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

=== 4.3.4 Conclusion on Coastal Risk: Reasons for Concern and Future Risks ===

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SLR projections for the 21st century, together with other ocean related changes (e.g., acidification and warming) and the possible increase in human-driven pressures at the coast (e.g., demographic and settlement patterns), make low-lying islands, coasts and communities relevant illustrations of some of the five Reasons for Concern (RFCs) developed by the IPCC since the Third Assessment Report (McCarthy et al., 2001; Smith et al., 2001) to assess risks from a global perspective. The AR5 Synthesis Report (IPCC, 2014) as well as the more recent SR1.5 (Hoegh-Guldberg et al., 2018) refined the RFC approach. The AR5 Synthesis Report (IPCC, 2014) developed two additional RFCs related to the coasts, subsequently updated along with the other RFCs (O’Neill et al., 2017) . One refers to risks to marine species arising from ocean acidification, and the other one refers to risks to human and natural systems from SLR. Despite the difficulty in attributing observed impacts to SLR per se (Section 4.3.3.1), O’Neill et al. (2017) estimate that risks related to SLR are already detectable globally and will increase rapidly, so that high risk may occur before a 1m rise level is reached. O’Neill et al. (2017) also suggest that limits to coastal protection and EbA by 2100 could occur in a 1 m SLR rise scenario. P revious assessments however left gaps, including quantifying the benefits from adaptation in terms of risk reduction.

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==== 4.3.4.1 Methodological Advances ====

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Rather than revisiting the AR5 and O’Neill et al. (2017) assessments from the particular perspective of risk related to SLR and for the global scale, this section provides a complementary perspective by assessing risks for specific geographies ( resource-rich coastal cities , urban atoll islands, large tropical agricultural deltas and selected Arctic communities), based on the methodological advances below.

'''''Scale of analysis and geographical scope''''' – To date, the RFCs and associated burning embers have been developed at a global scale (Oppenheimer et al., 2014; Gattuso et al., 2015; O’Neill et al., 2017) and do not address the spatial variability of risk highlighted in this report (Sections 4.3.2.7, 4.3.4, 5.3.7, Cross-Chapter Box 9, Box 4.1). In addition, assessments usually identify risks either for global human dimensions (e.g., to people, livelihood, breakdown of infrastructures, biodiversity, global economy, etc.; IPCC, 2014; Oppenheimer et al., 2014; O’Neill et al., 2017) or for ecosystems and ecosystem services (Gattuso et al., 2015; Hoegh-Guldberg et al., 2018) (Section 5.3.7). This section moves the focus from the global to more local scales by considering four generic categories of low-lying coastal areas (Figure 4.3, Panel B): selected Arctic communities remote from regions of rapid GIA, large tropical agricultural deltas, urban atoll islands, and resource-rich coastal cities . Each of these categories is informed by several real-world case studies.

'''''Risks considered''''' – In line with the AR5 (IPCC, 2014) , current and future risks result from the interaction of SLR-related hazards with the vulnerability of exposed ecosystems and societies. According to the specific scope of the chapter, this assessment focusses on the additional risks due to SLR and does not account for changes in extreme event climatology. Hazards considered are coastal flooding (Section 4.3.4.2), erosion (Section 4.3.4.3) and salinisation (Section 4.3.4.4). The proxies used to describe exposure and vulnerability are the density of assets at the coast (Section 4.3.2.2) and the level of degradation of natural buffering by marine and terrestrial ecosystems (Sections 4.3.2.3, 4.3.3.5.4, and 5.3.2 to 5.3.4). The assessment especially addresses risks to human assets at the coast, including populations, infrastructures and livelihoods. Specific metrics were developed (see SM4.3 for details), and their contribution to present-day observed impacts and to end-century risk have been assessed based on the authors’ expert judgment and a methodological grid presented in SM4.3 (SM4.3.1 to SM4.3.6). The author’s expert judgment draws on Sections 4.3.3.2 to 4.3.3.5 as well as additional literature for local scale perspectives (SM4.3.9).

'''''Sea level rise scenarios''''' – Based on the updates for ranges and mean values developed in this chapter (Section 4.2, Table 4.3), this assessment considers the end-century GMSL (2100) relative to 1986–2005 levels for two scenarios, SROCC RCP2.6 and SROCC RCP8.5. Both mean values and the SROCC RCP8.5 upper end of the  ''likely '' range are used to assess risk transitions (Figure 4.3, Panel A). For the sake of readability, the following values were used: 43 cm (mean SROCC RCP2.6), 84 cm (mean SROCC RCP8.5) and 110 cm (SROCC RCP8.5 upper end of  ''likely '' range). While GMSL serves as a representation of different possible climate change scenarios (see Panel A in Figure 4.3, Section 4.1.2), the assessment of additional risks due to SLR on specific geographies is developed against end-century relative SLR (RSL) in order to allow a geographically accurate approach (Panel B, Figure 4.3). Accordingly, risk was assessed to illustrative geographies based on RSLs for each of the two SROCC RCP scenarios and each of the real-world case studies to (SM4.3.6 and Table SM4.3.2; see dotted lines in Panel B of Figure 4.3). RSL observations include some or all of the following VLMs: both uplift (e.g., due to tectonics) and subsidence due to natural (e.g., tectonics, sediment compaction) and human (e.g., oil/gas/water extraction, mining activities) factors, as well as to GIA. However, in SROCC, numerical RSL projections only include GIA and the regional gravitational, rotational, and deformational responses (GRD, see Section 4.2.1.5) to ice mass loss. The main reason is the difficulty   of  project ing   the influence o n   some factors such as human interventions to the end of the century.

