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

==== 4.4.2.3 Ecosystem-based Adaptation ====

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<span id="observed-ecosystem-based-adaptation-across-geographies"></span>
===== 4.4.2.3.1 Observed ecosystem-based adaptation across geographies =====

Relative to hard adaptation measures whose global distribution is not known in detail (Scussolini et al., 2015 <sup>[[#fn:r1644|1644]]</sup> ), the current global distribution of coastal ecosystems is well-studied (e.g., for saltmarshes and mangroves, respectively; Giri et al., 2011 <sup>[[#fn:r1645|1645]]</sup> ; Mcowen et al., 2017 <sup>[[#fn:r1646|1646]]</sup> ). EbA, by definition, can only exist and function where the environmental conditions are appropriate for a given ecosystem. Mangroves, salt marshes and reefs occur along about 40–50% of the world’s coastlines (Wessel and Smith, 1996 <sup>[[#fn:r1647|1647]]</sup> ; Burke, 2011 <sup>[[#fn:r1648|1648]]</sup> ; Giri et al., 2011 <sup>[[#fn:r1649|1649]]</sup> ; Mcowen et al., 2017 <sup>[[#fn:r1650|1650]]</sup> ). However, there is no clear estimate on the global length of coastline covered by ecosystems relevant for EbA in the face of SLR in part because of a mismatch between the spatial resolutions of different estimates available. Mangroves occur on tropical and subtropical coasts, and cover 138,000–152,000 km <sup>2</sup> across about 120 countries (Spalding et al., 2010 <sup>[[#fn:r1651|1651]]</sup> ; Giri et al., 2011 <sup>[[#fn:r1652|1652]]</sup> ). At least 150,000 km of coastline in over 100 countries benefit from the presence of coral reefs (Burke, 2011 <sup>[[#fn:r1653|1653]]</sup> ) and these are estimated to protect over 100 million people from wave-induced flooding globally (Ferrario et al., 2014 <sup>[[#fn:r1654|1654]]</sup> ). The extent of other coastal habitats is less well known: salt marshes are estimated to occur in 99 countries, especially in temperature to high latitude locations, with nearly 5,500,000 ha mapped across 43 countries (Mcowen et al., 2017 <sup>[[#fn:r1655|1655]]</sup> ).

Since AR5 there has been growing recognition of the value of conserving existing coastal ecosystems, and where possible restoring them, for the flood protection and multiple other benefits they provide (Temmerman et al., 2013 <sup>[[#fn:r1656|1656]]</sup> ; Arkema et al., 2015 <sup>[[#fn:r1657|1657]]</sup> ). In parallel, EbA measures are increasingly being incorporated and required within national plans, strategies and targets (Lo, 2016), international adaptation funding mechanisms, such as the Adaptation Fund (AF; e.g., in Sri Lanka and India; Epple et al., 2016 <sup>[[#fn:r1658|1658]]</sup> ), and national natural capital valuations (Beck and Lange, 2016 <sup>[[#fn:r1659|1659]]</sup> ). Given their relative novelty, there is widespread interest in building and collecting knowledge of EbA implementation case-studies and examples (Table 4.7). Meanwhile, coastal communities around the globe are already implementing EbA responses at local scales, with emphasis on community participation and ownership and local priorities, needs and capacities (Reid, 2016 <sup>[[#fn:r1660|1660]]</sup> ; see Section 4.4.4.4).

EbA has been used as an integral part of some retreat, advance and accommodation responses. For example, on coastlines where high-risk properties are relocated inland, space can be made for ecosystem restoration to enhance natural biodiversity and provide coastal protection (French, 2006 <sup>[[#fn:r1661|1661]]</sup> ; Coastal Protection and Restoration Authority of Louisiana, 2017 <sup>[[#fn:r1662|1662]]</sup> ). There are also examples of ecosystem restoration to advance coastlines and build land elevation (Chung, 2006 <sup>[[#fn:r1663|1663]]</sup> ). EbA can also be an element of accommodation responses by, for example, restoring or creating marshes to provide space for flood water (Temmerman et al., 2013 <sup>[[#fn:r1664|1664]]</sup> ).

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<span id="projected-ecosystem-based-adaptation"></span>
===== 4.4.2.3.2 Projected ecosystem-based adaptation =====

While there are projections available of ecosystem responses to climate change and SLR (Section 4.3.3), to date, there are no large-scale projections available on the future extent of EbA. However, several coastal nations, particularly Small Island Developing States (SIDS) explicitly advocate EbA measures as a means to address future coastal hazard and SLR concerns. Based on Nationally Determined Contributions (NDCs) submitted to the United Nations Framework Convention on Climate Change (UNFCCC), more than 30 SIDS cite EbA as a preferred SLR response, with mangrove planting being the most common measure (Wong, 2018 <sup>[[#fn:r1665|1665]]</sup> ).

