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

===== 15.3.3.1.1 Submergence and flooding of islands and coastal areas =====

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Recent studies confirmed that observed ESL events causing extensive flooding generally resulted from compound effects, including the combination of SLR ( [[IPCC:Wg2:Chapter:Chapter-3#3.2.2.2|Section 3.2.2.2]] and Cross-Chapter Box SLR in Chapter 3) with ETCs, TCs and tropical depressions (WGI AR6 Sections 11.7.1 and 11.7.2, Seneviratne, 2021), ENSO-related high-water levels associated with high or spring tide and/or local human disturbances amplifying impacts ( ''high confidence'' ). For example, the major floods that occurred in 1987 and 2007 in the Maldives involved the combination of distant-source swells and high spring tides and the settlement of reclaimed low-lying areas (Box 15.1; [[#Wadey--2017|Wadey et al., 2017]] ). In the Tuamotu atolls, French Polynesia, the 1996 and 2011 floods were due to the combination of distant-source swells causing lagoon filling and the obstruction of inter-islet channels by human-built structures ( [[#Canavesio--2019|Canavesio, 2019]] ). In 2011, the flooding of the lagoon-facing coast of Majuro Atoll, Marshall Islands, resulted from the combination of high sea levels occurring during La Niña conditions and seasonally high tides ( [[#Ford--2018|Ford et al., 2018]] ). Another example is the widespread flooding caused by distant TC Pam (2015) in Kiribati and Tuvalu, which was attributed to the strong swell generated, the long duration of the event and exceptionally high regional sea levels ( [[#Hoeke--2021|Hoeke et al., 2021]] ). On high tropical islands, major floods often occurred during TC events, due to the cumulative effects of storm surge and river flooding, the impacts of which were exacerbated by human-induced changes to natural processes in urban areas. This, for example, occurred in 2014 (TC Bejisa) in Reunion Island, France, in a harbour area favourable to water accumulation ( [[#Duvat--2016|Duvat et al., 2016]] ); in 2015 (TC Pam) in Port Vila, Vanuatu, where urbanisation and human-induced changes to the river exacerbated flooding (Rey et al., 2017); and in 2017 (TC Irma) in Saint-Martin, Caribbean, where urbanisation had the same effect ( [[#Rey--2019|Rey et al., 2019]] ). Successive tropical depressions generating heavy rains were also involved in extensive flooding, for example, in 2012 in Fiji ( [[#Kuleshov--2014|Kuleshov et al., 2014]] ) and in 2014 in the Solomon Islands ( [[#Ha’apio--2019|Ha’apio et al., 2019]] ).

Reconstructions of past storm surges and modelling studies assessing storm surge risk similarly highlighted high variations of risk along island coasts, due to variations in exposure, topography and bathymetry ( ''high confidence'' ). For example, the storm surge caused by TC Oli (2010) on the high volcanic island of Tubuai, French Polynesia, ranged from a few centimetres to 2.5 m, depending on coast exposure ( [[#Barriot--2016|Barriot et al., 2016]] ). Investigating the contribution of reef characteristics to variations in wave-driven flooding on Roi-Namur Island, Kwajalein Atoll, Marshall Islands, [[#Quataert--2015|Quataert et al. (2015)]] found that the coasts fronted by narrow reefs with steep fore reef slopes and smoother reef flats are the most flood-prone. Modelling studies assessing storm surge risk in Fiji ( [[#McInnes--2014|McInnes et al., 2014]] ) and Samoa ( [[#McInnes--2016|McInnes et al., 2016]] ) confirmed the influence of coast exposure and water depth on risk distribution. In Apia, Samoa, Hoeke et al. (2015, p. 1117) found ‘differences in extreme sea levels in the order of 1 m at spatial scales of less than 1 km’ and estimated (p. 1131) that a ‘1 m SLR relative to constant topography increases wave energy reaching the shore by up to 200% during storm surges.’ These studies reaffirmed the main control exerted by SLR on ESL events and associated storm surges compared to ENSO ( ''high confidence'' ). In Hawaii and the Caribbean, SLR is projected to exponentially increase flooding, with nearly every centimetre of SLR causing a doubling of the probability of flooding ( [[#Taherkhani--2020|Taherkhani et al., 2020]] ). Simulations of SLR-induced flooding resulting from the combination of (a) direct marine flooding, (b) flow reversal in drainage networks caused by extreme tide levels and (c) the elevation of groundwater levels, at Honolulu, Hawaii, highlighted the major influence of this latter component (which is the most difficult to manage), as well as the increase of the proportion of triple-mechanism flooding as sea level rises ( [[#Habel--2020|Habel et al., 2020]] ). Where coral reefs buffer flooding through wave attenuation, flooding will be further aggravated by reef decline over time ( [[#15.3.3.1.3|Section 15.3.3.1.3]] ).

Larger-scale studies confirmed that projected changes in the wave climate superimposed on SLR will rapidly increase flooding in small islands, despite highly contrasting exposure profiles between ocean sub-regions ( ''high confidence'' ) ( [[#Shope--2016|Shope et al., 2016]] ; [[#Mentaschi--2017|Mentaschi et al., 2017]] ; [[#Shope--2017|Shope et al., 2017]] ; [[#Vitousek--2017|Vitousek et al., 2017]] ; [[#Morim--2019|Morim et al., 2019]] ). In particular, [[#Vitousek--2017|Vitousek et al. (2017)]] showed that even a 5–10-cm additional SLR (expected for ~2030–2050) will double flooding frequency in much of the Indian Ocean and Tropical Pacific, while TCs will remain the main driver of (rarer) flooding in the Caribbean Sea and Southern Tropical Pacific (Figure 15.3). Some Pacific atoll islands, which already experience major floods, will ''likely'' undergo annual wave-driven flooding over their entire surface from the 2060s–2070s ( [[#Storlazzi--2018|Storlazzi et al., 2018]] ) to 2090s ( [[#Beetham--2017|Beetham et al., 2017]] ) under RCP8.5, although future reef growth may delay the onset of flooding ( ''limited evidence, low agreement'' ) (key risk KR2 in Figure 15.5).

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