Sea Level Rise and Implications for Low-Lying Islands, SPM4 Coasts and Communities Coordinating Lead Authors: Michael Oppenheimer (USA), Bruce C. Glavovic (New Zealand/South Africa) Lead Authors: Jochen Hinkel (Germany), Roderik van de Wal (Netherlands), Alexandre K. Magnan (France), Amro Abd-Elgawad (Egypt), Rongshuo Cai (China), Miguel Cifuentes-Jara (Costa Rica), Robert M. DeConto (USA), Tuhin Ghosh (India), John Hay (Cook Islands), Federico Isla (Argentina), Ben Marzeion (Germany), Benoit Meyssignac (France), Zita Sebesvari (Hungary/Germany) Contributing Authors: Robbert Biesbroek (Netherlands), Maya K. Buchanan (USA), Ricardo Safra de Campos (UK), Gonéri Le Cozannet (France), Catia Domingues (Australia), Sönke Dangendorf (Germany), Petra Döll (Germany), Virginie K.E. Duvat (France), Tamsin Edwards (UK), Alexey Ekaykin (Russian Federation), Donald Forbes (Canada), James Ford (UK), Miguel D. Fortes (Philippines), Thomas Frederikse (Netherlands), Jean-Pierre Gattuso (France), Robert Kopp (USA), Erwin Lambert (Netherlands), Judy Lawrence (New Zealand), Andrew Mackintosh (New Zealand), Angélique Melet (France), Elizabeth McLeod (USA), Mark Merrifield (USA), Siddharth Narayan (US), Robert J. Nicholls (UK), Fabrice Renaud (UK), Jonathan Simm (UK), AJ Smit (South Africa), Catherine Sutherland (South Africa), Nguyen Minh Tu (Vietnam), Jon Woodruff (USA), Poh Poh Wong (Singapore), Siyuan Xian (USA) Review Editors: Ayako Abe-Ouchi (Japan), Kapil Gupta (India), Joy Pereira (Malaysia) Chapter Scientist: Maya K. Buchanan (USA) This chapter should be cited as: Oppenheimer, M., B.C. Glavovic , J. Hinkel, R. van de Wal, A.K. Magnan, A. Abd-Elgawad, R. Cai, M. Cifuentes-Jara, R.M. DeConto, T. Ghosh, J. Hay, F. Isla, B. Marzeion, B. Meyssignac, and Z. Sebesvari, 2019: Sea Level Rise and Implications for Low-Lying Islands, Coasts and Communities. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. In press. 321 Chapter 4 Sea Level Rise and Implications for Low-Lying Islands, Coasts and Communities Table of contents Executive Summary ................................................................................... 323 4.4 Responding to Sea Level Rise ................................................ 385 4.4.1 Introduction ................................................................................. 385 4.1 Synthesis ................................................................................................. 326 Box 4.3: Responses to Sea Level Rise .................................. 385 4.1.1 Purpose, Scope, and Structure 4.4.2 Observed and Projected Responses, of this Chapter ........................................................................... 326 their Costs, Benefits, Co-benefits, 4.1.2 Future Sea level Rise and Implications Drawbacks, Efficiency and Governance ................. 386 for Responses ......................................................................... 326 4.4.3 Governance Challenges in Responding 4.1.3 Sea Level Rise Impacts and Implications to Sea Level Rise ................................................................. 398 for Responses ......................................................................... 328 4.4.4 Planning, Engagement and Decision Tools 4.1.4 Response Options, Governance Challenges for Choosing Responses .................................................. 400 and Ways Forward ............................................................... 329 Box 4.4: Community Based Experiences: Canadian Arctic and Hawkes Bay, New Zealand ........ 406 4.2 Physical Basis for Sea Level Change 4.4.5 Enabling Conditions and Lessons and Associated Hazards ............................................................... 330 Learned From ‘Practice’ .................................................... 407 4.2.1 Processes of Sea Level Change ................................... 330 4.4.6 Towards Climate Resilient 4.2.2 Observed Changes in Sea Level Development Pathways .................................................... 410 (Past and Present) ................................................................ 332 4.2.3 Projections of Sea Level Change ................................ 344 References .............................................................................................................. 412 Box 4.1: Case Studies of Coastal Hazard and Response ........................................................................................... 363 Frequently Asked Questions .................................................................... 411 FAQ 4.1 What challenges does the 4.3 Exposure, Vulnerability, Impacts inevitability of sea level rise present and Risk Related to Sea Level Rise .................................... 