Editing IPCC:AR6/WGII/Chapter-16 (section)

===== 16.5.2.3.1 Risk to the integrity of low-lying coastal socio-ecological systems (RKR-A) =====

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RKR-A considers climate-change-related risks to low-lying coasts including their physical, ecological and human components. Low-lying systems are those occupying land below 10 m of elevation that is contiguous and hydrologically connected to the sea ( [[#McGranahan--2007|McGranahan et al., 2007]] ). The assessment builds on Key Risks identified in Chapters 3 and 15, Cross Chapter Paper 2 as well as in the SROCC ( [[#Magnan--2019|Magnan et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). It highlights risks to (i) natural coastal protection and habitats; (ii) lives, livelihoods, culture and well-being; and (iii) critical physical infrastructure; it therefore overlaps with several other RKRs (Figures 16.10, 16.11) but within a coastal focus. It encompasses all latitudes and considers multiple sources of climate hazards, including SLR, ocean warming and acidification, permafrost thaw, and sea ice loss and changes in weather extremes.

Severe risks to low-lying coasts involve irreversible long-term loss of land, critical ecosystem services, livelihoods, well-being or culture in relation to increasing combined drivers, including climate hazards and exposure and vulnerability conditions. The definition depends on the local context because of variation in the perception of tolerable risks and the limits to adaptation ( [[#Handmer--2019|Handmer and Nalau, 2019]] ). Accordingly, a qualitative range of consequences is presented here, in place of a quantitative global severe risk threshold.

The literature suggests that severe risks generally occur at the nexus of high levels and rates of anthropogenic-driven change in climate hazards ( [[#16.2.3.2|Section 16.2.3.2]] ), concentrations of people and tangible and intangible assets, non-climate hazards such as sediment mining and ecosystem degradation ( [[IPCC:Wg2:Chapter:Chapter-3#3.4.2.1|Section 3.4.2.1]] ), and the reaching of adaptation limits ( [[#16.4|Section 16.4]] ) ( ''medium evidence'' , ''high agreement'' ). In some Arctic communities and in communities reliant on warm-water coral reefs, even 1.5–2°C warming will lead to severe risks from loss of ecosystem services ( [[IPCC:Wg2:Chapter:Chapter-3#3.4.2.2|Section 3.4.2.2]] ; Cross-Chapter Paper 6) ( ''high confidence'' ). Loss of land is already underway globally due to accelerating coastal erosion and will be amplified by increased sea level extremes and permanent flooding ( ''high confidence'' ; Oppenheimer et al. 2019, Ranasinghe et al. 2021). Observed impacts of and projected increases in high-intensity extreme events (Ranasinghe et al. 2021) also provide evidence for severe risk to occur on livelihoods, infrastructure and well-being ( [[#16.5.2.3.3|Section 16.5.2.3.3]] ) by mid-century ( ''high confidence'' ). Consequently, the combination of high warming, continued coastal development and low adaptation levels will challenge the habitability of many low-lying coastal communities in both developing and developed countries over the course of this century ( ''limited evidence'' , ''high agreement'' ) ( [[#Duvat--2021|Duvat et al., 2021]] ; [[#Horton--2021|Horton et al., 2021]] ). In some contexts, climate risks are already considered severe ( ''medium evidence'' , ''medium agreement'' ), and in others, even lower warming will induce severe risks to habitability, which will not necessarily be offset by ambitious adaptation ( ''limited evidence'' , ''medium agreement'' ).

# Natural coastal protection and habitats—severe risks from the loss of shoreline protection from reductions in wave attenuation ( [[#Beck--2018|Beck et al., 2018]] , Sections 3.5.5.1, 3.5.4.5) and sediment delivery (Sections 3.4.2.5, 15.3.3) are already observed in some coastal systems ( [[#16.2.3.1|Section 16.2.3.1]] ) and occur broadly even with 1.5°C of global warming ( [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] ; [[#Bindoff--2019|Bindoff et al., 2019]] , [[IPCC:Wg2:Chapter:Chapter-3#3.4.2|Section 3.4.2]] ). These impacts are the consequence of warming and SLR on coastal ecosystems.