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<span id="cost-of-ecosystem-based-adaptation"></span>
===== 4.4.2.3.3 Cost of ecosystem-based adaptation =====

There is ''limited evidence and low agreement'' on the costs of ecosystem-based measures to make generally valid estimations of the unit costs across large spatial scales. The total cost of an ecosystem-based measure includes capital costs, maintenance costs, the cost of land and, in some situations, permitting costs (Bilkovic, 2017 <sup>[[#fn:r1666|1666]]</sup> ). The costs of restoring and maintaining coastal habitats depend on coastal setting, habitat type and project conditions. In general, unit restoration costs are lowest for mangroves, higher for salt marshes and oyster reefs, and highest for seagrass beds and coral reefs (Table 4.8).

The conservation of coral reefs and other coastal habitats may also entail substantial opportunity costs because alternative uses of this land, such as through agricultural production, industry and settlements, are generally of high economic value (Stewart et al., 2003 <sup>[[#fn:r1667|1667]]</sup> ; Balmford et al., 2004 <sup>[[#fn:r1668|1668]]</sup> ; Adams et al., 2011 <sup>[[#fn:r1669|1669]]</sup> ; Hunt, 2013 <sup>[[#fn:r1670|1670]]</sup> ). The high value of these alternative uses are the reason why globally, coastal ecosystems are amongst the ecosystems that face the highest rates of anthropogenic destruction, with estimated annual losses of 1–3% of mangroves area, 2–5% seagrass area and 4–9% corals (Duarte et al., 2013 <sup>[[#fn:r1671|1671]]</sup> ). Conserving these areas means reversing these trends.

Under the right conditions, and to some extent, EbA measures are free of maintenance costs, because they respond and adapt to changes in their coastal environment. However, maintenance can become important in the aftermath of damage by storms or human action, for example, when wetlands and reefs can be damaged by high winds, waves and surges, or affected by dredging operations (Smith III et al., 2009 <sup>[[#fn:r1672|1672]]</sup> ; Puotinen et al., 2016 <sup>[[#fn:r1673|1673]]</sup> ). At present, there is limited evidence about the conditions under which EbA measures can self-adapt and when they require human intervention to recover.

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'''Table 4.8:''' Costs of ecosystem-based adaptation (EbA). MPA is marine protected area.

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{| class="wikitable"
|-
| Type of measure
| Capital Costs
| Maintenance Costs
|-
| Wetland Conservation
| No data available
| Thinning, clearing debris after storms, etc.: Mangrove: 5000 USD ha –1 yr –1 in Florida (Lewis, 2001) to 11,000 ha –1 yr –1 (Aerts, 2018).
For mangroves globally, 7–85 USD ha –1 yr –1 (Aerts et al., 2018a);

For marshes in the Wadden Sea, 25 USD m –1 yr –1 (Vuik et al., 2019).

|-
| Wetland Restoration (Marshes/Mangroves, Maritime Forests)
| Wetlands: 85,000 – 230,000 USD ha –1 (Aerts et al., 2018a); Mangroves: USD 9000 ha –1 (median; Bayraktarov et al., 2016);
2000 – 13,000 USD ha –1 in American Samoa (Gilman and Ellison, 2007); Salt Marshes: 67,000 USD ha –1 (Bayraktarov et al., 2016); Brushwood dams for marsh restoration 150 m –1 (Vuik et al., 2019).

| Similar to maintenance costs for Wetland Conservation
|-
| Reef Conservation (Coral/ Oyster)
| For example, start-up costs for Reef MPAs: 96 – 40,000 USD km -2 (McCrea-Strub et al., 2011).
| For MPAs, 12 million USD yr -1 for the Great Barrier Reef (Balmford et al., 2004).
|-
| Reef Restoration (Coral/ Oyster)
| 165,600 USD ha –1 (median; Bayraktarov et al., 2016); Oyster Reefs: 66,800 USD ha –1 (median; Bayraktarov et al., 2016); Artificial Reefs in the UK 30,000–90,000 USD 100 m –1 (Aerts et al., 2018a)
| Similar to maintenance costs for Reef Conservation
|}
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<span id="effectiveness-of-ecosystem-based-adaptation"></span>
===== 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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<span id="co-benefits-and-drawbacks-of-ecosystem-based-adaptation"></span>
===== 4.4.2.3.5 Co-benefits and drawbacks of ecosystem-based adaptation =====

