Lead Authors: Marco Bindi (Italy), Sally Brown (UK), Ines Camilloni (Argentina), Arona Diedhiou (Ivory Coast/Senegal), Riyanti Djalante (Japan/Indonesia), Kristie L. Ebi (USA), Francois Engelbrecht (South Africa), Joel Guiot (France), Yasuaki Hijioka (Japan), Shagun Mehrotra (USA/India), Antony Payne (UK), Sonia I. Seneviratne (Switzerland), Adelle Thomas (Bahamas), Rachel Warren (UK), Guangsheng Zhou (China)

Impacts of 1.5°C Global Warming on Natural and Human Systems

The response of coastal wetlands to sea-level rise during the twenty-first century remains uncertain Global-scale projections suggest that between 20 and 90 per cent (for low and high sea-level rise scenarios, respectively) of the present-day coastal wetland area will be lost, which will in turn result in the loss of biodiversity and highly valued ecosystem services1-3 These projections do not necessarily take into account all essential geomorphological4-7 and socio-economic system feedbacks8 Here we present an integrated global modelling approach that considers both the ability of coastal wetlands to build up vertically by sediment accretion, and the accommodation space, namely, the vertical and lateral space available for fine sediments to accumulate and be colonized by wetland vegetation We use this approach to assess global-scale changes in coastal wetland area in response to global sea-level rise and anthropogenic coastal occupation during the twenty-first century On the basis of our simulations, we find that, globally, rather than losses, wetland gains of up to 60 per cent of the current area are possible, if more than 37 per cent (our upper estimate for current accommodation space) of coastal wetlands have sufficient accommodation space, and sediment supply remains at present levels In contrast to previous studies1-3, we project that until 2100, the loss of global coastal wetland area will range between 0 and 30 per cent, assuming no further accommodation space in addition to current levels Our simulations suggest that the resilience of global wetlands is primarily driven by the availability of accommodation space, which is strongly influenced by the building of anthropogenic infrastructure in the coastal zone and such infrastructure is expected to change over the twenty-first century Rather than being an inevitable consequence of global sea-level rise, our findings indicate that large-scale loss of coastal wetlands might be avoidable, if sufficient additional accommodation space can be created through careful nature-based adaptation solutions to coastal management

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Future response of global coastal wetlands to sea-level rise

Do Not Cite, Quote or Distribute 4-1 Total pages: 139 1 Chapter 4: Sea Level Rise and Implications for Low Lying Islands, Coasts and Communities 2 3 Coordinating Lead Authors: Michael Oppenheimer (USA), Bruce Glavovic (New Zealand), Tuhin Ghosh 4 (India) 5 6 Lead Authors: Amro Abd-Elgawad (Egypt), Rongshuo Cai (China), Miguel Cifuentes-Jara (Costa Rica), 7 Rob Deconto (USA), John Hay (Cook Islands), Jochen Hinkel (Germany), Federico Isla (Argentina), 8 Alexandre K. Magnan (France), Ben Marzeion (Germany), Benoit Meyssignac (France), Zita Sebesvari 9 (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), Virginie Duvat (France), Tamsin Edwards (UK), Miguel D. Fortes (Philippines), Thomas 13 Frederikse (Netherlands), Jean-Pierre Gattuso (France), Robert Kopp (USA), Erwin Lambert (Netherlands), 14 Elizabeth McLeod (USA), Mark Merrifield (USA), Siddharth Narayan (US), Robert J. Nicholls (UK), 15 Fabrice Renaud (UK), Jonathan Simm (UK), Jon Woodruff (USA), Poh Poh Wong (Singapore), Siyuan Xian 16 (USA) 17 18 Review Editors: Ayako Abe-Ouchi (Japan), Kapil Gupta (India), Joy Pereira (Malaysia) 19 20 Chapter Scientist Maya Buchanan (USA) 21 22 Date of Draft: 20 April 2018 23 24 Notes: TSU Compiled Version 25 26

