Editing IPCC:AR6/WGII/Chapter-3 (section)

===== 3.6.3.3.4 Mitigation =====

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Ocean and coastal NbS can contribute to global mitigation efforts, especially with ocean renewable energy and restoration and preservation of carbon ecosystems (see Box 3.4; [[#3.6.2.3|Section 3.6.2.3]] ). Technological, economic and financing barriers presently hamper development of renewable ocean energy (AR6 WGIII Chapter 6). Such development could help small nations reliant on imported fuel meet their climate-mitigation goals and decrease risk from global fuel-supply dynamics ( [[#Millar--2017|Millar et al., 2017]] ; [[#Chen--2018a|Chen et al., 2018a]] ), but progress is limited by lack of investment ( [[#Millar--2017|Millar et al., 2017]] ; [[#Lee--2020|Lee et al., 2020]] ) or equipment ( [[#Aderinto--2018|Aderinto and Li, 2018]] ; [[#Rusu--2018|Rusu and Onea, 2018]] ). Wave-energy installations, possibly co-located with wind turbines ( [[#Perez-Collazo--2018|Perez-Collazo et al., 2018]] ), are promising for both low- to middle-income nations and areas with significant island or remote coastal geographies ( [[#Lavidas--2016|Lavidas and Venugopal, 2016]] ; [[#Bergillos--2018|Bergillos et al., 2018]] ; [[#Jakimavičius--2018|Jakimavičius et al., 2018]] ; [[#Kompor--2018|Kompor et al., 2018]] ; [[#Penalba--2018|Penalba et al., 2018]] ; [[#Saprykina--2018|Saprykina and Kuznetsov, 2018]] ; [[#Lavidas--2019|Lavidas, 2019]] ). Wave-energy capture may also diminish storm-induced coastal erosion ( [[#Abanades--2018|Abanades et al., 2018]] ; [[#Bergillos--2018|Bergillos et al., 2018]] ). Tidal energy is a relatively new technology ( [[#Haslett--2018|Haslett et al., 2018]] ; [[#Liu--2018|Liu et al., 2018]] ; [[#Neill--2018|Neill et al., 2018]] ) with limiting siting requirements ( [[#Mofor--2013|Mofor et al., 2013]] ). Ocean renewable-energy expansion faces other technological obstacles including lack of implementable or scalable energy-capture devices, access to offshore sites, competing coastal uses, potential environmental impacts and lack of power-grid infrastructure at the coast ( [[#Aderinto--2018|Aderinto and Li, 2018]] ; [[#Neill--2018|Neill et al., 2018]] ).

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'''Cross-Chapter Box SLR | Sea Level Rise'''
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Authors: Gonéri Le Cozannet (France, Chapter 13, CCP4), Judy Lawrence (New Zealand, Chapter 11), David S. Schoeman (Australia, Chapter 3), Ibidun Adelekan (Nigeria, Chapter 9), Sarah R. Cooley (USA, Chapter 3), Bruce Glavovic (New Zealand/South Africa, Chapter 18, CCP2), Marjolijn Haasnoot (The Netherlands, Chapter 13, CCP2), Rebecca Harris (Australia, Chapter 2), Wolfgang Kiessling (Germany, Chapter 3), Robert E. Kopp (USA, WGI), Aditi Mukherji (Nepal, Chapter 4), Patrick Nunn (Australia, Chapter 15), Dieter Piepenburg (Germany, Chapter 13), Daniela Schmidt (UK/Germany, Chapter 13), Craig T. Simmons (Australia), Chandni Singh (India, Chapter 10, CCP2), Aimée Slangen (The Netherlands, WGI), Seree Supratid (Thailand, Chapter 4).

Sea level rise is already impacting ecosystems, human livelihoods, infrastructure, food security and climate mitigation at the coast and beyond. Ultimately, it threatens the existence of cities and settlements in low-lying areas, and some island nations and their cultural heritage (Chapters 9–15; Cross-Chapter Papers 2, 4; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). The challenge can be addressed by mitigation of climate change and coastal adaptation.