'''''Adaptation scenarios''''' – Risk will also depend on the effectiveness of coastal societies’ responses to both extreme events and slow onset changes. To capture the response dimension, four metrics have been considered that refer to the implementation of adequately calibrated hard, engineered coastal defences (Section 4.4.2.2), the restoration of the degraded ecosystems or the creation of new natural buffers areas (Section 4.4.2.2 and 4.4.2.3), planned and local-scale relocation (Section 4.4.2.6), and measures to limit human-induced subsidence (Sections 4.4.2.2, 4.4.2.5). On these bases, two contrasting adaptation scenarios were considered. The first one is called ‘No-to-moderate response’ (see (A) bars in Panel B, Figure 4.3) and represents a business-as-usual scenario where no major additional adaptation efforts compared to today are implemented. That is, neither substantial intensification of current actions nor new types of actions, e.g., only moderate raising of existing protections in high-density areas or sporadic episodes of relocation or beach nourishment where largescale efforts are not already underway. The second one, called ‘Maximum potential response’ (bars (B) in Figure 4.3), refers to an ambitious combination of both incremental and transformational adaptation (i.e., significantly upscaled effort); for example, relocation of entire districts or raised protections in some cities, or creation/restoration at a significant scale of beach-dune systems including indigenous vegetation.

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==== 4.3.4.2 Key Findings on Future Risks and Adaptation Benefits ====

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===== 4.3.4.2.1 Future risks =====

The findings suggest that risks from SLR are already detectable for all of the geographies considered (Panel B in Figure 4.3), and that risk is expected to increase over this century in virtually all low-lying coastal areas whatever their context-specificities or nature (island/continental, developed/developing county) (Cross-Chapter Box 9). In the absence of high adaptation (bars (A)), risk is expected to significantly increase in urban atoll islands and the selected Arctic coastal communities even in a SROCC RCP2.6 scenario, and all geographies are expected to experience almost high to very high risks at the upper ''likely'' range of SROCC RCP8.5. These results allow refining AR5 conclusions by showing, first, that high risk can indeed occur before the 1m rise benchmark (Oppenheimer et al., 2014 <sup>[[#fn:r1399|1399]]</sup> ; O’Neill et al., 2017) and, second, that risk as a function of SLR is highly variable from one geography to another. Some rationale is provided below for our assessment of illustrative geographies, summarising the more detailed description provided in SM4.3 (SM4.3.6 to SM4.3.8). Note however that the text below is not intended to be fully comprehensive and does not necessarily include all elements for which there is a substantive body of literature, nor does it necessarily include all elements which are of particular interest to decision makers.

'''''Resource-rich coastal cities''''' (SM4.3.8.1, Panel B in Figure 4.3) – Resource-rich coastal cities considered in this analysis are Shanghai, New York (see Box 4.1 for further details and references on Shanghai and NYC), and Rotterdam (Brinke et al., 2010 <sup>[[#fn:r1400|1400]]</sup> ; Hinkel et al., 2018 <sup>[[#fn:r1401|1401]]</sup> ). High, and in many cases, growing population density and total population, and high exposure of people and infrastructure to GMSL rise and ESL events characterise coastal megacities (Hanson et al., 2011 <sup>[[#fn:r1402|1402]]</sup> ). These are high concentrations of income and wealth in geographic terms but within relatively small area exhibit large distributional differences of both with important implications for emergency response and adaptation. Concentration translates into high exposure of monetary value to coastal hazards and the cities noted here have both historical and recent experience with damaging ESL events, such as Typhoon Winnie which struck Shanghai in 1997 (Xian et al., 2018 <sup>[[#fn:r1403|1403]]</sup> ), Hurricane Sandy in New York in 2012 (Rosenzweig and Solecki, 2014 <sup>[[#fn:r1404|1404]]</sup> ), and the North Sea storm of 1953 which impacted the Rotterdam area (Gerritsen, 2005 <sup>[[#fn:r1405|1405]]</sup> ; Jonkman et al., 2008 <sup>[[#fn:r1406|1406]]</sup> ). However, high density, limited space and high cost of land leads to development of below-ground space for transportation (e.g., subways, road tunnels; MTA, 2017) and storage, and even habitation, creating vulnerabilities not seen in low-density areas. Natural ecosystems within the megacity boundaries and nearby have been exploited for centuries and in some cases decimated or even extirpated (Hartig et al., 2002 <sup>[[#fn:r1407|1407]]</sup> ). Accordingly, they provide limited benefits in terms of coastal protection for the densest part of these cities but can be critically important for protection of lower-density areas, for example, wetlands and sandy beaches in the Jamaica Bay/Rockaway sector of New York that protect nearby residential communities (Hartig et al., 2002 <sup>[[#fn:r1408|1408]]</sup> ). Space limitations also constrain the potential benefits of EbA measures. Instead, resource-rich coastal cities depend largely on hard defences like sea walls and surge barriers for coastal protection (Section 4.4.2.2). Such defences are costly but generally cost effective due to the aforementioned concentration of population and value. However, barriers to planning and implementing adaptation include governance challenges (Section 4.4.2) such as limited control over finances and the intermittent nature of ESLs which inhibit focused attention over the long time scales needed to plan and implement hard defences (Section 4.4.2.2). As a result, coastal adaptation for resource-rich cities is uneven and the three presented here were selected with a view toward exhibiting a range of current and potential future effectiveness.

'''''Urban atoll islands''''' (SM4.3.8.2, Panel B in Figure 4.3) – The capital islands (or groups of islands) of three atoll nations in the Pacific and Indian Oceans are considered here: Fongafale (Funafuti Atoll, Tuvalu), the South Tarawa Urban District (Tarawa Atoll, Kiribati) and Male’ (North Kaafu Atoll, Maldives). Urban atoll islands have low elevation (&lt;4 m above mean sea level; in South Tarawa, e.g., lagoon sides where settlement concentrates are &lt;1.80 m in elevation) (Duvat, 2013) and are mainly composed of reef-derived unconsolidated material. Their future is of nation-wide importance as they concentrate populations, economic activities and critical infrastructure (airports, main harbours). They illustrate the prominence of anthropogenic-driven disturbances to marine and terrestrial ecosystems (e.g., mangrove clearing in South Tarawa or human-induced coral reef degradation through land reclamation in Male’; Duvat et al., 2013 <sup>[[#fn:r1409|1409]]</sup> ; Naylor, 2015 <sup>[[#fn:r1411|1411]]</sup> ) and therefore to services such as coastal protection delivered by the coral reef (i.e., wave energy attenuation that reduces flooding and erosion, and sediment provision that contributes to island persistence over time) (McLean and Kench, 2015 <sup>[[#fn:r1412|1412]]</sup> ; Quataert et al., 2015 <sup>[[#fn:r1413|1413]]</sup> ; Elliff and Silva, 2017 <sup>[[#fn:r1414|1414]]</sup> ; Storlazzi et al., 2018 <sup>[[#fn:r1415|1415]]</sup> ).