Warm-water coral reefs are at risk of widespread loss of structural complexity and reef accretion by 2050 under 1.5°C global warming ( [[IPCC:Wg2:Chapter:Chapter-3#3.4.2.1|Section 3.4.2.1]] ) ( ''high confidence'' ). Kelp forests may experience shifts in community structure ( [[#Arafeh-Dalmau--2019|Arafeh-Dalmau et al., 2019]] ; [[#Rogers-Bennett--2019|Rogers-Bennett and Catton, 2019]] ; [[#Smale--2020|Smale, 2020]] ; [[#Smith--2021|Smith et al., 2021]] ) with &gt;2°C of global warming especially at lower latitudes ( [[IPCC:Wg2:Chapter:Chapter-3#3.4.2.2|Section 3.4.2.2]] ) ( ''high confidence'' ). In addition, depending on the local tide and sediment conditions, SLR associated with &gt;1.5°C of global warming (SSP1–2.6; 3.4.2.5) is sufficient to initiate shifts to alternate states in some seagrass and coastal wetland systems ( [[#van%20Belzen--2017|van Belzen et al., 2017]] ; [[#El-Hacen--2018|El-Hacen et al., 2018]] , [[IPCC:Wg2:Chapter:Chapter-3#3.4.2.5|Section 3.4.2.5]] , Cross-Chapter Box SLR in Chapter 3), and submergence of some mangrove forests ( [[IPCC:Wg2:Chapter:Chapter-3#3.4.2.5|Section 3.4.2.5]] ). A striking example of risks becoming severe at higher levels of warming is the one of coral islands with low elevation ( [[IPCC:Wg2:Chapter:Chapter-15#15.3.4|Section 15.3.4]] , Box 15.1): the risk of loss of habitability transitions from Moderate-to-High under RCP2.6 for most island types (urban and rural) to High-to-Very High under RCP8.5 ( [[#Duvat--2021|Duvat et al., 2021]] ), even under a high adaptation scenario ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ), partly due to declining sediment supply ( [[#Perry--2018|Perry et al., 2018]] ) and increased annual flooding ( [[#Giardino--2018|Giardino et al., 2018]] ; [[#Storlazzi--2018|Storlazzi et al., 2018]] ).

More broadly, about 28,000 km 2 of land have been lost globally since the 1980s due to anthropogenic factors (e.g., coastal structures, disruption of sediment fluxes) and coastal hazards ( [[#Mentaschi--2018|Mentaschi et al., 2018]] ), and an additional loss of 6000–17,000 km 2 is estimated by the end of the century due to coastal erosion alone associated with SLR in combination with other drivers ( [[#Hinkel--2013|Hinkel et al., 2013]] ).