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Sea Level Rise and Implications for Low Lying Islands, Coasts and Communities

Increasing human populations around the global coastline have caused extensive loss, degradation and fragmentation of coastal ecosystems, threatening the delivery of important ecosystem services1. As a result, alarming losses of mangrove, coral reef, seagrass, kelp forest and coastal marsh ecosystems have occurred1–6. However, owing to the difficulty of mapping intertidal areas globally, the distribution and status of tidal flats—one of the most extensive coastal ecosystems—remain unknown7. Here we present an analysis of over 700,000 satellite images that maps the global extent of and change in tidal flats over the course of 33 years (1984–2016). We find that tidal flats, defined as sand, rock or mud flats that undergo regular tidal inundation7, occupy at least 127,921 km2 (124,286–131,821 km2, 95% confidence interval). About 70% of the global extent of tidal flats is found in three continents (Asia (44% of total), North America (15.5% of total) and South America (11% of total)), with 49.2% being concentrated in just eight countries (Indonesia, China, Australia, the United States, Canada, India, Brazil and Myanmar). For regions with sufficient data to develop a consistent multi-decadal time series—which included East Asia, the Middle East and North America—we estimate that 16.02% (15.62–16.47%, 95% confidence interval) of tidal flats were lost between 1984 and 2016. Extensive degradation from coastal development1, reduced sediment delivery from major rivers8,9, sinking of riverine deltas8,10, increased coastal erosion and sea-level rise11 signal a continuing negative trajectory for tidal flat ecosystems around the world. Our high-spatial-resolution dataset delivers global maps of tidal flats, which substantially advances our understanding of the distribution, trajectory and status of these poorly known coastal ecosystems. Analyses of over 700,000 satellite images to map the global extent of tidal flats over the past thirty years, and enable assessments of the status and likely future trajectories of these coastal ecosystems.

The global distribution and trajectory of tidal flats

The term Blue Carbon (BC) was first coined a decade ago to describe the disproportionately large contribution of coastal vegetated ecosystems to global carbon sequestration. The role of BC in climate change mitigation and adaptation has now reached international prominence. To help prioritise future research, we assembled leading experts in the field to agree upon the top-ten pending questions in BC science. Understanding how climate change affects carbon accumulation in mature BC ecosystems and during their restoration was a high priority. Controversial questions included the role of carbonate and macroalgae in BC cycling, and the degree to which greenhouse gases are released following disturbance of BC ecosystems. Scientists seek improved precision of the extent of BC ecosystems; techniques to determine BC provenance; understanding of the factors that influence sequestration in BC ecosystems, with the corresponding value of BC; and the management actions that are effective in enhancing this value. Overall this overview provides a comprehensive road map for the coming decades on future research in BC science.

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The future of Blue Carbon science

Sea-level rise can threaten the long-term sustainability of coastal communities and valuable ecosystems such as coral reefs, salt marshes and mangroves. Mangrove forests have the capacity to keep pace with sea-level rise and to avoid inundation through vertical accretion of sediments, which allows them to maintain wetland soil elevations suitable for plant growth. The Indo-Pacific region holds most of the world's mangrove forests, but sediment delivery in this region is declining, owing to anthropogenic activities such as damming of rivers. This decline is of particular concern because the Indo-Pacific region is expected to have variable, but high, rates of future sea-level rise. Here we analyse recent trends in mangrove surface elevation changes across the Indo-Pacific region using data from a network of surface elevation table instruments. We find that sediment availability can enable mangrove forests to maintain rates of soil-surface elevation gain that match or exceed that of sea-level rise, but for 69 per cent of our study sites the current rate of sea-level rise exceeded the soil surface elevation gain. We also present a model based on our field data, which suggests that mangrove forests at sites with low tidal range and low sediment supply could be submerged as early as 2070.

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Megan L. Saunders

Papers

The vulnerability of Indo-Pacific mangrove forests to sea-level rise