The controlling factors of urban atoll islands’ future habitability are the density of assets exposed to marine flooding and coastal erosion (SM4.3.8.2), future trends in these hazards, and ecosystem response to both ocean-climate related pressures and human activities. Urban atoll islands already experience coastal flooding, for example, in Male’ (Wadey et al., 2017 <sup>[[#fn:r1416|1416]]</sup> ) and Funafuti (Yamano et al., 2007 <sup>[[#fn:r1417|1417]]</sup> ; McCubbin et al., 2015 <sup>[[#fn:r1418|1418]]</sup> ). Coastal erosion is also a major concern along non-armoured shoreline in South Tarawa (Duvat et al., 2013 <sup>[[#fn:r1419|1419]]</sup> ) and Fongafale (Onaka et al., 2017 <sup>[[#fn:r1420|1420]]</sup> ), but not in Male’ where surrounding fortifications have extended along almost the entire shoreline from several decades (Naylor, 2015). Salinisation already affects groundwater lenses, but its contribution to risk varies from one case to another, from low in Male’ (relying on desalinised seawater) to important for human consumption and agriculture in South Tarawa (Bailey et al., 2014 <sup>[[#fn:r1422|1422]]</sup> ; Post et al., 2018 <sup>[[#fn:r1423|1423]]</sup> ).

Together, high population densities (from ~3,200 people per km 2 in South Tarawa to ~65,700 people per km 2 in Male’) (Government of the Maldives, 2014 <sup>[[#fn:r1424|1424]]</sup> ; McIver et al., 2015 <sup>[[#fn:r1425|1425]]</sup> ) and the concentration of critical infrastructure and settlements in naturally low-lying flood-prone areas already substantially contribute to coastal risk (Duvat et al., 2013 <sup>[[#fn:r1426|1426]]</sup> ; Field et al., 2017 <sup>[[#fn:r1427|1427]]</sup> ). Even stabilised densities in the future would translate into a substantial increase of risk under a 43cm GMSL rise. Risk will also be exacerbated by the negative effects of ocean warming and acidification, especially on coral reef and mangrove capacity to cope with SLR (Pendleton et al., 2016 <sup>[[#fn:r1428|1428]]</sup> ; Van Hooidonk et al., 2016 <sup>[[#fn:r1429|1429]]</sup> ; Perry and Morgan, 2017 <sup>[[#fn:r1430|1430]]</sup> ; Perry et al., 2018 <sup>[[#fn:r1431|1431]]</sup> ) (Sections 4.3.3.5, 5.3). In addition, even small values of SLR will significantly increase risk to atoll islands’ aquifers (Bailey et al., 2016; Storlazzi et al., 2018). Finally, land scarcity in atoll environments will exacerbate the importance of SLR induced damages (on housing, agriculture and infrastructure especially) and cascading impacts (on livelihoods, for example, as a result of groundwater and soil salinisation).

'''''Large tropical agricultural deltas''''' (SM4.3.8.3, Panel B in Figure 4.3) – River deltas considered in this analysis are the Mekong Delta and the Ganges-Brahmaputra-Meghna Delta. Both deltas are large, low-lying and dominated by agricultural production. The risk assessment to SLR considered the entire delta area (not only the coastal fringe; see SM4.3.6 for explanation). High population densities (1280 people per km 2  and 433 people per km 2  in the Ganges-Brahmaputra-Meghna and Mekong deltas, respectively) (Ericson et al., 2006 <sup>[[#fn:r1433|1433]]</sup> ; Government of the Maldives, 2014 <sup>[[#fn:r1434|1434]]</sup> ) and the removal of natural vegetation buffers contribute to high exposure rates to coastal flooding, erosion, and salinisation. Agricultural production contributes to GDP strongly (Smajgl et al., 2015 <sup>[[#fn:r1435|1435]]</sup> ; Hossain et al., 2018 <sup>[[#fn:r1436|1436]]</sup> ), making agricultural fields important assets. In both deltas, mangroves are partially degraded (Ghosh et al., 2018 <sup>[[#fn:r1437|1437]]</sup> ; Veettil et al., 2018 <sup>[[#fn:r1438|1438]]</sup> ) as well as other wetlands at the coast and further inland (Quan et al., 2018a <sup>[[#fn:r1439|1439]]</sup> ; Rahman et al., 2018 <sup>[[#fn:r1440|1440]]</sup> ). Currently, riverine flooding dominates in both deltas (Auerbach et al., 2015 <sup>[[#fn:r1441|1441]]</sup> ; Rahman and Rahman, 2015 <sup>[[#fn:r1442|1442]]</sup> ; Ngan et al., 2018 <sup>[[#fn:r1443|1443]]</sup> ). However, high tides and cyclones can generate large coastal flooding events, especially in the Ganges-Brahmaputra-Meghna Delta (Auerbach et al., 2015 <sup>[[#fn:r1444|1444]]</sup> ; Rahman and Rahman, 2015 <sup>[[#fn:r1445|1445]]</sup> ). Human-induced subsidence increases the likelihood of flooding in both deltas (Brown et al., 2018b <sup>[[#fn:r1446|1446]]</sup> ). Coastal and river bank erosion is already a problem in both delta (Anthony et al., 2015 <sup>[[#fn:r1447|1447]]</sup> ; Brown and Nicholls, 2015 <sup>[[#fn:r1448|1448]]</sup> ; Li et al., 2017 <sup>[[#fn:r1449|1449]]</sup> ) as well as salinity intrusion, which is impacting coastal aquifers, soils and surface waters (Anthony et al., 2015 <sup>[[#fn:r1450|1450]]</sup> ; Brown and Nicholls, 2015 <sup>[[#fn:r1451|1451]]</sup> ; Li et al., 2017 <sup>[[#fn:r1452|1452]]</sup> ). Salinisation of water and soil resources remains a coastal phenomenon (Smajgl et al., 2015 <sup>[[#fn:r1453|1453]]</sup> ), but salinity intrusion can reach far inland in some extreme years and significantly contribute to risk at the delta scale (Section 4.3.3.4.2). Both deltas are partly protected with hard engineered defences such as dikes and sluice gates to prevent riverine flooding, and polders and dikes in some coastal stretches to prevent salinity intrusion and storm surges (Smajgl et al., 2015 <sup>[[#fn:r1454|1454]]</sup> ; Rogers and Overeem, 2017 <sup>[[#fn:r1455|1455]]</sup> ; Warner et al., 2018a <sup>[[#fn:r1456|1456]]</sup> ). Today, in both deltas, the measures implemented to restore natural buffers are still limited to mangroves ecosystems (Quan et al., 2018a <sup>[[#fn:r1457|1457]]</sup> ; Rahman et al., 2018 <sup>[[#fn:r1458|1458]]</sup> ), and the measures aiming at reducing subsidence are underdeveloped (Schmidt, 2015 <sup>[[#fn:r1459|1459]]</sup> ; Schmitt et al., 2017 <sup>[[#fn:r1460|1460]]</sup> ). Assuming stable population densities in the future, coastal flooding will contribute increasingly to risk at the delta level (Brown and Nicholls, 2015 <sup>[[#fn:r1461|1461]]</sup> ; Brown et al., 2018a <sup>[[#fn:r1462|1462]]</sup> ; Dang et al., 2018 <sup>[[#fn:r1463|1463]]</sup> ). Coastal erosion will increase (Anthony et al., 2015 <sup>[[#fn:r1464|1464]]</sup> ; Liu et al., 2017a <sup>[[#fn:r1465|1465]]</sup> ; Uddin et al., 2019 <sup>[[#fn:r1466|1466]]</sup> ) and salinisation of coastal waters and soils will be more significant (Tran Anh et al., 2018 <sup>[[#fn:r1467|1467]]</sup> ; Vu et al., 2018 <sup>[[#fn:r1468|1468]]</sup> ; Rakib et al., 2019 <sup>[[#fn:r1469|1469]]</sup> ) and will strongly impact agriculture and water supply for the entire delta (Jiang et al., 2018 <sup>[[#fn:r1470|1470]]</sup> ; Timsina et al., 2018 <sup>[[#fn:r1471|1471]]</sup> ; Nhung et al., 2019 <sup>[[#fn:r1472|1472]]</sup> ). Without increased adaptation, coastal ecosystems will be largely destroyed at 110 cm of SLR (Schmitt et al., 2017 <sup>[[#fn:r1473|1473]]</sup> ; Mehvar et al., 2019 <sup>[[#fn:r1474|1474]]</sup> ; Mukul et al., 2019 <sup>[[#fn:r1475|1475]]</sup> ). Given the size of these deltas, it is only under high emission scenarios, that flooding, erosion and salinisation lead to high risk at the entire delta scale.