367 4 to coastal communities and how 4.3.1 Introduction ............................................................................... 367 can communities adapt? ....................................... 411 4.3.2 Dimensions of Exposure and Vulnerability to Sea Level Rise ................................................................... 367 Box 4.2: Methodological Advances in Exposure and Vulnerability Assessments .................. 368 4.3.3 Observed Impacts, and Current and Future Risk of Sea Level Rise ............................. 375 4.3.4 Conclusion on Coastal Risk: Reasons for Concern and Future Risks .................. 381 322 Sea Level Rise and Implications for Low-Lying Islands, Coasts and Communities Chapter 4 Executive Summary low-lying coastal areas more broadly, human-induced changes can be rapid and modify coastlines over short periods of time, outpacing This chapter assesses past and future contributions to global, regional the effects of SLR (high confidence). Adaptation can be undertaken in and extreme sea level changes, associated risk to low-lying islands, the short- to medium-term by targeting local drivers of exposure and coasts, cities, and settlements, and response options and pathways to vulnerability, notwithstanding uncertainty about local SLR impacts resilience and sustainable development along the coast. in coming decades and beyond (high confidence). {4.2.2.4, 4.3.1, 4.3.2.2, 4.3.2.3} Observations Coastal ecosystems are already impacted by the combination of SLR, other climate-related ocean changes, and adverse Global mean sea level (GMSL) is rising (virtually certain1) effects from human activities on ocean and land (high and accelerating (high confidence2). The sum of glacier and confidence). Attributing such impacts to SLR, however, remains ice sheet contributions is now the dominant source of GMSL challenging due to the influence of other climate-related and rise (very high confidence). GMSL from tide gauges and altimetry non-climatic drivers such as infrastructure development and observations increased from 1.4 mm yr–1 over the period 1901–1990 human-induced habitat degradation (high confidence). Coastal to 2.1 mm yr–1 over the period 1970–2015 to 3.2 mm yr–1 over ecosystems, including saltmarshes, mangroves, vegetated dunes and the period 1993–2015 to 3.6 mm yr–1 over the period 2006–2015 sandy beaches, can build vertically and expand laterally in response (high confidence). The dominant cause of GMSL rise since 1970 is to SLR, though this capacity varies across sites (high confidence). anthropogenic forcing (high confidence). {4.2.2.1.1, 4.2.2.2} These ecosystems provide important services that include coastal protection and habitat for diverse biota. However, as a consequence GMSL was considerably higher than today during past climate of human actions that fragment wetland habitats and restrict states that were warmer than pre-industrial, including the landward migration, coastal ecosystems progressively lose their Last Interglacial (LIG; 129–116 ka), when global mean surface ability to adapt to climate-induced changes and provide ecosystem temperature was 0.5ºC–1.0ºC warmer, and the mid-Pliocene services, including acting as protective barriers (high confidence). Warm Period (mPWP; ~3.3 to 3.0 million years ago), 2ºC–4ºC {4.3.2.3} warmer. Despite the modest global warmth of the Last Interglacial, GMSL was likely 6–9 m higher, mainly due to Coastal risk is dynamic and increased by widely observed contributions from the Greenland and Antarctic ice sheets changes in coastal infrastructure, community livelihoods, (GIS and AIS, respectively), and unlikely more than 10m higher agriculture and habitability (high confidence). As with coastal (medium confidence). Based on new understanding about ecosystems, attribution of observed changes and associated geological constraints since the IPCC 5th Assessment Report risk to SLR remains challenging. Drivers and processes inhibiting (AR5), 25 m is a plausible upper bound on GMSL during the attribution include demographic, resource and land use changes and 4 mPWP (low confidence). Ongoing uncertainties in palaeo sea level anthropogenic subsidence. {4.3.3, 4.3.4} reconstructions and modelling hamper conclusions regarding the total magnitudes and rates of past sea level rise (SLR). Furthermore, A diversity of adaptation responses to coastal impacts the long (multi-millennial) time scales of these past climate and and risks have been implemented around the world, but sea level changes, and regional climate influences from changes in mostly as a reaction to current coastal risk or experienced Earth’s orbital