SECOND ORDER DRAFT Chapter 4 IPCC SR Ocean and Cryosphere 1 2 Chapter 4: Sea Level Rise and Implications for Low Lying Islands, Coasts and Communities 3 4 Coordinating Lead Authors: Michael Oppenheimer (USA), Bruce Glavovic (New Zealand) 5 6 Lead Authors: Amro Abd-Elgawad (Egypt), Rongshuo Cai (China), Miguel Cifuentes-Jara (Costa Rica), 7 Rob Deconto (USA), Tuhin Ghosh (India), John Hay (Cook Islands), Jochen Hinkel (Germany), Federico 8 Isla (Argentina), Alexandre K. Magnan (France), Ben Marzeion (Germany), Benoit Meyssignac (France), 9 Zita Sebesvari (Hungary), AJ Smit (South Africa), Roderik van de Wal (Netherlands) 10 11 Contributing Authors: Maya Buchanan (USA), Gonéri Le Cozannet (France), Catia Domingues 12 (Australia), Petra Döll (Germany), Virginie K.E. Duvat (France), Tamsin Edwards (UK), Alexey Ekaykin 13 (Russian Federation), Miguel D. Fortes (Philippines), Thomas Frederikse (Netherlands), Jean-Pierre Gattuso 14 (France), Robert Kopp (USA), Erwin Lambert (Netherlands), Andrew Mackintosh (New Zealand), 15 Angélique Melet (France), Elizabeth McLeod (USA), Mark Merrifield (USA), Siddharth Narayan (US), 16 Robert J. Nicholls (UK), Fabrice Renaud (UK), Jonathan Simm (UK), Jon Woodruff (USA), Poh Poh Wong 17 (Singapore), Siyuan Xian (USA) 18 19 Review Editors: Ayako Abe-Ouchi (Japan), Kapil Gupta (India), Joy Pereira (Malaysia) 20 21 Chapter Scientist: Maya Buchanan (USA) 22 23 Date of Draft: 16 November 2018 24 25 Notes: TSU Compiled Version 26 27 28 Table of Contents 29 30 Executive Summary ......................................................................................................................................... 2 31 4.1 Purpose, Scope, and Structure of the Chapter ...................................................................................... 6 32 4.1.1 Themes of this Chapter ................................................................................................................... 6 33 4.1.2 Advances in this Chapter Beyond AR5 and SR1.5 ......................................................................... 7 34 4.2 Physical Basis for Sea Level Change and Associated Hazards .......................................................... 10 35 4.2.1 Processes of Sea Level Change .................................................................................................... 10 36 4.2.2 Observed Changes in Sea Level (Past and Present) .................................................................... 13 37 4.2.3 Projections of Sea Level Change ................................................................................................. 31 38 4.2.4 Synthesis of the Physics of Sea Level for Low-lying Islands and Coasts ..................................... 57 39 Box 4.1: Case Studies of Coastal Hazard and Response ............................................................................ 59 40 4.3 Exposure, Vulnerability, Impacts and Risk Related to Sea Level Rise ............................................ 66 41 4.3.1 Introduction .................................................................................................................................. 66 42 Box 4.2: Methodological Advances in Exposure and Vulnerability Assessments .................................... 66 43 4.3.2 Dimensions Exposure and Vulnerability to Sea Level Rise ......................................................... 69 44 4.3.3 Observed Impacts, and Current and Future Risk of Sea Level Rise ............................................ 77 45 4.3.4 Conclusion on Coastal Risk ............................................................................................................ 86 46 4.4 Responses to Sea Level Rise .................................................................................................................. 89 47 4.4.1 Introduction .................................................................................................................................. 89 48 4.4.2 Types of Responses ....................................................................................................................... 90 49 4.4.3 Observed and Projected Responses, their Costs, Benefits, Co-benefits, Negative Consequences, 50 Efficiency and Governance .......................................................................................................... 92 51 4.4.4 Approaches for Making Social Choices and Appraising and Institutionalizing Adaptation 52 Pathways .................................................................................................................................... 112 53 4.4.5 Implementing Sea Level Rise Responses at the Local Level: Enablers, Barriers and Lessons 54 Learned ....................................................................................................................................... 