'''Current impacts of sea level rise'''
The rate of global mean SLR was 1.35 mm yr –1 (0.78–1.92 mm yr –1 , ''very likely'' range) during 1901–1990, faster than during any century in at least 3000 years ( ''high confidence'' ) (WGI AR6 Chapter 9; [[#Stanley--1994|Stanley and Warne, 1994]] ; [[#Woodroffe--2016|Woodroffe et al., 2016]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). Global mean SLR has accelerated to 3.25 mm yr –1 (2.88–3.61 mm yr –1 , ''very likely'' range) during 1993–2018 ( ''high confidence'' ). Extreme sea levels have increased consistently across most regions (WGI AR6 Chapter 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). The largest observed changes in coastal ecosystems are being caused by the concurrence of human activities, waves, current-induced sediment transport and extreme storm events ( ''medium confidence'' ) (Chapters 3, 15, 16; [[#Takayabu--2015|Takayabu et al., 2015]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ; [[#Duvat--2019|Duvat, 2019]] ; [[#Murray--2019|Murray et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). Early impacts of accelerating SLR detected at sheltered or subsiding coasts include chronic flooding at high tides, wetland salinisation and ecosystem transitions, increased erosion and coastal flood damage (Chapters 3, 11, 13–16; WGI AR6 Chapter 9; [[#Sweet--2014|Sweet and Park, 2014]] ; [[#Moftakhari--2015|Moftakhari et al., 2015]] ; [[#Nunn--2017|Nunn et al., 2017]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Sharples--2020|Sharples et al., 2020]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Strauss--2021|Strauss et al., 2021]] ). The exposure of many coastal populations and ecosystems to SLR is high: economic development is disproportionately concentrated in and around coastal cities and settlements ( ''virtually certain'' ) (Chapters 3, 9–15; Cross-Chapter Papers 2, 4).

'''Projected risks to coastal communities, infrastructure and ecosystems'''
Risks from SLR are ''very likely'' to increase by one order of magnitude well before 2100 without adaptation and mitigation action as agreed by parties to the Paris Agreement ( ''very high confidence'' ). Global mean SLR is ''likely'' to continue accelerating under SSP1-2.6 and more strongly forced scenarios (Figure BoxSLR1; WGI AR6 Chapter 9; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), increasing the risk of chronic coastal flooding at high tide, serious flooding during extreme events such as swells, storms and hurricanes, and erosion, and coastal ecosystem losses across many low-lying and erodible coasts ( ''very high confidence'' ) (Chapters 3, 9–15; Cross-Chapter Paper 2; [[#Hinkel--2014|Hinkel et al., 2014]] ; [[#McLachlan--2018|McLachlan and Defeo, 2018]] ; [[#Kulp--2019|Kulp and Strauss, 2019]] ; [[#Vousdoukas--2020b|Vousdoukas et al., 2020b]] ). The compounding of rainfall, river flooding, rising water tables, coastal surges and waves are projected to exacerbate SLR impacts on low-lying areas and rivers further inland (Chapters 4, 11–15; [[#Bevacqua--2020|Bevacqua et al., 2020]] ).

There is ''high confidence'' that coastal risks will increase by at least one order of magnitude over the 21st century due to committed SLR ( [[#Hinkel--2013|Hinkel et al., 2013]] ; [[#Hinkel--2014|Hinkel et al., 2014]] ; [[#IPCC--2019b|IPCC, 2019b]] ). Exposure of population and economic assets to coastal hazards is projected to increase over the next decades, particularly in coastal regions with fast-growing populations in Africa, Southeast Asia and Small Islands ( ''medium evidence'' ) (Chapters 9–15; Cross-Chapter Papers 2, 4; [[#Neumann--2015|Neumann et al., 2015]] ; [[#Jones--2016|Jones and O’Neill, 2016]] ; [[#Merkens--2016|Merkens et al., 2016]] ; [[#Merkens--2018|Merkens et al., 2018]] ; [[#Rasmussen--2020|Rasmussen et al., 2020]] ). For RCP8.5, 2.5–9% of the global population and 12–20% of the global gross domestic product is projected to be exposed to coastal flooding by 2100 ( [[#Kulp--2019|Kulp and Strauss, 2019]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ; [[#Rohmer--2021|Rohmer et al., 2021]] ). Above 3°C global warming levels (GWL) and with low adaptation, SLR may cause disruptions to ports and coastal infrastructure ( [[#Camus--2019|Camus et al., 2019]] ; [[#Christodoulou--2019|Christodoulou et al., 2019]] ; [[#Verschuur--2020|Verschuur et al., 2020]] ; [[#Yesudian--2021|Yesudian and Dawson, 2021]] ), which in turn may cascade and amplify across sectors and regions, generating impacts to financial systems (Chapters 11, 13; [[#Mandel--2021|Mandel et al., 2021]] ). Depending on the hydrogeological context, SLR causes salinisation of groundwater, estuaries, wetlands and soils, adding constraints to water management and livelihoods in agriculture sectors, for example, in deltas (Chapters 9, 15; Cross-Chapter Paper 4; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Nicholls--2020|Nicholls et al., 2020]] ).