'''''Arctic communities''''' (SM4.3.8.4, Panel B in Figure 4.3) – Five small indigenous settlements located on the Arctic Coastal Plain are considered in this analysis: Bykovsky (Lena Delta, Russian Federation), Shishmaref and Kivalina (Alaska, USA), and Shingle Point and Tuktoyaktuk (Mackenzie Delta, Canada). They lie on exposed coasts composed of unlithified ice-rich sediments in permafrost, in areas with seasonal sea ice and slow to moderate SLR. These communities have populations ranging from 380 to 900 (fewer and seasonal at Shingle Point) that are heavily dependent on marine subsistence resources (Forbes, 2011 <sup>[[#fn:r1476|1476]]</sup> ; Ford et al., 2016a <sup>[[#fn:r1477|1477]]</sup> ). Shishmaref and Kivalina are located on low-lying barrier islands highly susceptible to rising sea level (Marino, 2012 <sup>[[#fn:r1478|1478]]</sup> ; Bronen and Chapin, 2013 <sup>[[#fn:r1479|1479]]</sup> ; Fang et al., 2018 <sup>[[#fn:r1480|1480]]</sup> ; Rolph et al., 2018 <sup>[[#fn:r1481|1481]]</sup> ). Shingle Point is situated on an active gravel spit; Tuktoyaktuk is built on low ground with high concentrations of massive ice; and Bykovsky is mostly situated on an ice-rich eroding terrace about 20 m above sea level. All the selected communities are remote from regions of rapid positive GIA; many other areas in the Arctic experience rapid GIA uplift (James et al., 2015 <sup>[[#fn:r1482|1482]]</sup> ; Forbes et al., 2018 <sup>[[#fn:r1483|1483]]</sup> ) and have very low sensitivity to SLR, which may in fact help to reduce shoaling.

Especially in the Arctic, anthropogenic drivers in recent decades resulted in the induced settlement of indigenous peoples in marginalised climate-sensitive communities (Ford et al., 2016b) and the construction of infrastructure in nearshore areas, with the assumption of stable coastlines. This resulted in increased exposure to coastal hazards. Coastal erosion is already a major problem in all of the case studies, where space for building is usually limited. Accelerating permafrost thaw is promoting rapid erosion of ice-rich sediments, e.g., at Bykovsky (Myers, 2005 <sup>[[#fn:r1485|1485]]</sup> ; Lantuit et al., 2011 <sup>[[#fn:r1486|1486]]</sup> ; Vanderlinden et al., 2018 <sup>[[#fn:r1487|1487]]</sup> ) and Tuktoyaktuk (Lamoureux et al., 2015 <sup>[[#fn:r1488|1488]]</sup> ; Ford et al., 2016a <sup>[[#fn:r1489|1489]]</sup> ). Related to this, Kivalina, Shishmaref, Shingle Point, Tuktoyaktuk, and parts of the Lena delta (less so for Bykovsky) are already facing high risk of flooding. Shishmaref, for example, experienced 10 flooding events between 1973 and 2015 that resulted in emergency declarations (Bronen and Chapin, 2013 <sup>[[#fn:r1490|1490]]</sup> ; Lamoureux et al., 2015 <sup>[[#fn:r1491|1491]]</sup> ; Irrgang et al., 2019 <sup>[[#fn:r1492|1492]]</sup> ). There is however no evidence of salinisation in the selected communities, but brackish water flooding of the outer Mackenzie Delta caused by a 1999 storm surge (a rare event due to upwelling ahead of the storm) led to widespread die-off of vegetation with negative ecosystem impacts (Pisaric et al., 2011 <sup>[[#fn:r1493|1493]]</sup> ; Kokelj et al., 2012 <sup>[[#fn:r1494|1494]]</sup> ).

