Editing IPCC:AR6/SROCC/Chapter-4 (section)

== 4.1 Synthesis ==

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=== 4.1.1 Purpose, Scope, and Structure of this Chapter ===

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This chapter assesses the literature published since the AR5 on past and future contributions to global, regional and ESL changes, associated risk to low-lying islands, coasts, cities and settlements, and response options and pathways to resilience and sustainable development along the coast. The chapter follows the risk framework of AR5, in which risk is assessed in terms of hazard, exposure and vulnerability (Cross-Chapter Box 1 Chapter 1; Box 4.1), and is structured as follows (Figure 4.1):

* Section 4.1 (this section) presents a high-level synthesis of our assessment and provides entry points to more specific content found in the other sections.
* Section 4.2 assesses the current understanding of processes contributing to mean and extreme SLR globally, regionally and locally, with an emphasis on new insights about the AIS contribution.
* Section 4.3 assesses how mean and extreme sea level changes translate into coastal hazards (e.g., flooding, erosion and salinity intrusion), how these interact with socioeconomic drivers of coastal exposure and vulnerability, and how this interaction translates into observed impacts and projected risks for ecosystems, natural resources and human systems.
* Section 4.4 assesses the cost, effectiveness, co-benefits, efficiency, and technical limits of different types of SLR responses and identifies governance challenges (also called barriers) associated with choosing and implementing responses. Next, planning, public participation, conflict resolution and decision analysis methods for addressing the identified governance challenges are assessed, as well as practical lessons learned in local cases.

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=== 4.1.2 Future Sea level Rise and Implications for Responses ===

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For understanding responses to climate-change induced SLR, two aspects of sea level are important to note initially:

# Climate-change induced GMSL rise is caused by thermal expansion of ocean water and ocean mass gain, the latter primarily due to a decrease in land-ice mass. However, responses to SLR are local and hence always based on RSL experienced at a particular location. GMSL is modified regionally by climate processes and locally by a variety of factors, some driven or influenced by human activity. Of particular relevance for responding to SLR is anthropogenic subsidence, which can lead to rates of RSL rise that exceed those of climate-induced SLR by an order of magnitude, specifically in delta regions and near cities (4.2.2.4). In these subsiding regions, one available response to prepare for future climate-induced SLR is to manage and reduce anthropogenic subsidence (4.4.2).
# The combination of gradual change of mean sea level with ESL events such as tides, surges and waves causes coastal impacts (4.2.3). ESL events at the coast that are rare today will become more frequent in the future, which means that for many locations, the main starting point for coastal planning and decision making is information on current and future ESL events. One important response for preparing for future SLR is to improve observational systems (tide gauges, wave buoys and remote sensing techniques), because in many places around the world current frequencies and intensities of ESL events are not well understood due to a lack of observational data (4.2.3.4.1).

After an increase of sea level from 1–2 mm yr –1 in most regions over the past century, rates of 3–4 mm yr –1 are now being experienced that will further increase to 4–9 mm yr –1 under RCP2.6 and 10–20 mm yr –1 at the end of the century under RCP8.5. Nevertheless, up to 2050, uncertainty in climate change-driven future sea level is relatively small, which provides a robust basis for short-term ( ≤ 30 years) adaptation planning. GMSL will rise between 0.24 m (0.17–0.32 m, ''likely'' range) under RCP2.6 and 0.32 m (0.23–0.40 m, ''likely'' range) under RCP8.5 ( ''medium confidence'' ; 4.2.3). The combined effect of mean and extreme sea levels results in events which are rare in the historical context (return period of 100 years or larger; probability &lt;0.01 yr –1 ) occurring yearly at some locations by the middle of this century under all emission scenarios (4.2.3.4.1; ''high confidence'' ). This includes, for instance, those parts of the intertropical low-lying coasts that are currently exposed to storm surges only infrequently. Hence, additional adaptation is needed irrespective of the uncertainties in future global GHG emissions and the Antarctic contribution to SLR.

