Editing IPCC:AR6/SROCC/Chapter-5 (section)

=== 5.3.3 Sandy Beaches ===

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Sandy beaches represent 31% of the world’s ice-free shoreline (Luijendijk et al., 2018 <sup>[[#fn:r990|990]]</sup> ). They provide habitat for dune vegetation, benthic fauna and sea birds, nesting areas for marine turtles (Defeo et al., 2009 <sup>[[#fn:r991|991]]</sup> ), and several key ecosystem services (Drius et al., 2019 <sup>[[#fn:r992|992]]</sup> ) (Section 5.4.1.2). Sandy beach ecosystems are physically dynamic, where sediment movement is a key driver of benthic flora and fauna zonation (Schlacher and Thompson, 2013 <sup>[[#fn:r993|993]]</sup> ; van Puijenbroek et al., 2017 <sup>[[#fn:r994|994]]</sup> ). In AR5 WGII (Wong et al., 2014b <sup>[[#fn:r995|995]]</sup> ) and SR15 (Hoegh-Guldberg et al., 2018 <sup>[[#fn:r996|996]]</sup> ), climate impacts on sandy beach ecosystems were not assessed individually but together with other coastal systems that included beaches, barriers, sand dunes, rocky coasts, aquifers and lagoons. Those assessments concluded with ''high confidence'' that SLR, storminess, wave energy and weathering regimes will continue to erode coastal shorelines and affect the soil accretion and land-based ecosystems, with highly site-specific effects ( ''high confidence'' ). Infrastructure and geological constraints reduce shoreline movement and cause coastal squeeze ( ''high confidence'' ). Assessment in Section 4.3.3.3 supports the conclusions in AR5 and SR15 regarding the erosion of sandy coastlines. This section specifically assesses the combined climate and non-climatic impacts on sandy beach biodiversity, ecosystem structure and functioning.

Worldwide, sandy beaches show vegetation transformations caused by erosion following locally severe wave events (Castelle et al., 2017 <sup>[[#fn:r997|997]]</sup> ; Delgado-Fernandez et al., 2019 <sup>[[#fn:r998|998]]</sup> ; Zinnert et al., 2019 <sup>[[#fn:r999|999]]</sup> ) (Table SM5.7). The original dense vegetation is replaced by sparser vegetation (Zinnert et al., 2019 <sup>[[#fn:r1000|1000]]</sup> ) and has a generally slow recovery (multiple years to decades) (Castelle et al., 2017 <sup>[[#fn:r1001|1001]]</sup> ). In some instances, the changes persist over decades, resulting in a regime shift in the beach morphology (Kuriyama and Yanagishima, 2018 <sup>[[#fn:r1002|1002]]</sup> ). Such changes in vegetation and beach morphology in response to local disturbances were also related to shifts in the associated fauna composition (Carcedo et al., 2017 <sup>[[#fn:r1003|1003]]</sup> ; Delgado-Fernandez et al., 2019 <sup>[[#fn:r1004|1004]]</sup> ). Direct attribution of these observed events to climate change is not available despite early evidence (since the 1970s) and an emerging literature (Section 4.3.3.1, Table SM5.7).

Sandy beaches show similar patterns of biogeographical shifts following warming, with increased dominance of species more tolerant to higher temperatures, as observed in other ocean ecosystems (Section 5.2.3.1.1, Table SM5.7). Examples of these observed shifts in abundance and distribution of benthic fauna in sandy beaches are found in the Pacific and Atlantic coasts of North and South America, and in Australia, including increased mortality of clam populations close to their upper temperature limits with low population recovery (Orlando et al., 2019 <sup>[[#fn:r1005|1005]]</sup> ), and poleward expansion of crabs since the 1980s that were related to warming (Schoeman et al., 2015 <sup>[[#fn:r1006|1006]]</sup> ) (Table SM5.7). Also, mass mortalities of beach clams have occurred during warm phases of El Niño events (Orlando et al., 2019 <sup>[[#fn:r1007|1007]]</sup> )(Table SM5.7), parasite infestations on dense populations (Vázquez et al., 2016 <sup>[[#fn:r1008|1008]]</sup> ) and high wave exposure (Turra et al., 2016 <sup>[[#fn:r1009|1009]]</sup> ).