Permafrost thaw is already accelerating due to increasing ground temperatures that weaken the mechanical stability of frozen ground (Section 3.4.2.2). Arctic SLR and sea surface warming have the potential to substantially contribute to this thawing (Forbes, 2011 <sup>[[#fn:r1495|1495]]</sup> ; Barnhart et al., 2014b <sup>[[#fn:r1496|1496]]</sup> ; Lamoureux et al., 2015 <sup>[[#fn:r1497|1497]]</sup> ; Fritz et al., 2017 <sup>[[#fn:r1498|1498]]</sup> ). An additional factor unique to the polar regions is the decrease in seasonal sea ice extent in the Arctic (Sections 3.2.1 and 3.2.2), which together with a lengthening open water season, provides less protection from storm impacts, particularly later in the year when storms are prevalent (Forbes, 2011 <sup>[[#fn:r1499|1499]]</sup> ; Lantuit et al., 2011 <sup>[[#fn:r1500|1500]]</sup> ; Barnhart et al., 2014a <sup>[[#fn:r1501|1501]]</sup> ; Melvin et al., 2017 <sup>[[#fn:r1502|1502]]</sup> ; Fang et al., 2018 <sup>[[#fn:r1503|1503]]</sup> ; Forbes, 2019 <sup>[[#fn:r1504|1504]]</sup> ) and therefore reduces the physical protection of the land (Section 6.3.1.3).

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B in Figure 4.3), and that risk is expected to increase over this century in virtually all low-lying coastal areas whatever their context-specificities or nature (island/continental, developed/developing county) (Cross-Chapter Box 9). In the absence of high adaptation (bars (A)), risk is expected to significantly increase in urban atoll islands and the selected Arctic coastal communities even in a SROCC RCP2.6 scenario, and all geographies are expected to experience almost high to very high risks at the upper ''likely'' range of SROCC RCP8.5. These results allow refining AR5 conclusions by showing, first, that high risk can indeed occur before the 1m rise benchmark (Oppenheimer et al., 2014; O’Neill et al., 2017) and, second, that risk as a function of SLR is highly variable from one geography to another. Some rationale is provided below for our assessment of illustrative geographies, summarising the more detailed description provided in SM4.3 (SM4.3.6 to SM4.3.8). Note however that the text below is not intended to be fully comprehensive and does not necessarily include all elements for which there is a substantive body of literature, nor does it necessarily include all elements which are of particular interest to decision makers.

'''''Resource-rich coastal cities''''' (SM4.3.8.1, Panel B in Figure 4.3) – Resource-rich coastal cities considered in this analysis are Shanghai, New York (see Box 4.1 for further details and references on Shanghai and NYC), and Rotterdam (Brinke et al., 2010 <sup>[[#fn:r1400|1400]]</sup> ; Hinkel et al., 2018 <sup>[[#fn:r1401|1401]]</sup> ). High, and in many cases, growing population density and total population, and high exposure of people and infrastructure to GMSL rise and ESL events characterise coastal megacities (Hanson et al., 2011 <sup>[[#fn:r1402|1402]]</sup> ). These are high concentrations of income and wealth in geographic terms but within relatively small area exhibit large distributional differences of both with important implications for emergency response and adaptation. Concentration translates into high exposure of monetary value to coastal hazards and the cities noted here have both historical and recent experience with damaging ESL events, such as Typhoon Winnie which struck Shanghai in 1997 (Xian et al., 2018 <sup>[[#fn:r1403|1403]]</sup> ), Hurricane Sandy in New York in 2012 (Rosenzweig and Solecki, 2014 <sup>[[#fn:r1404|1404]]</sup> ), and the North Sea storm of 1953 which impacted the Rotterdam area (Gerritsen, 2005 <sup>[[#fn:r1405|1405]]</sup> ; Jonkman et al., 2008 <sup>[[#fn:r1406|1406]]</sup> ). However, high density, limited space and high cost of land leads to development of below-ground space for transportation (e.g., subways, road tunnels; MTA, 2017) and storage, and even habitation, creating vulnerabilities not seen in low-density areas. Natural ecosystems within the megacity boundaries and nearby have been exploited for centuries and in some cases decimated or even extirpated (Hartig et al., 2002 <sup>[[#fn:r1407|1407]]</sup> ). Accordingly, they provide limited benefits in terms of coastal protection for the densest part of these cities but can be critically important for protection of lower-density areas, for example, wetlands and sandy beaches in the Jamaica Bay/Rockaway sector of New York that protect nearby residential communities (Hartig et al., 2002 <sup>[[#fn:r1408|1408]]</sup> ). Space limitations also constrain the potential benefits of EbA measures. Instead, resource-rich coastal cities depend largely on hard defences like sea walls and surge barriers for coastal protection (Section 4.4.2.2). Such defences are costly but generally cost effective due to the aforementioned concentration of population and value. However, barriers to planning and implementing adaptation include governance challenges (Section 4.4.2) such as limited control over finances and the intermittent nature of ESLs which inhibit focused attention over the long time scales needed to plan and implement hard defences (Section 4.4.2.2). As a result, coastal adaptation for resource-rich cities is uneven and the three presented here were selected with a view toward exhibiting a range of current and potential future effectiveness.

