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

===== 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).

<div id="section-4-3-4-2-key-findings-on-future-risks-and-adaptation-benefits-block-2"></div>

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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