Editing IPCC:AR6/WGII/Chapter-15 (section)

===== 15.3.3.1.2 Reef island destabilisation and coastal erosion =====

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Over the past three to five decades, shoreline changes were dominated by stability on reef islands and erosion on high islands; attribution of observed erosion to SLR and other climate change-related drivers is challenged by the complex interplay of multiple climatic, ecological and human drivers ( ''high confidence'' ). Since the 1950s–1970s, and even in regions exhibiting higher than global-averaged SLR rates, atoll islands maintained their land area ( ''high confidence).'' A literature review including 709 Indian Ocean and Pacific Ocean atoll islands showed that 73.1% of these islands were stable in area, while, respectively, 15.5% and 11.4% increased and decreased in area ( [[#Duvat--2018|Duvat, 2018]] ). The rates of change did not correlate with SLR rates, suggesting that the impact of SLR on island land area was obscured by other climate drivers and human disturbances on some islands ( ''high confidence'' ) ( [[#Kench--2015|Kench et al., 2015]] ; [[#McLean--2015|McLean and Kench, 2015]] ; [[#Duvat--2018|Duvat, 2018]] ). However, reef island disappearance and reduction in land area was clearly observed in New Caledonia and the Solomon Islands, and was attributed to the synergistic interactions of gradual SLR with stronger trade winds causing higher sea levels and local tectonics in the Solomon Islands ( [[#Albert--2016|Albert et al., 2016]] ; [[#Garcin--2016|Garcin et al., 2016]] ). Despite important knowledge gaps on coastal erosion in high tropical islands, recent studies confirmed increasing shoreline retreat and beach loss over the past decades, mainly due to TC and ETC waves and human disturbances ( ''high confidence'' ) (e.g., in the Caribbean region: Anguilla, Saint-Kitts, Nevis, Montserrat, Dominica and Grenada ( [[#Cambers--2009|Cambers, 2009]] ; [[#Reguero--2018|Reguero et al., 2018]] )), and Pacific (Hawaii ( [[#Romine--2013|Romine and Fletcher, 2013]] ); Tubuai, French Polynesia ( [[#Salmon--2019|Salmon et al., 2019]] )) and Indian Oceans (Anjouan, Comoros ( [[#Ratter--2016|Ratter et al., 2016]] ).

Despite storm-induced erosion prevailing along some shoreline sections, recent studies reaffirmed the contribution of TC and ETC waves to coastal and reef island vertical building through massive reef-to-island sediment transfer ''(high confidence'' ). For example, TC Ophelia (1958) and Category 5 TC Fantala (2016), which eroded the islands of Jaluit Atoll, Marshall Islands ( [[#Ford--2016|Ford and Kench, 2016]] ), and Farquhar Atoll, Seychelles ( [[#Duvat--2017c|Duvat et al., 2017c]] ), respectively, also contributed to island and beach expansion. Likewise, tropical depressions can have constructional effects, as reported on Fakarava Atoll, French Polynesia ( [[#Duvat--2020b|Duvat et al., 2020b]] ). On Saint-Martin/Sint Maarten and Saint-Barthélemy, the 2017 hurricanes, which caused marked shoreline retreat at most beach sites, also enabled beach formation and beach ridge development along some natural coasts ( [[#Duvat--2019a|Duvat et al., 2019a]] ; [[#Pillet--2019|Pillet et al., 2019]] ). Similarly, El Niño and La Niña were involved in rapid and highly contrasting shoreline changes ( ''high confidence'' ), including reef island accretion in the Ryukyu Islands, Japan ( [[#Kayanne--2016|Kayanne et al., 2016]] ), beach shifts on Maiana and Aranuka atolls, Kiribati (Rankey, 2011), and beach erosion on Hawaii, USA ( [[#Barnard--2015|Barnard et al., 2015]] ). These contrasting shoreline responses were, respectively, due to coral reef degradation from past bleaching events providing material to islands, wave directional shifts, and increased wave energy. The role of bleaching events in increasing short-term sediment generation in atoll contexts was confirmed by a study conducted on Gaafu Dhaalu Atoll, Maldives, which reported an increase of sediment production from ~0.5 kg CaCO 3 m –2 yr -1 to ~3.7 kg CaCO 3 m –2 yr -1 between 2016 (pre-bleaching) and 2019 (bleaching + 3 years) ( [[#Perry--2020|Perry et al., 2020]] ).