'''''Urban atoll islands''''' (SM4.3.8.2, Panel B in Figure 4.3) – The capital islands (or groups of islands) of three atoll nations in the Pacific and Indian Oceans are considered here: Fongafale (Funafuti Atoll, Tuvalu), the South Tarawa Urban District (Tarawa Atoll, Kiribati) and Male’ (North Kaafu Atoll, Maldives). Urban atoll islands have low elevation (&lt;4 m above mean sea level; in South Tarawa, e.g., lagoon sides where settlement concentrates are &lt;1.80 m in elevation) (Duvat, 2013 <sup>[[#fn:r1409|1409]]</sup> ) and are mainly composed of reef-derived unconsolidated material. Their future is of nation-wide importance as they concentrate populations, economic activities and critical infrastructure (airports, main harbours).They illustrate the prominence of anthropogenic-driven disturbances to marine and terrestrial ecosystems (e.g., mangrove clearing in South Tarawa or human-induced coral reef degradation through land reclamation in Male’; Duvat et al., 2013 <sup>[[#fn:r1410|1410]]</sup> ; Naylor, 2015 <sup>[[#fn:r1411|1411]]</sup> ) and therefore to services such as coastal protection delivered by the coral reef (i.e., wave energy attenuation that reduces flooding and erosion, and sediment provision that contributes to island persistence over time) (McLean and Kench, 2015 <sup>[[#fn:r1412|1412]]</sup> ; Quataert et al., 2015 <sup>[[#fn:r1413|1413]]</sup> ; Elliff and Silva, 2017 <sup>[[#fn:r1414|1414]]</sup> ; Storlazzi et al., 2018 <sup>[[#fn:r1415|1415]]</sup> ).

The controlling factors of urban atoll islands’ future habitability are the density of assets exposed to marine flooding and coastal erosion (SM4.3.8.2), future trends in these hazards, and ecosystem response to both ocean-climate related pressures and human activities. Urban atoll islands already experience coastal flooding, for example, in Male’ (Wadey et al., 2017 <sup>[[#fn:r1416|1416]]</sup> ) and Funafuti (Yamano et al., 2007 <sup>[[#fn:r1417|1417]]</sup> ; McCubbin et al., 2015 <sup>[[#fn:r1418|1418]]</sup> ). Coastal erosion is also a major concern along non-armoured shoreline in South Tarawa (Duvat et al., 2013 <sup>[[#fn:r1419|1419]]</sup> ) and Fongafale (Onaka et al., 2017 <sup>[[#fn:r1420|1420]]</sup> ), but not in Male’ where surrounding fortifications have extended along almost the entire shoreline from several decades (Naylor, 2015 <sup>[[#fn:r1421|1421]]</sup> ). Salinisation already affects groundwater lenses, but its contribution to risk varies from one case to another, from low in Male’ (relying on desalinised seawater) to important for human consumption and agriculture in South Tarawa (Bailey et al., 2014 <sup>[[#fn:r1422|1422]]</sup> ; Post et al., 2018 <sup>[[#fn:r1423|1423]]</sup> ).

Together, high population densities (from ~3,200 people per km <sup>2</sup> in South Tarawa to ~65,700 people per km <sup>2</sup> in Male’) (Government of the Maldives, 2014 <sup>[[#fn:r1424|1424]]</sup> ; McIver et al., 2015 <sup>[[#fn:r1425|1425]]</sup> ) and the concentration of critical infrastructure and settlements in naturally low-lying flood-prone areas already substantially contribute to coastal risk (Duvat et al., 2013 <sup>[[#fn:r1426|1426]]</sup> ; Field et al., 2017 <sup>[[#fn:r1427|1427]]</sup> ). Even stabilised densities in the future would translate into a substantial increase of risk under a 43cm GMSL rise. Risk will also be exacerbated by the negative effects of ocean warming and acidification, especially on coral reef and mangrove capacity to cope with SLR (Pendleton et al., 2016 <sup>[[#fn:r1428|1428]]</sup> ; Van Hooidonk et al., 2016 <sup>[[#fn:r1429|1429]]</sup> ; Perry and Morgan, 2017 <sup>[[#fn:r1430|1430]]</sup> ; Perry et al., 2018 <sup>[[#fn:r1431|1431]]</sup> ) (Sections 4.3.3.5, 5.3). In addition, even small values of SLR will significantly increase risk to atoll islands’ aquifers (Bailey et al., 2016 <sup>[[#fn:r1431|1431]]</sup> ; Storlazzi et al., 2018 <sup>[[#fn:r1432|1432]]</sup> ). Finally, land scarcity in atoll environments will exacerbate the importance of SLR induced damages (on housing, agriculture and infrastructure especially) and cascading impacts (on livelihoods, for example, as a result of groundwater and soil salinisation).