Beyond 2050, uncertainty in climate change induced SLR increases substantially due to uncertainties in emission scenarios and the associated climate changes, and the response of the AIS in a warmer world. Combining process-model based studies in which there is ''medium confidence'' , it is found that GMSL is projected to rise between 0.43 m (0.29–0.59 m, ''likely'' range) under RCP 2.6 and 0.84 m (0.61–1.10 m, ''likely'' range) under RCP 8.5 by 2100 (Figure 4.3). The range that needs to be considered for planning and implementing coastal responses depends on the risk tolerance of stakeholders (i.e., those deciding and those affected by a decision; 4.4.4.3.2). Stakeholders that are risk tolerant (e.g., those planning for investments that can be very easily adapted to unforeseen conditions) may prefer to use the ''likely'' ranges of RCP2.6 and RCP8.5 for long-term adaptation planning. Stakeholders with a low risk tolerance (e.g., those planning for coastal safety in cities and long term investment in critical infrastructure) may also consider SLR above this range, because there is a 17% chance that GMSL will exceed 0.59 m under RCP2.6 and 1.10 m under RCP8.5 in 2100. Process-model based studies cannot yet provide this information, but expert elicitation studies show that a GMSL of 2 m in 2100 cannot be ruled out (4.2.3).

Despite the large uncertainty in late 21st century SLR, progress in adaptation planning and implementation is feasible today and may be economically beneficial. Many coastal decisions with time horizons of decades to over a century are made today (e.g., critical infrastructure, coastal protection works, city planning, etc.) and accounting for relative SLR can improve these decisions. Decision-analysis methods specifically targeting situations of large uncertainty are available and, combined with suitable planning, public participation and conflict resolution processes, can improve outcomes ( ''high confidence'' ; 4.4.4.2, 4.4.4.3). For example, adaptation pathway analysis recognises and enables sequenced long-term decision making in the face of dynamic coastal risk characterised by deep uncertainty ( ''medium evidence, high agreement'' ; 4.4.4.3.4). The use of these decision-analysis tools can be integrated into statutory land use or spatial planning provisions to formalise these decisions and enable effective implementation by relevant governing authorities (4.4.4.2).

Beyond 2100, sea level will continue to rise for centuries and will remain elevated for thousands of years ( ''high confidence;'' 4.2.3.5). Only a few modelling studies are available for SLR beyond 2100. However, all studies agree that the difference in GMSL between RCP2.6 and RCP8.5 increases substantially on multi-centennial and millennial time scales ( ''very high confidence'' ). On a millennial time scale, this difference is about 10 metres in some model simulations, whereas it is only several decimetres at the end of 21st century. The larger the emissions the larger the risks associated with SLR as already assessed in SR1.5. Under RCP8.5 the few available studies indicate a ''likely'' range of 2.3–5.4 m ( ''low confidence'' ) in 2300. With strong mitigation efforts (RCP2.6), SLR will be kept to a ''likely'' range of 0.6–1.1 m (Figure 4.2). Regardless, ambitious and sustained adaptation efforts are needed to reduce risks.

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'''Figure 4.1'''

<span id="figure-4.1-schematic-illustration-of-the-interconnection-of-chapter-4-themes-including-drivers-of-sea-level-rise-slr-and-extreme-sea-level-hazards-section-4.2-exposure-vulnerability-impacts-and-risk-related-to-slr-section-4.3-and-responses-associated-governance-challenges-and-practises-and-tools-for-enabling-social-choices-and-addressing-governance-challenges-section-4.4."></span>
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'''Figure 4.1 | Schematic illustration of the interconnection of Chapter 4 themes, including drivers of sea level rise (SLR) and (extreme) sea level hazards (Section 4.2), exposure, vulnerability, impacts and risk related to SLR (Section 4.3), and responses, associated governance challenges and practises and tools for enabling social choices and addressing governance challenges (Section 4.4).'''
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[[File:636c2af83c3fe98ec1d45645d066b94c IPCC-SROCC-CH_4_1-3000x1698.jpg]]

Figure 4.1 | Schematic illustration of the interconnection of Chapter 4 themes, including drivers of sea level rise (SLR) and (extreme) sea level hazards (Section 4.2), exposure, vulnerability, impacts and risk related to SLR (Section 4.3), and responses, associated governance challenges and practises and tools for enabling social choices and addressing governance challenges (Section 4.4).