There is ''high confidence'' that accelerating SLR and increased wave height will affect the geomorphology of reef islands ( [[#Baldock--2015|Baldock et al., 2015]] ; [[#Costa--2019|Costa et al., 2019]] ; [[#Tuck--2019|Tuck et al., 2019]] ) and coastal systems on high islands ( [[#Grady--2013|Grady et al., 2013]] ; [[#Barnard--2015|Barnard et al., 2015]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ), and that the responses of these systems will highly depend on changes in boundary conditions (wave regime and direction, exposure to extreme events, impacts of ocean warming and acidification on supporting ecosystems, bathymetry and reef flat roughness) and the degree of disturbance of their natural dynamics by human activities ( [[#Smithers--2014|Smithers and Hoeke, 2014]] ; [[#McLean--2015|McLean and Kench, 2015]] ; [[#Bheeroo--2016|Bheeroo et al., 2016]] ; [[#Ratter--2016|Ratter et al., 2016]] ; [[#Shope--2016|Shope et al., 2016]] ; [[#Duvat--2017a|Duvat et al., 2017a]] ; [[#Kench--2017|Kench and Mann, 2017]] ; [[#Kench--2018|Kench et al., 2018]] ; [[#Duvat--2019a|Duvat et al., 2019a]] ). Reef islands and beach and beach-dune systems that are not disturbed by human activities are, respectively, expected to migrate lagoonward ( [[#Webb--2010|Webb and Kench, 2010]] ; [[#Albert--2016|Albert et al., 2016]] ; [[#Beetham--2017|Beetham et al., 2017]] ; [[#Costa--2019|Costa et al., 2019]] ; [[#Tuck--2019|Tuck et al., 2019]] ) and landward ( [[#Bindoff--2019|Bindoff et al., 2019]] ), and to also experience increased erosion as well as changes in configuration, volume and elevation ( [[#Kench--2017|Kench and Mann, 2017]] ; [[#Tuck--2019|Tuck et al., 2019]] ) ( [[#Bramante--2020|Bramante et al., 2020]] ; [[#Kane--2020|Kane and Fletcher, 2020]] ). Small reef islands and narrow coastal systems affected by human disturbances will increasingly be at risk of disappearance due to SLR (KR2 in Figure 15.5), enhanced sediment loss caused by extreme events ( [[#Duvat--2019a|Duvat et al., 2019a]] ) and/or human activities ( ''high confidence'' ), as reported in Hawaii ( [[#Romine--2013|Romine and Fletcher, 2013]] ), Puerto Rico ( [[#Jackson--2012|Jackson et al., 2012]] ), Sicily ( [[#Anfuso--2012|Anfuso et al., 2012]] ), and Takuu, Papua New Guinea ( [[#Mann--2014|Mann and Westphal, 2014]] ). SLR will also increase coastal erosion in the Mediterranean Sea, (e.g., in the Aegean Archipelago, Greece ( [[#Monioudi--2017|Monioudi et al., 2017]] ), and Mallorca, Spain ( [[#Enríquez--2017|Enríquez et al., 2017]] ).

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[[File:a1cb2010756818ae2f65cc852f12fad6 IPCC_AR6_WGII_Figure_15_005.png]]

'''Figure 15.5 |''' '''Key risks in small islands'''. KR1 to KR8 are interconnected as shown by ''arrows'' , which causes risk accumulation leading to reduced island habitability. The main interconnections are shown in this figure: for example, loss of marine and coastal and terrestrial biodiversity and ecosystem services (KR1 and KR3, respectively) are projected to cause the submergence of reef islands (KR2), water insecurity (KR4), destruction of settlements and infrastructure (KR5), degradation of human health and well-being (KR6), economic decline and livelihood failure (KR7), and loss of cultural resources and heritage (KR8). Importantly, KRs result from both direct effects (e.g., decrease in rainfall will increase water insecurity) and indirect effects (e.g., loss of terrestrial biodiversity and ecosystem services will increase water insecurity, which will in turn cause the degradation of human health and well-being).

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