# Impacts to lives, livelihoods, culture and well-being—in the absence of effective adaptation, changing extreme and slow-onset hazards combined with anthropogenic drivers (e.g., increased population pressure at the coast between +5% and +13.6% by 2100 compared with today, [[#Jones--2016|Jones and O’Neill, 2016]] ) will lead to loss of lives, livelihoods, health, well-being and/or culture ( [[#McGregor--2016|McGregor et al., 2016]] ; [[#Pinnegar--2019|Pinnegar et al., 2019]] ; [[#Pugatch--2019|Pugatch, 2019]] ; [[#Schneider--2020|Schneider and Asch, 2020]] ; [[#Thomas--2020|Thomas and Benjamin, 2020]] ; [[#McNamara--2021|McNamara et al., 2021]] ) ( ''high confidence'' ). Catastrophic examples that may foreshadow the future include Hurricane Sandy in 2012 ( [[#Strauss--2021|Strauss et al., 2021]] ) and Super Typhoon Haiyan in 2013 (&gt;6,000 deaths and inequities in access to safe housing; Trenberth et al. 2015) (Sections 6.2.2, 6.3.5.1). Although there is no unique definition of ‘intolerable’ loss, risks are generally expected to become severe over this century ( [[#Tschakert--2017|Tschakert et al., 2017]] ; [[#Dannenberg--2019|Dannenberg et al., 2019]] ; [[#Tschakert--2019|Tschakert et al., 2019]] ). Globally, with High warming, 90–380 million more people will be exposed to annual flood levels by the mid- and end-century, respectively, compared with 250 million people today ( [[#Kulp--2019|Kulp and Strauss, 2019]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ), with potential implications on forced displacement or migration ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Wrathall--2019|Wrathall et al., 2019]] ; [[#Hauer--2020|Hauer et al., 2020]] ; [[#Lincke--2021|Lincke and Hinkel, 2021]] , [[#16.5.2.3.8|Section 16.5.2.3.8]] ). Some of the largest fish-producing and fish-dependent ecoregions have already experienced losses of up to 35% in marine fisheries productivity due to warming ( [[#Free--2019|Free et al., 2019]] ), and about 11% of the global population will face increasing nutritional risks if current trajectories continue ( [[#Golden--2016|Golden et al., 2016]] ). While difficult to measure, current climate-driven losses to (Indigenous) knowledge, traditions ( [[#Tschakert--2019|Tschakert et al., 2019]] ; [[#Pearson--2021|Pearson et al., 2021]] ) and well-being ( [[#Ebi--2017|Ebi et al., 2017]] ; [[#Cunsolo--2018|Cunsolo and Ellis, 2018]] ; [[#Jaakkola--2018|Jaakkola et al., 2018]] ) indicate such risk as already severe in some regions ( ''limited evidence'' , ''medium agreement'' ), jeopardising communities’ realisation of their rights to food, health and culture. In the Arctic, climate-driven changes to ice and weather regimes have substantially affected traditional coastal-based hunting and fishing activities ( [[#Fawcett--2018|Fawcett et al., 2018]] ; [[#Galappaththi--2019|Galappaththi et al., 2019]] ; [[#Huntington--2020|Huntington et al., 2020]] ; [[#Nuttall--2020|Nuttall, 2020]] , Cross-Chapter Paper 6), and where permafrost thaw, SLR and coastal erosion are contributing to threatening cultural sites ( [[#Hollesen--2018|Hollesen et al., 2018]] ; [[#Fenger-Nielsen--2020|Fenger-Nielsen et al., 2020]] ).
# Critical physical infrastructure—severe risks are also illustrated through damages that lead to possibly long-lasting disruption of key services like transportation as well as energy generation and distribution in coastal areas ( [[#16.5.2.3.3|Section 16.5.2.3.3]] ) under all RCPs (Section [https://www.ipcc.ch/chapter/16#CCP2.2 CCP2.2.3] ) and if no additional adaptation ( ''medium confidence'' ). Critical transport infrastructure is already suffering from structural failures in polar regions, for instance, due to permafrost thaw and increased erosion associated with ocean warming, storm surge flooding and loss of sea ice ( [[#Melvin--2017|Melvin et al., 2017]] ; [[#Fang--2018|Fang et al., 2018]] , Sections 14.5.2.8, 16.2.3.2, Cross-Chapter Paper 6). One hundred airports are projected to be below mean sea level in 2100 with 2°C of warming (i.e., 0.62 m SLR, [[#Yesudian--2021|Yesudian and Dawson, 2021]] ), including in small islands ( [[#Monioudi--2018|Monioudi et al., 2018]] ; [[#Storlazzi--2018|Storlazzi et al., 2018]] ) and megacities. Projections show San Francisco International Airport, for instance, to be inundated by 2100 under the upper likely range of SLR in RCP8.5 (also considering subsidence trends, [[#Shirzaei--2018|Shirzaei and Bürgmann, 2018]] ). On the energy side, it is estimated that with 1.8 m SLR, for example, 4 out of 13 US nuclear power plant facilities will become exposed to storm surges and 3 others will be surrounded or submerged by seawater ( [[#Jordaan--2019|Jordaan et al., 2019]] ; [[#Jenkins--2020|Jenkins et al., 2020]] ).

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