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'''Figure 4.2'''

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'''Figure 4.2 | Projected sea level rise (SLR) until 2300. The inset shows an assessment of the likely range of the projections for RCP2.6 and RCP8.5 up to 2100 (medium confidence). Projections for longer time scales are highly uncertain but a range is provided (4.2.3.6; low confidence). For context, results are shown from other estimation […]'''
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[[File:9a7523b4e542c402e911b12dbb20767d IPCC-SROCC-CH_4_2-3000x1354.jpg]]

Figure 4.2 | Projected sea level rise (SLR) until 2300. The inset shows an assessment of the likely range of the projections for RCP2.6 and RCP8.5 up to 2100 (medium confidence). Projections for longer time scales are highly uncertain but a range is provided (4.2.3.6; low confidence). For context, results are shown from other estimation approaches in 2100 and 2300. The two sets of two bars labelled B19 are from an expert elicitation for the Antarctic component (Bamber et al., 2019 <sup>[[#fn:r1|1]]</sup> ), and reflect the likely range for a 2oC and 5oC temperature warming (low confidence; details section 4.2.3.3.1). The bar labelled “prob.” indicates the likely range of a set of probabilistic projections (4.2.3.2). The arrow indicated by S18 shows the result of an extensive sensitivity experiment with a numerical model for the Antarctic Ice Sheet (AIS) combined, like the results from B19 and “prob.”, with results from Church et al. (2013) <sup>[[#fn:r2|2]]</sup> for the other components of SLR. S18 also shows the likely range.

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=== 4.1.3 Sea Level Rise Impacts and Implications for Responses ===

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Rising mean and increasingly extreme sea level threaten coastal zones through a range of coastal hazards including (i) the permanent submergence of land by higher mean sea levels or mean high tides; (ii) more frequent or intense coastal flooding; (iii) enhanced coastal erosion; (iv) loss and change of coastal ecosystems; (v) salinisation of soils, ground and surface water; and (vi) impeded drainage. At the century scale and without adaptation, the vast majority of low-lying islands, coasts and communities face substantial risk from these coastal hazards, whether they are urban or rural, continental or island, at any latitude, and irrespective of their level of development (Section 4.3.4; Figure 4.3; ''high confidence'' ). In the absence of an ambitious increase in adaptation efforts compared to those currently underway, high to very high risks are expected in many coastal geographies at the upper end of the RCP8.5 ''likely'' range. These include resource-rich coastal cities, urban atoll islands, densely populated deltas, and Arctic communities (Chapter 4 Box 4; Figure 4.3 and Section 4.3.4). At the same time coastal protection is very effective and cost-efficient for cities but not for less densely populated rural areas. Some geographies, such as urban atoll islands and Arctic communities face high risk even under RCP2.6 ( ''medium confidence'' ).

In many places, however, non SLR-related, local environmental and human dimensions of exposure and vulnerability play a critical role in increasing exposure and vulnerability to coastal hazards (Section 4.3.2.5). For example, the ability of morphological and ecological systems (Sections 4.3.3.3 and 4.3.3.5) to protect human settlements and infrastructure by attenuating ESL events and stabilising shorelines is progressively being lost due to coastal squeeze, pollution, habitat degradation and fragmentation (Section 4.3.3.5.4; ''high confidence'' ). Hence, an important near term response to RSL rise is to reduce these adverse environmental and human dimensions of exposure and vulnerability. In addition, the drivers of exposure and vulnerability vary across different coastal contexts ranging from resource-rich cities to small islands (Sections 4.3.3, 4.3.4). Accordingly, effective responses need to be context-specific, and address the locality-specific drivers of risk.

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'''Figure 4.3'''

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'''Figure 4.3 | Additional risk related to sea level rise (SLR) for low-lying coastal areas by the end of the 21st century. Section 4.3.4 provides a synthesis of the assessment methodology and the findings, while SM4.3 provides details. Left-hand panel describes global mean sea level (GMSL) rise observations for the Present-Day (1986–2005) and projections under […]'''
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[[File:24764f4424275dbd01b62a4b9a512a87 IPCC-SROCC-CH_4_SM_4_4-3000x1155.jpg]]