Coastal ecosystems can migrate landward or grow vertically in response to SLR, but their resilience and capacity to keep up with SLR will be compromised by ocean warming and other drivers, depending on regions and species, for example, above 1.5°C for coral reefs ( ''high confidence'' ) (Chapters 3, 16; [[#IPCC--2018|IPCC, 2018]] ; [[#Melbourne--2018|Melbourne et al., 2018]] ; [[#Perry--2018|Perry et al., 2018]] ; [[#IPCC--2019b|IPCC, 2019b]] ; [[#Cornwall--2021|Cornwall et al., 2021]] ). Sediments and space for landward retreat are crucial for mangroves, salt marshes and beach ecosystems ( ''high confidence'' ) (Chapter 3; [[#Peteet--2018|Peteet et al., 2018]] ; [[#Schuerch--2018|Schuerch et al., 2018]] ; [[#FitzGerald--2019|FitzGerald and Hughes, 2019]] ; [[#Friess--2019|Friess et al., 2019]] ; [[#Leo--2019|Leo et al., 2019]] ; [[#Schuerch--2019|Schuerch et al., 2019]] ). Loss of habitat is accompanied by loss of associated ecosystem services, including wave-energy attenuation, habitat provision for biodiversity, food production and carbon storage (Chapter 3; Cross-Chapter Box NATURAL in Chapter 2).

Under a high-emissions, low-likelihood/high-impact scenario, where ''low confidence'' ice-sheet mass loss occurs, global mean SLR could exceed the ''likely'' range by more than one additional metre in 2100 (Figure BoxSLR1b; Cross-Chapter Box DEEP in Chapter 17; WGI AR6 Technical Summary and Chapter 9; [[#Arias--2021|Arias et al., 2021]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). This is a reason for concern given that rapid SLR after the last glacial–interglacial transition caused a drowning of coral reefs ( ''high confidence'' ) ( [[#Camoin--2015|Camoin and Webster, 2015]] ; [[#Sanborn--2017|Sanborn et al., 2017]] ; [[#Webster--2018|Webster et al., 2018]] ), extensive loss of coastal land and islands, habitats and associated biodiversity ( ''high confidence'' ) (AR6 WGI Chapter 9; [[#Fruergaard--2015|Fruergaard et al., 2015]] ; [[#Fernández-Palacios--2016|Fernández-Palacios et al., 2016]] ; [[#Hamilton--2019|Hamilton et al., 2019]] ; [[#Helfensdorfer--2019|Helfensdorfer et al., 2019]] ; [[#Kane--2020|Kane and Fletcher, 2020]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), and triggered Neolithic migrations in Europe and Australia ( ''medium confidence'' ) (Cross-Chapter Box PALEO in Chapter 1; [[#Turney--2007|Turney and Brown, 2007]] ; [[#Brisset--2018|Brisset et al., 2018]] ; [[#Williams--2018|Williams et al., 2018]] ).

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Cross-Chapter Box SLR

At centennial time scales, projected SLR represents an existential threat for island nations, low-lying coastal zones and the communities, infrastructure, and cultural heritage therein (Chapters 9–15; Cross-Chapter Paper 4). Even if climate warming is stabilised at 2°C to 2.5°C GWL, coastlines will continue to reshape over millennia, affecting at least 25 megacities and drowning low-lying areas where 0.6–1.3 billion people lived in 2010 ( ''medium confidence'' ) (WGI AR6 Chapter 9; [[#Marzeion--2014|Marzeion and Levermann, 2014]] ; [[#Clark--2016|Clark et al., 2016]] ; [[#Kulp--2019|Kulp and Strauss, 2019]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Strauss--2021|Strauss et al., 2021]] ).

'''Solutions, opportunities and limits to adaptation'''
The ability to adapt to current coastal impacts, to cope with future coastal risks and to prevent further acceleration of SLR beyond 2050 depends on immediate mitigation and adaptation actions ( ''very high confidence'' ). The most urgent adaptation challenge is chronic flooding at high tide (Chapters 10, 11, 13–15). Reducing the acceleration of SLR beyond 2050 will only be achieved with fast and profound mitigation of climate change ( [[#Nicholls--2018|Nicholls et al., 2018]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). Until 2050, adaptation planning and implementation needs are projected to increase significantly in most inhabited coastal regions (see Figure BoxSLR1; WGI AR6 Chapter 9; [[#IPCC--2019b|IPCC, 2019b]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). For SSP1-2.6 and more strongly forced scenarios, SLR rates continue to increase (WGI AR6 Chapter 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), and so do the scale and frequency of adaptation interventions needed in coastal zones (Figure BoxSLR1; [[#Haasnoot--2020|Haasnoot et al., 2020]] ).

