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

=== 4.9.8 Avoiding coastal maladaptation ===

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Coastal degradation – for example, beach erosion, coastal squeeze, and coastal biodiversity loss – as a result of rising sea levels is a major concern for low lying coasts and small islands ( ''high confidence'' ). The contribution of climate change to increased coastal degradation has been well documented in AR5 (Nurse et al. 2014 <sup>[[#fn:r1545|1545]]</sup> ; Wong et al. 2014 <sup>[[#fn:r1546|1546]]</sup> ) and is further discussed in Section 4.4.1.3 as well as in the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). However, coastal degradation can also be indirectly induced by climate change as the result of adaptation measures that involve changes to the coastal environment, for example, coastal protection measures against increased flooding and erosion due to sea level rise, and storm surges transforming the natural coast to a ‘stabilised’ coastline (Cooper and Pile 2014 <sup>[[#fn:r1547|1547]]</sup> ; French 2001 <sup>[[#fn:r1548|1548]]</sup> ). Every kind of adaptation response option is context-dependent, and, in fact, sea walls play an important role for adaptation in many places. Nonetheless, there are observed cases where the construction of sea walls can be considered ‘maladaptation’ (Barnett and O’Neill 2010 <sup>[[#fn:r1549|1549]]</sup> ; Magnan et al. 2016 <sup>[[#fn:r1659|1659]]</sup> ) by leading to increased coastal degradation, such as in the case of small islands where, due to limitations of space, coastal retreat is less of an option than in continental coastal zones. There is emerging literature on the implementation of alternative coastal protection measures and mechanisms on small islands to avoid coastal degradation induced by sea walls (e.g., Mycoo and Chadwick 2012; Sovacool 2012 <sup>[[#fn:r1551|1551]]</sup> ).

In many cases, increased rates of coastal erosion due to the construction of sea walls are the result of the negligence of local coastal morphological dynamics and natural variability as well as the interplay of environmental and anthropogenic drivers of coastal change ( ''medium evidence, high agreement'' ). Sea walls in response to coastal erosion may be ill-suited for extreme wave heights under cyclone impacts and can lead to coastal degradation by keeping overflowing sea water from flowing back into the sea, and therefore affect the coastal vegetation through saltwater intrusion, as observed in Tuvalu (Government of Tuvalu 2006 <sup>[[#fn:r1552|1552]]</sup> ; Wairiu 2017 <sup>[[#fn:r1553|1553]]</sup> ). Similarly, in Kiribati, poor construction of sea walls has resulted in increased erosion and inundation of reclaimed land (Donner 2012 <sup>[[#fn:r1554|1554]]</sup> ; Donner and Webber 2014 <sup>[[#fn:r1555|1555]]</sup> ). In the Comoros and Tuvalu, sea walls have been constructed from climate change adaptation funds and ‘often by international development organisations seeking to leave tangible evidence of their investments’ (Marino and Lazrus 2015 <sup>[[#fn:r1556|1556]]</sup> , p. 344). In these cases, they have even increased coastal erosion, due to poor planning and the negligence of other causes of coastal degradation, such as sand mining (Marino and Lazrus 2015; Betzold and Mohamed 2017 <sup>[[#fn:r1557|1557]]</sup> ; Ratter et al. 2016 <sup>[[#fn:r1558|1558]]</sup> ). On the Bahamas, the installation of sea walls as a response to coastal erosion in areas with high wave action has led to the contrary effect and has even increased sand loss in those areas (Sealey 2006 <sup>[[#fn:r1559|1559]]</sup> ). The reduction of natural buffer zones – such as beaches and dunes – due to vertical structures, such as sea walls, increased the impacts of tropical cyclones on Reunion Island (Duvat et al. 2016 <sup>[[#fn:r1560|1560]]</sup> ). Such a process of ‘coastal squeeze’ (Pontee 2013 <sup>[[#fn:r1561|1561]]</sup> ) also results in the reduction of intertidal habitat zones, such as wetlands and marshes (Zhu et al. 2010 <sup>[[#fn:r1562|1562]]</sup> ). Coastal degradation resulting from the construction of sea walls, however, is not only observed in SIDS, as described above, but also on islands in the Global North, for example, the North Atlantic (Muir et al. 2014 <sup>[[#fn:r1563|1563]]</sup> ; Young et al. 2014 <sup>[[#fn:r1564|1564]]</sup> ; Cooper and Pile 2014 <sup>[[#fn:r1565|1565]]</sup> ; Bush 2004 <sup>[[#fn:r1566|1566]]</sup> ).

The adverse effects of coastal protection measures may be avoided by the consideration of local social-ecological dynamics, including critical study of the diverse drivers of ongoing shoreline changes, and the appropriate implementation of locally adequate coastal protection options (French 200 <sup>[[#fn:r1567|1567]]</sup> 1; Duvat 2013 <sup>[[#fn:r1568|1568]]</sup> ). Critical elements for avoiding maladaptation include profound knowledge of local tidal regimes, availability of relative sea level rise scenarios and projections for extreme water levels. Moreover, the downdrift effects of sea walls need to be considered, since undefended coasts may be exposed to increased erosion (Zhu et al. 2010 <sup>[[#fn:r1569|1569]]</sup> ). In some cases, it may be possible to keep intact and restore natural buffer zones as an alternative to the construction of hard engineering solutions. Otherwise, changes in land use, building codes, or even coastal realignment can be an option in order to protect and avoid the loss of the buffer function of beaches (Duvat et al. 2016 <sup>[[#fn:r1570|1570]]</sup> ; Cooper and Pile 2014 <sup>[[#fn:r1571|1571]]</sup> ). Examples in Barbados show that combinations of hard and soft coastal protection approaches can be sustainable and reduce the risk of coastal ecosystem degradation while keeping the desired level of protection for coastal users (Mycoo and Chadwick 2012 <sup>[[#fn:r1572|1572]]</sup> ). Nature-based solutions and approaches such as ‘building with nature’ (Slobbe et al. 2013 <sup>[[#fn:r1573|1573]]</sup> ) may allow for more sustainable coastal protection mechanisms and avoid coastal degradation. Examples from the Maldives, several Pacific islands and the North Atlantic show the importance of the involvement of local communities in coastal adaptation projects, considering local skills, capacities, as well as demographic and socio-political dynamics, in order to ensure the proper monitoring and maintenance of coastal adaptation measures (Sovacool 2012 <sup>[[#fn:r1574|1574]]</sup> ; Muir et al. 2014 <sup>[[#fn:r1575|1575]]</sup> ; Young et al. 2014 <sup>[[#fn:r1576|1576]]</sup> ; Buggy and McNamara 2016 <sup>[[#fn:r1577|1577]]</sup> ; Petzold 2016 <sup>[[#fn:r1578|1578]]</sup> ).

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