Figure 4.3 | Additional risk related to sea level rise (SLR) for low-lying coastal areas by the end of the 21st century. Section 4.3.4 provides a synthesis of the assessment methodology and the findings, while SM4.3 provides details. Left-hand panel describes global mean sea level (GMSL) rise observations for the Present-Day (1986–2005) and projections under RCP2.6 and RCP8.5 by 2100 relative to the Present-Day according to advances in this chapter. Relative sea level (RSL) changes at specific locations are represented by the coloured blocs (range of the real-world case studies used) and coloured dotted lines (mean) at the background of the middle panel, which describes risk to illustrative geographies as assessed in this chapter. Each illustrative geography is supported by real-world case studies described in the literature (Box 4.1, 4.3.4.1 and Table SM4.2.5): three for resource-rich coastal cities, three for urban atoll islands, two for large tropical agricultural deltas, five for Arctic communities. N.B. (1): Only Arctic communities remote from regions of rapid glacial-isostatic adjustment have been selected for this assessment. N.B (2): according to the specific scope of the chapter, this assessment focuses on the additional risks due to SLR and does not account for changes in extreme event climatology (Sections 4.2.3.4.1 to 4.2.3.4.3, 6.3.1.1 to 6.3.1.3), which in some cases would imply a different level of risk than assessed here. The middle panel also distinguishes between two adaptation scenarios. (A) ‘No-to- moderate response’ represents a business-as-usual scenario where no major additional adaptation efforts compared to today’s level of effort are implemented (i.e., neither further significant intensification of action nor new types of actions). (B) ‘Maximum potential response’ represents the opposite situation, that is, an ambitious combination of both incremental and transformational adaptation that leads to significant additional efforts compared to today and to (A). Here, the authors assume adaptation implemented at its full potential, that is, the extent of adaptation that is technologically possible, with minimal financial, social and political barriers.

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=== 4.1.4 Response Options, Governance Challenges and Ways Forward ===

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Responding to SLR refers to reducing hazards, exposure and vulnerability of low-lying coastal areas. It can be approached in fundamentally different ways and five major categories are described in this chapter (Box 4.3): Protection reduces coastal risk and impacts by blocking the inland propagation and other effects of mean or extreme sea levels hazards (e.g., through dikes, seawalls, storm surge barriers, breakwaters, beach-dune systems, etc.). Advance creates new land by building seawards (e.g., reclamation of new land above sea levels or planting vegetation with the specific intention to support natural accretion of land). Ecosystem-based adaptation (EbA) provides a combination of the benefits of protect and advance strategies based on the conservation and restoration of ecosystems such as reefs and coastal vegetation. Accommodation includes a diverse set of biophysical and institutional responses to reduce vulnerability of coastal residents, human activities, ecosystems and the built environment (e.g., raising buildings, planting salt tolerant crops, insurance and EWS for ESL events). Retreat reduces exposure to coastal hazards by moving people, assets and human activities out of the exposed coastal area.

Each type of response has particular advantages and disadvantages, and may play a synergistic role in an integrated and sequenced response to SLR. For example, hard protection needs less space and its effectiveness is more predictable than for EbA ( ''high confidence'' ; 4.4.2.2.4, 4.4.2.3.4). EbA has advantages of contributing to conservation goals and providing additional ecosystem services such as carbon sequestration and improved water quality (4.4.2.2.5). EbA can become more effective over time, because coastal ecosystems can migrate inland with rising sea levels, provided this is not restricted by infrastructure (4.4.2.2.4). In practise, hard, sediment-based and ecosystem-based protection responses are often combined and there is ''high agreement'' that such hybrid approaches are a promising way forward (4.4.2.3.1). Advance is an option widely practised when land is scarce and offers the opportunity to finance adaptation through land sale revenues, but can also increase exposure and destroy coastal wetlands and their protective function (4.4.2.4). Accommodation measures such as flood proofing buildings, flood forecasting, early warning and emergency planning have high benefit-cost ratios, which means that implementing them is much cheaper than doing nothing (4.4.2.5.6). Retreat, and avoidance of development in some locations, are the only types of responses that eliminate residual risks, assuming there is sufficiently safe alternative land to retreat to or develop (4.4.2.6, Cross-Chapter Box 9).

