fn:r1687|1687]]</sup> ). The effectiveness of EbA measures, however, varies considerably depending on storm, wetland, reef and landscape parameters (Koch et al., 2009 <sup>[[#fn:r1700|1700]]</sup> ; Loder et al., 2009 <sup>[[#fn:r1701|1701]]</sup> ; Wamsley et al., 2010 <sup>[[#fn:r1702|1702]]</sup> ; Pinsky et al., 2013 <sup>[[#fn:r1703|1703]]</sup> ; Quataert et al., 2015 <sup>[[#fn:r1704|1704]]</sup> ), which makes it difficult to extrapolate the physical and economic benefits across geographies. Depending on these parameters, rates of surge attenuation can vary between 5–70 cm km <sup>-1</sup> (Krauss et al., 2009 <sup>[[#fn:r1705|1705]]</sup> ; Vuik et al., 2015 <sup>[[#fn:r1706|1706]]</sup> ).

Critical gaps remain in our understanding about those parameters that together affect the success of ecosystem-based measures including choice of species and restoration techniques, lead time, natural variability and residual risk, temperature, salinity, wave energy and tidal range (Smith, 2006 <sup>[[#fn:r1707|1707]]</sup> ; Stiles Jr, 2006 <sup>[[#fn:r1708|1708]]</sup> ). Among reasons commonly cited for the failure of mangrove restoration projects are poor choice of mangrove species, planting in the wrong tidal zones and in areas of excessive wave energy (Primavera and Esteban, 2008 <sup>[[#fn:r1709|1709]]</sup> ; Bayraktarov et al., 2016 <sup>[[#fn:r1710|1710]]</sup> ; Kodikara et al., 2017 <sup>[[#fn:r1711|1711]]</sup> ).

The effectiveness of ecosystem-based measures also exhibits high seasonal, annual and longer-term variability. For example, marsh and seagrass wetlands typically have lower densities in winter which reduces their coastal protection capacity (Möller and Spencer, 2002 <sup>[[#fn:r1712|1712]]</sup> ; Paul and Amos, 2011 <sup>[[#fn:r1713|1713]]</sup> ; Schoutens et al., 2019 <sup>[[#fn:r1714|1714]]</sup> ). In the long-term, there is ''limited evidence'' and ''low agreement'' on how changes in sea level, sediment inputs, ocean temperature and ocean acidity will influence the extent, distribution and health of marsh and mangrove wetlands, coral reefs and oyster reefs (Hoegh-Guldberg et al., 2007 <sup>[[#fn:r1715|1715]]</sup> ; Lovelock et al., 2015 <sup>[[#fn:r1716|1716]]</sup> ; Crosby et al., 2016 <sup>[[#fn:r171|171]]</sup> ; Albert et al., 2017 <sup>[[#fn:r1718|1718]]</sup> ). EbA measures may have differential lead times before they are effective. For example, newly planted mangroves provide less wave attenuation until they mature (~3–5 years; Mazda et al., 1997 <sup>[[#fn:r1719|1719]]</sup> ). In contrast, a reef restoration project that uses submerged concrete structures performs as a breakwater as soon as the sub-structure is in place (Reguero et al., 2018a <sup>[[#fn:r1720|1720]]</sup> ).

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