World Ocean Assessment II, Chapter 09, Pressures from changes in climate and atmosphere

The Second World Ocean Assessment WORLD OCEAN ASSESSMENT II Volume II The Second World Ocean Assessment WORLD OCEAN ASSESSMENT II Volume II Cover photo: Yung-Sen Wu United Nations World Oceans Day Photo Competition United Nations publication Sales no.: E.21.V.5 ISBN: 978-92-1-1-130422-0 eISBN: 978-92-1-1-604006-2 Copyright © United Nations, 2021 All rights reserved Printed at the United Nations, New York Ŵŵŵŵiii Contents Volume I Page Foreword by the Secretary-General ............................................. iii Summary ................................................................. v Preface ................................................................. vii Part one: Summary ........................................................... 1 Chapter 1: Overall summary.................................................... 3 Keynote points ..................................................... 5 1. Introduction..................................................... 5 2. Drivers ......................................................... 6 3. Cleaning up the ocean............................................ 7 4. Protecting marine ecosystems..................................... 10 5. Understanding of the ocean for sustainable management ............. 13 6. Promoting safety from the ocean .................................. 15 7. Sustainable food from the ocean................................... 16 8. Sustainable economic use of the ocean............................. 19 9. Effective implementation of international law as reƽected in the United Nations Convention on the Law of the Sea ................. 21 Part two: Introduction......................................................... 37 Chapter 2: Approach to the assessment ......................................... 39 Keynote points ..................................................... 41 1. Purpose of the second World Ocean Assessment ..................... 41 2. Primary audience and framework of the second World Ocean Assessment 42 3. Preparation of the second World Ocean Assessment .................. 43 4. Terminology .................................................... 44 5. Acknowledgements.............................................. 45 References......................................................... 45 Chapter : ScientiƼc understandinK ofɸthe ocean.................................. 47 Keynote points ..................................................... 49 1. Introduction..................................................... 49 2. Description of changes in data, technology and models since the Ƽrst World Ocean Assessment and their consequences for overall understanding, including socioeconomic consequences............... 50 ivŵŵŵ World Ocean Assessment II:ŴVolume II Page 3. Key region-speciƼc changes and consequences...................... 51 4. Outlook for scientiƼc understanding of the ocean..................... 56 5. Key remaining knowledge gaps .................................... 56 6. Key remaining capacity-building gaps............................... 57 References......................................................... 58 Part three: (rivers of chanKes in theɸmarine environment .......................... 63 Chapter 4: Drivers ............................................................ 65 Keynote points ..................................................... 67 1. Introduction..................................................... 67 2. Drivers of change in the marine environment......................... 69 3. Key region-speciƼc issues or aspects associated with drivers .......... 73 4. Outlook ........................................................ 74 5. Key remaining knowledge and capacity-building gaps................. 76 References......................................................... 77 Part four: Current state of the marine environment andɸits trends ................... 81 Chapter : 8rends in the physical and chemical state ofɸthe ocean................... 83 Keynote points ..................................................... 85 1. Introduction..................................................... 85 2. Physical and chemical state of the ocean............................ 87 3. Knowledge gaps................................................. 100 4. Summary....................................................... 101 References......................................................... 103 Chapter : 8rends in the biodiversity ofɸtheɸmain ta\a of marine biota ................ 111 Introduction........................................................ 113 Chapter 6A: Plankton phytoplankton, ^ooplankton, microbes andɸviruses......... 115 Keynote points ..................................................... 117 1. Introduction..................................................... 117 2. Summary of chapter 6 of the Ƽrst World Ocean Assessment ........... 118 3. Regions targeted in the present World Ocean Assessment ............. 119 4. Estimating plankton diversity ..................................... 120 5. Microbial plankton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 6. Metazoan zooplankton ........................................... 124 7. Documented trends.............................................. 125 8. Outlook ........................................................ 128 References......................................................... 130 Ŵŵŵŵv Contents Chapter 6B: Marine invertebrates ............................................ 141 Keynote points ..................................................... 143 1. Introduction..................................................... 143 2. Summary of the situation recorded in the Ƽrst World Ocean Assessment . 143 3. Description of environmental changes 2010Ɓ2020................... 144 4. International and governmental responses .......................... 151 5. Achievement of relevant Sustainable Development Goals and contribution to Aichi Biodiversity Target 11 ..................................... 153 6. Key remaining knowledge gaps and capacity-building gaps ............ 153 References......................................................... 154 Addendum by the Group of Experts of the Regular Process for Global Reporting and Assessment of the State of the Marine Environment, including Socioeconomic Aspects.................................. 158 References......................................................... 159 Chapter 6C: Fishes ........................................................ 161 Keynote points ..................................................... 163 1. Introduction..................................................... 163 2. Documented change in the state of Ƽsh biodiversity . . . . . . . . . . . . . . . . . . 165 3. Consequences of biodiversity change on human communities, economies andɸwell-being ........................................ 168 4. Key region-speciƼc changes and consequences...................... 169 5. Outlook ........................................................ 171 References......................................................... 172 Chapter 6D: Marine mammals............................................... 