There is high confidence that ecosystem-based measures provide multiple co-benefits such as sequestering carbon (Siikamäki et al., 2012 <sup>[[#fn:r1721|1721]]</sup> ; Hamilton and Friess, 2018 <sup>[[#fn:r1722|1722]]</sup> ), income from tourism (Carr and Mendelsohn, 2003 <sup>[[#fn:r1723|1723]]</sup> ; Spalding et al., 2017 <sup>[[#fn:r1724|1724]]</sup> ), enhancing coastal fishery productivity (Carrasquilla-Henao and Juanes, 2017 <sup>[[#fn:r1725|1725]]</sup> ; Taylor et al., 2018 <sup>[[#fn:r1726|1726]]</sup> ), improving water quality (Coen et al., 2007 <sup>[[#fn:r1727|1727]]</sup> ; Lamb et al., 2017 <sup>[[#fn:r1728|1728]]</sup> ), providing raw material for food, medicine, fuel and construction (Hussain and Badola, 2010 <sup>[[#fn:r1729|1729]]</sup> ; Uddin et al., 2013 <sup>[[#fn:r1730|1730]]</sup> ), and a range of intangible and cultural benefits (Scyphers et al., 2015 <sup>[[#fn:r1731|1731]]</sup> ) that help improve the resilience of communities vulnerable to sea level hazards (Sutton-Grier et al., 2015 <sup>[[#fn:r1732|1732]]</sup> ).

In comparison to hard structures like seawalls, EbA measures, particularly coastal wetlands, require more land (The Royal Society Science Policy Centre, 2014), and competition for land is often why the ecosystems have declined in the first place (4.4.2.3.1). On developed coasts, this land is often not available. In such cases, hybrid measures that either combine EbA measures with structural measures like mangrove forests in front of dikes (Dasgupta et al., 2019 <sup>[[#fn:r1733|1733]]</sup> ), or build ecological enhancements into engineered structures can provide an effective solution. Like any other feature that interacts with coastal processes, natural wetlands and reefs can increase flooding in some instances, for example, due to the redistribution or acceleration of flows in channels within a wetland system (Marsooli et al., 2016 <sup>[[#fn:r1734|1734]]</sup> ), or an increase in infragravity wave (i.e., surface gravity waves with frequencies lower than wind waves) energy behind a reef (Roeber and Bricker, 2015 <sup>[[#fn:r1735|1735]]</sup> ).

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<span id="governance-of-ecosystem-based-adaptation"></span>
===== 4.4.2.3.6 Governance of ecosystem-based adaptation =====

The coastal protection benefits of natural ecosystems are increasingly being recognised within international discourse and national coastal adaptation, resilience and sustainable development plans and strategies (Section 4.4.2.3.1). In general, obtaining permits for EbA remains more difficult compared to established hard protection measures, in places like the USA (Bilkovic, 2017 <sup>[[#fn:r1736|1736]]</sup> ). However, there are examples of instruments specifically tailored to retain the protective function of EbA (Borges et al., 2009 <sup>[[#fn:r1737|1737]]</sup> ; Government of India, 2018 <sup>[[#fn:r1738|1738]]</sup> ). The Living Shorelines Regulations of the state government of Maryland in the USA (Maryland DEP, 2013 <sup>[[#fn:r1739|1739]]</sup> ), for instance, requires that private properties must include marsh creation or other non-structural measures when stabilising their shorelines, unless a waiver is obtained.

There are an increasing number of public and private financial mechanisms and policy instruments to encourage the use and implementation of EbA measures (Colgan et al., 2017 <sup>[[#fn:r1740|1740]]</sup> ; Sutton-Grier et al., 2018 <sup>[[#fn:r1741|1741]]</sup> ). For example, a regulation by the Federal Emergency Management Agency (FEMA) of the USA, allows proponents of hazard mitigation projects, such as state, territorial and local governments, to take into account the co-benefits of EbA when assessing benefit-cost ratios of FEMA-funded recovery projects (FEMA, 2015 <sup>[[#fn:r1742|1742]]</sup> ). International guidelines are being developed for designing and implementing EbA measures, with the intention to support wider implementation of these responses (Hardaway Jr and Duhring, 2010 <sup>[[#fn:r1743|1743]]</sup> ; Van Slobbe et al., 2013; Van Wesenbeeck et al., 2017; Bridges et al., 2018 <sup>[[#fn:r1744|1744]]</sup> ).

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===== 4.4.2.3.7 Economic efficiency of ecosystem-based adaptation =====

There is ''limited evidence'' regarding the economic efficiency of EbA, mainly due to the ''low agreement'' about EbA effectiveness (Section 4.4.2.3.2) and costs (Section 4.4.2.3.2). A study of coastal protection measures on the Gulf of Mexico coastline, USA, estimated that EbA measures have average benefit-cost ratios above 3.5 for 2030 flood risk conditions, assuming a discount rate of 2% (Reguero et al., 2018b <sup>[[#fn:r1745|1745]]</sup> ; see Section 4.4.2.3.2). This study also finds that EbA are nearly four times more cost-efficient along developed coastlines as compared to conservation-priority areas because protection benefits are higher in the former case due to the level of asset exposure.

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