'''''Large tropical agricultural deltas''''' (SM4.3.8.3, Panel B in Figure 4.3) – River deltas considered in this analysis are the Mekong Delta and the Ganges-Brahmaputra-Meghna Delta. Both deltas are large, low-lying and dominated by agricultural production. The risk assessment to SLR considered the entire delta area (not only the coastal fringe; see SM4.3.6 for explanation). High population densities (1280 people per km <sup>2</sup>  and 433 people per km <sup>2</sup>  in the Ganges-Brahmaputra-Meghna and Mekong deltas, respectively) (Ericson et al., 2006 <sup>[[#fn:r1433|1433]]</sup> ; Government of the Maldives, 2014 <sup>[[#fn:r1434|1434]]</sup> ) and the removal of natural vegetation buffers contribute to high exposure rates to coastal flooding, erosion, and salinisation. Agricultural production contributes to GDP strongly (Smajgl et al., 2015 <sup>[[#fn:r1435|1435]]</sup> ; Hossain et al., 2018 <sup>[[#fn:r1436|1436]]</sup> ), making agricultural fields important assets. In both deltas, mangroves are partially degraded (Ghosh et al., 2018 <sup>[[#fn:r1437|1437]]</sup> ; Veettil et al., 2018 <sup>[[#fn:r1438|1438]]</sup> ) as well as other wetlands at the coast and further inland (Quan et al., 2018a <sup>[[#fn:r1439|1439]]</sup> ; Rahman et al., 2018 <sup>[[#fn:r1440|1440]]</sup> ). Currently, riverine flooding dominates in both deltas (Auerbach et al., 2015 <sup>[[#fn:r1411|1411]]</sup> ; Rahman and Rahman, 2015 <sup>[[#fn:r1442|1442]]</sup> ; Ngan et al., 2018 <sup>[[#fn:r1443|1443]]</sup> ). However, high tides and cyclones can generate large coastal flooding events, especially in the Ganges-Brahmaputra-Meghna Delta (Auerbach et al., 2015 <sup>[[#fn:r1444|1444]]</sup> ; Rahman and Rahman, 2015 <sup>[[#fn:r1445|1445]]</sup> ). Human-induced subsidence increases the likelihood of flooding in both deltas (Brown et al., 2018b <sup>[[#fn:r1446|1446]]</sup> ). Coastal and river bank erosion is already a problem in both delta (Anthony et al., 2015 <sup>[[#fn:r1447|1447]]</sup> ; Brown and Nicholls, 2015 <sup>[[#fn:r1448|1448]]</sup> ; Li et al., 2017 <sup>[[#fn:r1449|1449]]</sup> ) as well as salinity intrusion, which is impacting coastal aquifers, soils and surface waters (Anthony et al., 2015 <sup>[[#fn:r1450|1450]]</sup> ; Brown and Nicholls, 2015 <sup>[[#fn:r1451|1451]]</sup> ; Li et al., 2017 <sup>[[#fn:r1452|1452]]</sup> ). Salinisation of water and soil resources remains a coastal phenomenon (Smajgl et al., 2015 <sup>[[#fn:r1453|1453]]</sup> ), but salinity intrusion can reach far inland in some extreme years and significantly contribute to risk at the delta scale (Section 4.3.3.4.2). Both deltas are partly protected with hard engineered defences such as dikes and sluice gates to prevent riverine flooding, and polders and dikes in some coastal stretches to prevent salinity intrusion and storm surges (Smajgl et al., 2015 <sup>[[#fn:r1454|1454]]</sup> ; Rogers and Overeem, 2017 <sup>[[#fn:r1455|1455]]</sup> ; Warner et al., 2018a <sup>[[#fn:r1456|1456]]</sup> ). Today, in both deltas, the measures implemented to restore natural buffers are still limited to mangroves ecosystems (Quan et al., 2018a <sup>[[#fn:r1457|1457]]</sup> ; Rahman et al., 2018 <sup>[[#fn:r1458|1458]]</sup> ), and the measures aiming at reducing subsidence are underdeveloped (Schmidt, 2015 <sup>[[#fn:r1459|1459]]</sup> ; Schmitt et al., 2017 <sup>[[#fn:r1460|1460]]</sup> ). Assuming stable population densities in the future, coastal flooding will contribute increasingly to risk at the delta level (Brown and Nicholls, 2015 <sup>[[#fn:r1461|1461]]</sup> ; Brown et al., 2018a <sup>[[#fn:r1462|1462]]</sup> ; Dang et al., 2018 <sup>[[#fn:r1463|1463]]</sup> ). Coastal erosion will increase (Anthony et al., 2015 <sup>[[#fn:r1464|1464]]</sup> ; Liu et al., 2017a <sup>[[#fn:r1465|1465]]</sup> ; Uddin et al., 2019 <sup>[[#fn:r1466|1466]]</sup> ) and salinisation of coastal waters and soils will be more significant (Tran Anh et al., 2018 <sup>[[#fn:r1467|1467]]</sup> ; Vu et al., 2018 <sup>[[#fn:r1468|1468]]</sup> ; Rakib et al., 2019 <sup>[[#fn:r1469|1469]]</sup> ) and will strongly impact agriculture and water supply for the entire delta (Jiang et al., 2018 <sup>[[#fn:r1470|1470]]</sup> ; Timsina et al., 2018 <sup>[[#fn:r1471|1471]]</sup> ; Nhung et al., 2019 <sup>[[#fn:r1472|1472]]</sup> ). Without increased adaptation, coastal ecosystems will be largely destroyed at 110 cm of SLR (Schmitt et al., 2017 <sup>[[#fn:r1473|1473]]</sup> ; Mehvar et al., 2019 <sup>[[#fn:r1474|1474]]</sup> ; Mukul et al., 2019 <sup>[[#fn:r1475|1475]]</sup> ).Given the size of these deltas, it is only under high emission scenarios, that flooding, erosion and salinisation lead to high risk at the entire delta scale.

'''''Arctic communities''''' (SM4.3.8.4, Panel B in Figure 4.3) – Five small indigenous settlements located on the Arctic Coastal Plain are considered in this analysis: Bykovsky (Lena Delta, Russian Federation), Shishmaref and Kivalina (Alaska, USA), and Shingle Point and Tuktoyaktuk (Mackenzie Delta, Canada). They lie on exposed coasts composed of unlithified ice-rich sediments in permafrost, in areas with seasonal sea ice and slow to moderate SLR. These communities have populations ranging from 380 to 900 (fewer and seasonal at Shingle Point) that are heavily dependent on marine subsistence resources (Forbes, 2011 <sup>[[#fn:r1476|1476]]</sup> ; Ford et al., 2016a <sup>[[#fn:r1477|1477]]</sup> ). Shishmaref and Kivalina are located on low-lying barrier islands highly susceptible to rising sea level (Marino, 2012 <sup>[[#fn:r1478|1478]]</sup> ; Bronen and Chapin, 2013 <sup>[[#fn:r1479|1479]]</sup> ; Fang et al., 2018 <sup>[[#fn:r1480|1480]]</sup> ; Rolph et al., 2018 <sup>[[#fn:r1481|1481]]</sup> ). Shingle Point is situated on an active gravel spit; Tuktoyaktuk is built on low ground with high concentrations of massive ice; and Bykovsky is mostly situated on an ice-rich eroding terrace about 20 m above sea level. All the selected communities are remote from regions of rapid positive GIA; many other areas in the Arctic experience rapid GIA uplift (James et al., 2015 <sup>[[#fn:r1482|1482]]</sup> ; Forbes et al., 2018 <sup>[[#fn:r1483|1483]]</sup> ) and have very low sensitivity to SLR, which may in fact help to reduce shoaling.