Risks can be anticipated, planned and decided upon, and adaptation interventions can be implemented over the coming decades considering their often long lead- and lifetimes, irrespective of the large uncertainty about SLR beyond 2050 ( ''high confidence'' ) (Figure BoxSLR1; Cross-Chapter Box DEEP in Chapter 17; Cross-Chapter Paper 2; Chapters 11, 13; [[#Haasnoot--2018|Haasnoot et al., 2018]] ; [[#Stephens--2018|Stephens et al., 2018]] ; [[#Stammer--2019|Stammer et al., 2019]] ). Adaptation capacity and governance to manage risks from projected SLR typically require decades to implement and institutionalise ( ''high confidence'' ) (Chapters 11, 13; [[#Haasnoot--2021|Haasnoot et al., 2021]] ). Without considering both short- and long-term adaptation needs, including beyond 2100, communities are increasingly confronted with a shrinking solution space, and adverse consequences are disproportionately borne by exposed and socially vulnerable people (Chapters 1, 8). Sea level rise is ''likely'' to compound social conflict in some settings ( ''high confidence'' ) ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ).

Coastal impacts of SLR can be avoided by preventing new development in exposed coastal locations (Chapters 3, 9–15; Cross-Chapter Paper 2; [[#Doberstein--2019|Doberstein et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). For existing developments, a range of near-term adaptation options exists, including: (a) engineered, sediment- or ecosystem-based protection; (b) accommodation and land-use planning, to reduce the vulnerability of people and infrastructure; (c) advance through, for example, land reclamation; and (d) retreat through planned relocation or displacements and migrations due to SLR (Chapters 9–15; Cross-Chapter Paper 2; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). Only avoidance and relocation can remove coastal risks for the coming decades, while other measures only delay impacts for a time, have increasing residual risk or perpetuate risk and create ongoing legacy effects and ''virtually certain'' property and ecosystem losses ( ''high confidence'' ) (Cross-Chapter Paper 2; [[#Siders--2019|Siders et al., 2019]] ). Large-scale relocation has immense cultural, political, social and economic costs, and equity implications, which can be reduced by fast implementation of climate mitigation and adaptation policies (Chapter 8; Cross-Chapter Paper 2; [[#Gibbs--2015|Gibbs, 2015]] ; [[#Haasnoot--2021|Haasnoot et al., 2021]] ). While relocation may currently appear socially unacceptable, economically inefficient or technically infeasible today ( [[#Lincke--2021|Lincke and Hinkel, 2021]] ), it becomes the only feasible option as protection costs become unaffordable and the limits to accommodation become obvious (Chapters 11, 13, 15; [[#Hino--2017|Hino et al., 2017]] ; [[#Siders--2019|Siders et al., 2019]] ; [[#Strauss--2021|Strauss et al., 2021]] ). Effective responses to rising sea level involve locally applicable combinations of decision analysis, land-use planning, public participation and conflict resolution approaches; together these can anticipate change and help to chart adaptation pathways, over time addressing the governance challenges due to rising sea level ( ''high confidence'' ) ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ).

Ecosystem-based adaptation can reduce impacts on human settlements and bring substantial co-benefits, such as ecosystem services restoration and carbon storage, but they require space for sediment and ecosystems and have site-specific physical limits, at least above 1.5°C GWL ( ''high confidence'' ) (Cross-Chapter Box NATURAL in Chapter 2; Chapters 3, 9, 11, 15; [[#Herbert--2015|Herbert et al., 2015]] ; [[#Brown--2019|Brown et al., 2019]] ; [[#Van%20Coppenolle--2019|Van Coppenolle and Temmerman, 2019]] ; [[#Watanabe--2019|Watanabe et al., 2019]] ; [[#Neijnens--2021|Neijnens et al., 2021]] ). For example, planting and conserving vegetation helps sediment accumulation by dissipating wave energy and reducing impacts of storms, at least at present-day sea levels ( ''high confidence'' ) ( [[#Temmerman--2013|Temmerman et al., 2013]] ; [[#Narayan--2016|Narayan et al., 2016]] ; [[#Romañach--2018|Romañach et al., 2018]] ; [[#Laengner--2019|Laengner et al., 2019]] ; [[#Leo--2019|Leo et al., 2019]] ). Coastal wetlands and ecosystems can be preserved by landward migration ( [[#Schuerch--2018|Schuerch et al., 2018]] ; [[#Schuerch--2019|Schuerch et al., 2019]] ) or sediment supply ( [[#VanZomeren--2018|VanZomeren et al., 2018]] ), but they can be seriously damaged by coastal defences designed to protect infrastructure (Chapters 3, 13; [[#Cooper--2020b|Cooper et al., 2020b]] ). Sediment nourishment can prevent erosion, but it can also negatively impact beach amenities and ecosystems through ongoing dredging, pumping and deposition of sand and silts ( [[#VanZomeren--2018|VanZomeren et al., 2018]] ; [[#de%20Schipper--2021|de Schipper et al., 2021]] ; [[#Harris--2021|Harris et al., 2021]] ).