Human disturbances have caused coastal squeeze and morphological changes in sandy beaches (Martínez et al., 2017 <sup>[[#fn:r1010|1010]]</sup> ; Rêgo et al., 2018 <sup>[[#fn:r1011|1011]]</sup> ; Delgado-Fernandez et al., 2019 <sup>[[#fn:r1012|1012]]</sup> ). Along with SLR and climate-driven intensification of waves and offshore winds, these hazards have increased erosion rates suggesting a reduced resilience due to insufficient sediment supply and accretion capacity (Castelle et al., 2017 <sup>[[#fn:r1013|1013]]</sup> ; Houser et al., 2018 <sup>[[#fn:r1014|1014]]</sup> ; Kuriyama and Yanagishima, 2018 <sup>[[#fn:r1015|1015]]</sup> ). Narrow sandy beaches such as those in south California (Vitousek et al., 2017 <sup>[[#fn:r1016|1016]]</sup> ) or central Chile (Martínez et al., 2017 <sup>[[#fn:r1017|1017]]</sup> ) are particularly vulnerable to climate hazards when combined with human disturbances and where landward retreat of beach profile and benthic organisms is constrained due to increasing urbanisation (Hubbard et al., 2014 <sup>[[#fn:r1018|1018]]</sup> ) (Section 4.3.2.3).

Notwithstanding the uncertainty in projecting future interactions of SLR with other natural and human impacts on sandy shorelines (Le Cozannet et al., 2019; Orlando et al., 2019 <sup>[[#fn:r1019|1019]]</sup> ), they are expected to continue to reduce their area and change their topography due to SLR and increased extreme climatic erosive events. This will be especially important in low-lying coastal areas with high population and building densities ( ''medium confidence'' , SM 4.2). Megafauna that use sandy beaches during vulnerable parts of their life cycles could be particularly impacted (Laloë et al., 2017 <sup>[[#fn:r1020|1020]]</sup> ). For example, the modelled incubation temperatures of green turtles have increased by 1°C since the mid-1970s, resulting in an average 20% increase in the proportion of female hatchlings over this period (Patrício et al., 2019 <sup>[[#fn:r1021|1021]]</sup> ). By 2100, global temperatures will approach lethal levels for incubation in existing nesting sites, and hatchling success is expected to drop to 32% under RCP8.5 scenario, with 93% of the hatchlings expected to be female (76% under RCP4.5). A possible microhabitat adaptation such as shadowed vegetated areas, however, could allow for continued male production throughout the 21st century (Patrício et al., 2019 <sup>[[#fn:r1022|1022]]</sup> ). In addition, a projected global mean SLR of ~1.2 m under the upper likely range of RCP8.5 by 2100 implies a loss of 59% and 67% in the present nesting area of the green turtle and the loggerhead respectively in the Mediterranean (Varela et al., 2019 <sup>[[#fn:r1023|1023]]</sup> ), and a loss of 43% in the nesting area of green turtles in West Africa (Patrício et al., 2019 <sup>[[#fn:r1024|1024]]</sup> ). Moreover, benthic crustaceans of sandy beaches, including isopods, crabs and amphipods, generally follow the temperature-body size gradient in which body size decreases towards warmer lower-latitude regions (Jaramillo et al., 2017 <sup>[[#fn:r1025|1025]]</sup> ). Assuming that the physiological underpinning of the relationship between body size and temperature can be applied to warming (see Section 5.2.2, ''medium confidence'' ), the body size of sandy beach crustaceans is expected to decrease under warming ( ''low evidence, medium agreement'' ).

Overall, changes in sandy beach morphology have been observed from climate related events, such as storm surges, intensified offshore winds, and from coastal degradation caused by humans ( ''high confidence'' ), with impacts on beach habitats (e.g., benthic megafauna) ( ''medium confidence'' ). The direct influence of contemporary SLR on shoreline behaviour is emerging, but attribution of such changes to SLR remains difficult (Section 4.3.3.1). Projected changes in mean and extreme sea levels (Section 4.2.3) and warming (Section 5.2.1) under RCP8.5 are expected to result in high risk of impacts on sandy beach ecosystems by the end of the 21st century ( ''medium confidence'' , Figure 5.16), taking account of the slow recovery rate of sandy beach vegetation, the direct loss of habitats and the high climatic sensitivity of some fauna. Under RCP2.6, the risk of impacts on sandy beaches is expected to be only slightly higher than the present day level ( ''low confidence'' , Figure 5.16). However, pervasive coastal urbanisation lowers the buffering capacity and recovery potential of sandy beach ecosystems to impacts from SLR and warming and thus is expected to limit their resilience to climate change ( ''high confidence'' ).

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