Especially in the Arctic, anthropogenic drivers in recent decades resulted in the induced settlement of indigenous peoples in marginalised climate-sensitive communities (Ford et al., 2016b) and the construction of infrastructure in nearshore areas, with the assumption of stable coastlines. This resulted in increased exposure to coastal hazards. Coastal erosion is already a major problem in all of the case studies, where space for building is usually limited. Accelerating permafrost thaw is promoting rapid erosion of ice-rich sediments, e.g., at Bykovsky (Myers, 2005 <sup>[[#fn:r1485|1485]]</sup> ; Lantuit et al., 2011 <sup>[[#fn:r1486|1486]]</sup> ; Vanderlinden et al., 2018 <sup>[[#fn:r1487|1487]]</sup> ) and Tuktoyaktuk (Lamoureux et al., 2015 <sup>[[#fn:r1488|1488]]</sup> ; Ford et al., 2016a <sup>[[#fn:r1489|1489]]</sup> ). Related to this, Kivalina, Shishmaref, Shingle Point, Tuktoyaktuk, and parts of the Lena delta (less so for Bykovsky) are already facing high risk of flooding. Shishmaref, for example, experienced 10 flooding events between 1973 and 2015 that resulted in emergency declarations (Bronen and Chapin, 2013 <sup>[[#fn:r1490|1490]]</sup> ; Lamoureux et al., 2015 <sup>[[#fn:r1491|1491]]</sup> ; Irrgang et al., 2019 <sup>[[#fn:r1492|1492]]</sup> ). There is however no evidence of salinisation in the selected communities, but brackish water flooding of the outer Mackenzie Delta caused by a 1999 storm surge (a rare event due to upwelling ahead of the storm) led to widespread die-off of vegetation with negative ecosystem impacts (Pisaric et al., 2011 <sup>[[#fn:r1493|1493]]</sup> ; Kokelj et al., 2012 <sup>[[#fn:r1494|1494]]</sup> ).

Permafrost thaw is already accelerating due to increasing ground temperatures that weaken the mechanical stability of frozen ground (Section 3.4.2.2). Arctic SLR and sea surface warming have the potential to substantially contribute to this thawing (Forbes, 2011 <sup>[[#fn:r1495|1495]]</sup> ; Barnhart et al., 2014b <sup>[[#fn:r1496|1496]]</sup> ; Lamoureux et al., 2015 <sup>[[#fn:r1497|1497]]</sup> ; Fritz et al., 2017 <sup>[[#fn:r1498|1498]]</sup> ). An additional factor unique to the polar regions is the decrease in seasonal sea ice extent in the Arctic (Sections 3.2.1 and 3.2.2), which together with a lengthening open water season, provides less protection from storm impacts, particularly later in the year when storms are prevalent (Forbes, 2011 <sup>[[#fn:r1499|1499]]</sup> ; Lantuit et al., 2011 <sup>[[#fn:r1500|1500]]</sup> ; Barnhart et al., 2014a <sup>[[#fn:r1501|1501]]</sup> ; Melvin et al., 2017 <sup>[[#fn:r1502|1502]]</sup> ; Fang et al., 2018 <sup>[[#fn:r1503|1503]]</sup> ; Forbes, 2019 <sup>[[#fn:r1504|1504]]</sup> ) and therefore reduces the physical protection of the land (Section 6.3.1.3).

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<span id="adaptation-benefits"></span>
===== 4.3.4.2.2 Adaptation benefits =====

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The assessment also shows that benefits in terms of risk reduction over this century are to be expected from ambitious adaptation efforts (bars (B), Sections 4.4.2, 4.4.3 and 4.4.3). In the case of resource-rich coastal cities especially, adequately engineered coastal defences can play a decisive role in reducing risk (Section 4.4.2.2, Box 4.1), for example from high to moderate at the SROCC RCP8.5 upper ''likely'' range. In other contexts, such as atoll islands for example, while engineered protection structures will reduce risk of flooding, they will not necessarily prevent seawater infiltration due to the permeable nature of the island substratum. So even adequate coastal protection would not eliminate risk (SM4.3.8.3). In urban atoll islands, large tropical agricultural deltas and the selected Arctic communities, ambitious adaptation efforts mixing adequate coastal defences, the restoration and creation of buffering ecosystems (e.g., coral reefs), and a moderate amount of relocation are expected to reduce risk. For resource-rich coastal cities , adequately engineered hard protection can virtually eliminate risk of flooding up to 84 cm except for residual risk of structural failure (Sections 4.4.2 to 4.4.5). Benefits are relatively important in a 84 cm SLR scenario, as they reduce risk from high-to-very-high to moderate-to-high (atolls, Arctic) and from moderate-to-high to moderate (deltas). These benefits become more modest when approaching the upper ''likely'' range of SROCC RCP8.5, and risk tends to return to high-to-very-high (atolls, Arctic) levels once the 110 cm rise in sea level is reached. Noteworthy in urban atoll islands, intensified proactive coastal relocation (e.g., relocation of buildings and infrastructures that are very close to the shoreline) is expected to play a substantial role in risk reduction under all SLR scenarios. Proactive relocation can indeed compensate for the increasing extent of coastal flooding and associated damages (SM4.3.8.3). When taken to the extreme, relocation could lead to the elimination of risk in situ, for example in the case of the relocation of the full population of urban atoll islands either elsewhere in the country (e.g., on another island) or abroad (i.e., international migration). This is an extreme situation where it is hard to distinguish whether the measure is an impact of SLR (and ocean change more broadly), for example, displacement, or an adaptation solution. In addition, relocation of people displaces pressure to destination areas, with a potential increase of risk for the latter. In other words, the broader ‘coastal retreat’ category (Section 4.4.2.6) raises the issue of the ‘limits to adaptation’, which is not represented in Figure 4.3.

These conclusions must be nuanced, first, by the fact that our assessment does not consider either financial or social aspects that can act as limiting factors to the development of adaptation options (Sections 4.4.3 and 4.4.5), for instance, hard engineering coastal defences (Hurlimann et al., 2014 <sup>[[#fn:r1505|1505]]</sup> ; Jones et al., 2014 <sup>[[#fn:r1506|1506]]</sup> ; Elrick-Barr et al., 2017 <sup>[[#fn:r1507|1507]]</sup> ; Hinkel et al., 2018 <sup>[[#fn:r1508|1508]]</sup> ) . However, from a general perspective, these findings suggest that although ambitious adaptation will not necessarily eradicate end-century risk from SLR across all low-lying coastal areas around the world, it will help to buy time in many locations and therefore contribute to developing a robust foundation for adaptation beyond 2100. Second, the future of other climate-related drivers of risk (such as ESL, waves and cyclones; Sections 4.2.3.4.1 to 4.2.3.4.3, 6.3.1.1 to 6.3.1.3) is not fully and systematically included in each risk assessment above, so that much larger risks than assessed here are to be expected.

<span id="responding-to-sea-level-rise"></span>