[[File:9d7ef33b97d234e8d52d2d6d35c26042 IPCC_AR6_WGII_Figure_3_Cross-Chapter_Box_SLR_1.png]]

'''Figure Cross-Chapter BoxSLR.1 |''' '''The challenge of coastal adaptation in the era of sea level rise (SLR):''' '''(a)''' '''typical time scales for the planning, implementation (grey triangles) and operational lifetime of current coastal risk-management measures (blue bars);''' '''(b)''' '''global sea level projections, which are representative of relative SLR projected for 60–70% of global shorelines, within ±20% errors (WGI AR6 Chapter 9; Fox-Kemper et al.''' ''',''' 2021); '''(c)''' frequency of illustrative adaptation decisions to +0.5 m of SLR under different SSP-RCP scenarios. In response to accelerated SLR, adaptation either occurs earlier and faster, or accounts for higher amounts of SLR (e.g., to +1 m instead of to +0.5 m). Adaptation to +0.5 m from today’s sea levels have a lifetime of 90 years for SSP1-2.6, but lifetime is reduced to 60 years for SSP5-8.5 and 30 years for a high-end scenario involving ''low confidence'' processes. Adaptations to +0.5 m are comparable to, for example, the Thames Barrier in the United Kingdom or the Delta Works in the Netherlands, which primarily had an intended lifetime of 100–200 years. Adaptation measures to +0.2 m may include nourishment or wetland or setback zones.

There is increasing evidence that current governance and institutional arrangements are unable to address the escalating risks in low-lying coastal areas worldwide ( ''high confidence'' ). Barriers to adaptation, such as decision making driven by short-term thinking or vested interests, funding limitations and inadequate financial policies and insurance, can be addressed equitably and sustainably through implementation of suites of adaptation options and pathways (Chapters 11, 13, 17–18; Cross-Chapter Paper 2). Improved coastal adaptation governance can be supported by approaches that consider changing risks over time, such as ‘dynamic adaptation pathways’ planning (Chapters 11, 13, 18; Cross-Chapter Box DEEP in Chapter 17). Integrated coastal zone management and land-use and infrastructure planning are starting to consider SLR by, for example, monitoring early signals ( [[#Haasnoot--2018|Haasnoot et al., 2018]] ; [[#Stephens--2018|Stephens et al., 2018]] ; [[#Kool--2020|Kool et al., 2020]] ), updating sea level projections ( [[#Stephens--2017|Stephens et al., 2017]] ; [[#Hinkel--2019|Hinkel et al., 2019]] ; [[#Kopp--2019|Kopp et al., 2019]] ; [[#Stammer--2019|Stammer et al., 2019]] ), considering uncertainties of sea level projections and coastal impacts (e.g., [[#Stephens--2017|Stephens et al., 2017]] ; [[#Jevrejeva--2019|Jevrejeva et al., 2019]] ; [[#Rohmer--2019|Rohmer et al., 2019]] ), as well as engaging with communities, practitioners and scientists, recognising the values of current and future generations (e.g., [[#Nicholls--2014|Nicholls et al., 2014]] ; [[#Buchanan--2016b|Buchanan et al., 2016b]] ). While there is ''high agreement'' that the majority of adaptation needs are not met yet, there is ''robust evidence'' of SLR increasingly being considered in coastal adaptation decision making and being embedded in national and local guidance and regulations ( [[#Nicholls--2014|Nicholls et al., 2014]] ; [[#Le%20Cozannet--2017|Le Cozannet et al., 2017]] ; [[#Lawrence--2018|Lawrence et al., 2018]] ; [[#Kopp--2019|Kopp et al., 2019]] ; [[#McEvoy--2021|McEvoy et al., 2021]] ).

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Cross-Chapter Box SLR

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