Editing IPCC:AR6/WGI/Chapter-12 (section)

=== 12.3.5 Coastal ===

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The SROCC included in-depth discussions of threats facing the world’s coastlines ( [[#IPCC--2019b|IPCC, 2019b]] ) and [[IPCC:Wg1:Chapter:Chapter-9#9.6|Section 9.6]] provides further discussion on coastal processes. Here we note major connections between coastal CIDs and ecosystem and societal assets near coastlines.

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==== 12.3.5.1 Relative Sea Level ====

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Sea level rise hazards for coastal ecosystems, infrastructure, farmland, cities and settlements in a particular region are often driven by regional changes in relative sea level (RSL) that account for land uplift or subsidence and thus represent local asset vulnerability better than global mean sea level (Box 9.1; [[#Hallegatte--2013|Hallegatte et al., 2013]] ; [[#Hinkel--2013|Hinkel et al., 2013]] ; [[#McInnes--2016|McInnes et al., 2016]] ; [[#Weatherdon--2016|Weatherdon et al., 2016]] ; [[#Brown--2018|Brown et al., 2018]] ; [[#IPCC--2019b|IPCC, 2019b]] ; [[#Rasoulkhani--2020|Rasoulkhani et al., 2020]] ). Vertical land motion (i.e., land subsidence) caused by local fluid (gas or groundwater) extraction can also have a large influence on relative sea levels ( [[#Minderhoud--2020|Minderhoud et al., 2020]] ). Several indices have been suggested to signify coastal inundation, including a threshold when the local land elevation falls below the local mean higher high water (MHHW) that is close to the ‘high tide’ level ( [[#Kulp--2019|Kulp and Strauss, 2019]] ) or a threshold when flooding occurs about once every two weeks ( [[#Sweet--2014|Sweet and Park, 2014]] ; [[#Dahl--2017b|Dahl et al., 2017b]] ). RSL rise (or RSLR) can drive increased inland penetration of above-ground and subterranean salt water fronts (i.e., salinity intrusion) affecting coastal ecosystems, agriculture and water resources ( [[#Ferguson--2012|Ferguson and Gleeson, 2012]] ; [[#Kirwan--2013|Kirwan and Megonigal, 2013]] ; [[#Rotzoll--2013|Rotzoll and Fletcher, 2013]] ; [[#Chen--2016|Chen et al., 2016]] ; [[#Colombani--2016|Colombani et al., 2016]] ; [[#Holding--2016|Holding et al., 2016]] ; [[#Sawyer--2016|Sawyer et al., 2016]] ; [[#Mohammed--2018|Mohammed and Scholz, 2018]] ). The rate of RSLR can determine the survival and net pressure on niche coastal ecosystems such as mangroves, tidal flats, sea grasses and coral reefs ( [[#Hubbard--2008|Hubbard et al., 2008]] ; [[#Craft--2009|Craft et al., 2009]] ; [[#Bell--2013|Bell et al., 2013]] ; [[#Kirwan--2013|Kirwan and Megonigal, 2013]] ; [[#Alongi--2015|Alongi, 2015]] ; [[#Ellison--2015|Ellison, 2015]] ; [[#Lovelock--2015|Lovelock et al., 2015]] ; [[#Ward--2016|Ward et al., 2016]] ; [[#Lee--2018|Lee et al., 2018]] ).

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==== 12.3.5.2 Coastal Flood ====

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Episodic coastal flooding of coastal communities, farmland, buildings, transportation routes, industry and other infrastructure is caused by extreme total water levels (ETWL), which is the combination of RSL, tides, storm surge and high wave setup at the shoreline ( [[#Vitousek--2017|Vitousek et al., 2017]] ; [[#Melet--2018|Melet et al., 2018]] ; [[#Vousdoukas--2018|Vousdoukas et al., 2018]] , 2020a; [[#Koks--2019|Koks et al., 2019]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). Coastal settlement and infrastructure design often uses coastal flooding metrics such as the ETWL frequency distribution or the 100-year average return interval storm tide (storm surge + high tide) level ( [[#McInnes--2016|McInnes et al., 2016]] ; [[#Mills--2016|Mills et al., 2016]] ; [[#Walsh--2016b|Walsh et al., 2016b]] ; [[#Zheng--2017|Zheng et al., 2017]] ). The duration of floods that overtop coastal protection, due to extreme coastal water levels (ECWL), is important for port and harbour operations and coastal energy infrastructure thresholds ( [[#Bilskie--2016|Bilskie et al., 2016]] ; [[#Camus--2017|Camus et al., 2017]] ). Frequent inundation by salt water can also have significant impacts on water resources, crops, aquaculture and transportation systems due to corrosion and undercutting of coastal roads, bridges and rails ( [[#Zimmerman--2010|Zimmerman and Faris, 2010]] ; N. [[#Ahmed--2019|]] [[#Ahmed--2019|Ahmed et al., 2019]] ; [[#Gopalakrishnan--2019|Gopalakrishnan et al., 2019]] ).

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==== 12.3.5.3 Coastal Erosion ====

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Effective management of coastal ecosystems, cities, settlements, beaches and infrastructure requires information about coastal erosion driven by storm surge, waves and sea level rise ( [[#Dawson--2009|Dawson et al., 2009]] ; [[#Hinkel--2013|Hinkel et al., 2013]] ; [[#Harley--2017|Harley et al., 2017]] ; [[#Mentaschi--2017|Mentaschi et al., 2017]] ). Coastal erosion is generally accompanied by shoreline retreat, which can occur as a gradual process (e.g., due to sea level rise) or as an episodic event due to storm surge and/or extreme waves, especially when combined with high tide ( [[#Ranasinghe--2016|Ranasinghe, 2016]] ). The most commonly used shoreline retreat index is the magnitude of shoreline retreat by a pre-determined planning horizon such as 50 or 100 years into the future. Commonly used metrics for episodic coastal erosion include the beach erosion volume due to the 100-year recurrence storm wave height, the full exceedance probability distribution of coastal erosion volume ( [[#Li--2014a|Li et al., 2014a]] ; [[#Pender--2015|Pender et al., 2015]] ; [[#Ranasinghe--2017|Ranasinghe and Callaghan, 2017]] ) and the cumulative storm energy and storm power index ( [[#Godoi--2018|Godoi et al., 2018]] ). The destruction or overtopping of barrier islands may lead to irreversible changes in the physical system as well as in coastal ecosystems ( [[#Carrasco--2016|Carrasco et al., 2016]] ; [[#Zinnert--2019|Zinnert et al., 2019]] ). Shoreline position change rates along inlet-interrupted coasts may also be affected by changes in river flows and fluvial sediment supply ( [[#Hinkel--2013|Hinkel et al., 2013]] ; [[#Bamunawala--2018|Bamunawala et al., 2018]] ; [[#Ranasinghe--2019|Ranasinghe et al., 2019]] ). Permafrost thaw and Arctic sea ice decline also reduce natural coastal protection from wave erosion for communities and industry ( [[#Forbes--2011|Forbes, 2011]] ; [[#Melvin--2017|Melvin et al., 2017]] ).

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