Given diverse geographies and contexts (4.1.3), and the pros and cons of different responses, there is no silver bullet for responding to SLR. Rather, each coastal locality requires a tailor-made response that uses an appropriate mix of measures, sequenced over time as sea level rises. Possible integrated response strategies are illustrated for two contrasting types of settlements: densely populated urban and sparsely populated rural coasts.

For densely populated urban low elevation areas, including continental and island cities and megacities, hard protection has played and will continue to play the central role in response strategies (4.4.2.2, Box 4.1). In general, it is technologically feasible and economically efficient to protect large parts of cities against 21st century SLR ( ''high confidence'' ; 4.4.2.2.4, 4.4.2.7). However, questions of affordability remain for poorer and developing regions (4.3.3.4, 4.4.2.2.3). In cities, advance can offer a way to finance coastal protection through revenues generated from newly created land (4.4.2.4), but raises equity concerns with regard to the distribution of costs and access to the new land (4.4.2.4.6). Where space is available, EbA can supplement hard protection (4.4.2.3), except in situations where other human interventions, like infrastructure and pollution, interfere with EbA, especially for RCP8.5 (Cross-Chapter Box 9). Retreat may currently be favoured over rebuilding in the aftermath of major flooding disasters, but in densely populated areas protected by hard infrastructure, general retreat need not be considered until later in the century once it is known whether or not SLR will reach the higher end of the projections (1.1 m or more by 2100; 4.4.2.6).

Along sparsely populated rural coasts, safeguarding communities by conserving coastal ecosystems and natural morphodynamic processes, and restoring those already degraded, is the central element of an integrated strategy. Intact coastal ecosystems can protect settlements and, in some contexts, natural sedimentation processes and avoiding sand mining can help to raise exposed land (4.4.2.2). Hard coastal protection can lead to flooding or erosion elsewhere (4.4.2.2.5), and the destruction of ecosystems and the coastal protection they offer (4.3.3.5). Ecosystem health can be further maintained by reducing non-climatic drivers such as those that interrupt sediment flows in deltas and estuaries (4.3.2.3). Hard protection may be appropriate for areas containing high value assets (e.g., settlements and cultural sites). Retreat is worth considering now where coastal population size and density is low, risks are already high, and the economic, cultural and sociopolitical impacts of retreat and resettlement are carefully considered and addressed by at-risk communities and their governing authorities.

Designing and implementing an appropriate mix of responses is not only a technical task but also an inherently political and value-laden social choice that involves trade-offs between multiple values, goals and interests (Section 4.4.3). Specifically, distinctive features of SLR together with this complex nature of social choices give rise to five overarching governance challenges (Section 4.4.3.3):

# '''''Time horizon and uncertainty''''' associated with SLR beyond 2050 challenge standard planning and decision making practises ( ''high confidence'' ).
# '''''Cross-scale and cross-domain coordination''''' linking differing jurisdictional levels, sectors and policy domains is often needed for effective responses ( ''medium confidence'' ).
# '''''Equity and social vulnerability''''' are often negatively affected by SLR and also responses to SLR, which can undermine societal aspirations such as achieving the SDGs ( ''high confidence'' ).
# '''''Social conflict''''' (i.e., nonviolent struggle between groups, organisations and communities over values, interests, resources, and influence or power) caused or exacerbated by SLR could escalate over time and become very difficult to resolve ( ''high confidence'' ).
# '''''Complexity''''' , reinforced by the combination of the above challenges, makes it difficult to understand and address SLR ( ''high confidence'' ).

These governance challenges can be addressed through an integrated combination of well-established and emerging planning, public participation and conflict resolution practices (Section 4.4.4.2), decision analysis methods (Section 4.4.4.3) and enabling conditions (Section 4.4.5). For example, iterative planning and flexible, adaptive and robust decision making (RDM) can help coastal communities to plan for the future and account for SLR uncertainty. Planning can also enable thinking and action across spatial, temporal and governance scales and thus help to coordinate roles and responsibilities across multiple governance levels. Public participation approaches can be designed to account for divergent perspectives in making difficult social choices, enhancing social learning, experimentation and innovation in developing locally appropriate SLR responses. Conflict resolution approaches have considerable potential to improve adaptation prospects by harnessing the productive potential of nonviolent conflict.

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