122 55 FAQ 4.1: What challenges does certain sea level rise present to coastal communities? ....................... 130 56 References ..................................................................................................................................................... 132 57 Do Not Cite, Quote or Distribute 4-1 Total pages: 174 SECOND ORDER DRAFT Chapter 4 IPCC SR Ocean and Cryosphere 1 Executive Summary 2 3 Sea Level Change 4 5 1) Observations show that global mean sea level (GMSL) is rising and accelerating and it will continue 6 to rise over many centuries. Glaciers and polar ice sheets are now the dominant source of sea level rise 1 7 (very high confidence ). During the Last Interglacial period (LIG) 130 to 115 thousand years ago, these 8 polar ice sheets contributed 6-9 m to sea level above present-day at a time when Earth was 0.5°C–1°C 9 warmer than preindustrial time (high confidence), providing additional evidence of high sensitivity to modest 10 warming (medium confidence). However, differences in incident solar radiation between the LIG and today 11 complicate the relationship between past temperature change and sea level. The Mid Pliocene Warm Period 12 (~3 million years ago) was 1.9°C–3.6°C warmer than pre-industrial, and sea level was higher than during the 13 LIG (high confidence), but the maximum level remains deeply uncertain. The rate of past ice-sheet mass 14 change and hence the rate of sea level rise during these periods remain very uncertain {4.2.2.1, 4.2.2.2, 15 4.2.2.6}. 16 17 2) Human activity has been the dominant cause of global mean sea level rise since 1970 (high 18 confidence). Sea level rise (SLR) at the end of the 21st century will be strongly dependent on the global 19 emission scenario followed, particularly as a result of Antarctica’s contribution under RCP8.5 (high 20 confidence). Greenhouse gas mitigation envisioned in low-emission scenarios (e.g., RCP2.6) sharply reduces 21 but does not eliminate risk to low-lying coastlines and islands from SLR and extreme sea levels (ESL). 22 Lower emission scenarios lead to slower rates of SLR resulting in a wider range of adaptation options. For 23 the first half of the 21st century differences among the scenarios are small. Few individual ESL events or 24 regional-to-local mean sea level changes have as yet been attributed to climate change {4.2.1.2, 4.2.2.6}. 2 25 3) Different modelling studies demonstrate that under high emissions scenarios, Antarctica will likely 26 contribute several tens of centimetres of sea level rise by the end of the century (medium confidence). 27 Projections of global mean SLR under RCP2.6 result in 0.39 m (0.26–0.52 m, likely range) for the period 28 2081–2100, and 0.42 m (0.28–0.57 m) in 2100. Projections of global mean SLR under RCP4.5 results in 29 0.48 m (0.34–0.63 m, likely range) for the period 2081–2100, and 0.55 m (0.39–0.71 m) in 2100. Projections 30 of global mean SLR under RCP8.5 results in 0.78 m (0.47–1.09 m, likely range) for the period 2081–2100, 31 and 0.97 m (0.55–1.40 m) in 2100. The rate of SLR is estimated to be 19 mm/yr under RCP8.5 in 2100. This 32 magnitude, rate and range for RCP8.5 are higher than earlier assessments {4.2.3.1}. 33 4) Processes controlling the timing of future ice-shelf collapse and a possible Marine Ice Cliff 34 Instability (MICI) make Antarctica’s contribution to future sea level rise deeply uncertain for 35 outcomes with SLR higher than the likely range (Cross-Chapter Box 3 in Chapter 1). Long term evolution 36 of the Antarctic Ice Sheet beyond the end of the 21st century remains deeply uncertain as ice sheet models 37 lack realistic representations of some of the underlying physical processes. The few studies available 38 addressing century to millennial timescales indicate multi-metre rise in sea level for RCP8.5 (medium 39 confidence) {4.2.3.1}. 40 41 5) Due to projected global mean sea level rise, extreme sea level events that are historically rare (e.g., 42 those that, in the past, have been associated with surges from intense cyclones), will become common 43 by 2100 under all RCPs, leading to severe flooding in the absence of strong adaptation (high 44 confidence). In RCP8.5, many small islands and megacities will experience such events annually by 2050 45 {4.2.3.4}. 1 FOOTNOTE: In this Report, the following summary terms are used to describe the available evidence: limited, medium, or robust; and for the degree of agreement: low, medium, or high. A level of confidence is expressed using five qualifiers: very low, low, medium, high, and very high, and typeset in italics, e.g., medium confidence. For a given evidence and agreement statement, different confidence levels can be assigned,