177 Keynote points ..................................................... 179 1. Introduction..................................................... 179 2. Cetaceans...................................................... 181 3. Pinnipeds....................................................... 184 4. Sirenians ....................................................... 186 5. Otters and polar bear............................................. 186 6. Consequences of changes on human communities, economies andɸwell-being................................................... 187 7. Outlook ........................................................ 188 8. Key remaining knowledge gaps .................................... 189 9. Key remaining capacity-building gaps............................... 189 References......................................................... 190 Page viŵŵŵ World Ocean Assessment II:ŴVolume II Chapter 6E: Marine reptiles ................................................. 195 Keynote points ..................................................... 197 1. Introduction..................................................... 197 2. Conservation status of marine reptiles.............................. 197 3. Regional trends.................................................. 199 4. Threats......................................................... 201 5. Economic and social consequences of the changes to marine reptile populations..................................................... 203 6. Key knowledge and capacity-building gaps .......................... 204 References......................................................... 205 Chapter 6F: Seabirds....................................................... 211 Keynote points ..................................................... 213 1. Introduction..................................................... 213 2. Description of environmental changes between 2010 and 2020......... 214 3. Consequences of changes in seabird populations on human communities, economies andɸwell-being ............................ 217 4. Outlook ........................................................ 218 5. Key remaining knowledge gaps .................................... 219 6. Key remaining capacity-building gaps............................... 220 References......................................................... 220 Chapter 6G: Marine plants and macroalgae.................................... 225 Keynote points ..................................................... 227 1. Introduction..................................................... 227 2. Mangroves ..................................................... 227 3. Salt marsh plants................................................ 229 4. Seagrasses ..................................................... 230 5. Macroalgae..................................................... 232 6. Consequences of changes on human communities, economies andɸwell-being................................................... 240 7. Key remaining knowledge and capacity-building gaps................. 241 8. Outlook ........................................................ 241 References......................................................... 242 Chapter 7: Trends in the state of biodiversity in marine habitats..................... 251 Introductionɸ ....................................................... 253 Chapter 7A: Intertidal zone.................................................. 255 Keynote points ..................................................... 257 1. Introduction..................................................... 257 2. Description of the environmental changes between 2010 and 2020...... 260 Page Ŵŵŵŵvii Contents 3. Economic and social consequences................................ 261 4. Key region-speciƼc changes and consequences...................... 261 5. Outlook ........................................................ 262 6. Key remaining knowledge gaps .................................... 263 7. Key remaining capacity-building gaps............................... 263 References......................................................... 264 Chapter 7B: Biogenic reefs and sandy, muddy and rocky shore substrates ......... 267 Keynote points ..................................................... 269 1. Introduction..................................................... 269 2. Documented change in state of biogenic reefs and sandy, muddy and rocky shore substrates ....................................... 272 3. Consequences of the changes on human communities, economies andɸwell-being................................................... 275 4. Key region-speciƼc changes and consequences...................... 277 5. Outlook ........................................................ 279 6. Key remaining knowledge and capacity-building gaps................. 280 References......................................................... 281 Chapter 7C: Atoll and island lagoons ......................................... 289 Keynote points ..................................................... 291 1. Introduction..................................................... 291 2. Documented changes in state of atolls and island lagoons............. 292 3. Consequences of the changes on human communities, economies andɸwell-being................................................... 295 4. Key region-speciƼc changes and consequences ..................... 296 5. Outlook ........................................................ 296 6. Key remaining knowledge gaps .................................... 297 7. Key remaining capacity-building gaps............................... 298 References......................................................... 299 Chapter 7D: Tropical and subtropical coralɸreefs ............................... 305 Keynote points ..................................................... 307 1. Introduction..................................................... 307 2. Description of environmental changes between 2010 and 2020......... 308 3. Description of economic and social consequences and/or other economic or social changes ...................................... 309 4. Key region-speciƼc changes and consequences...................... 310 5. Outlook ........................................................ 312 6. Key remaining knowledge gaps .................................... 313 7. Key remaining capacity-building gaps............................... 313 References......................................................... 314 Page viiiŵŵŵ World Ocean Assessment II:ŴVolume II Page Chapter 7E: Cold water corals ............................................... 321 Keynote points ..................................................... 323 1. Introduction and summary of the Ƽrst World Ocean Assessment ........ 323 2. Description of environmental changes between 2010 and 2020......... 324 3. Economic and social consequences................................ 329 4. Key region-speciƼc changes and consequences...................... 330 5. Outlook ........................................................ 330 6. Key remaining knowledge gaps .................................... 331 7. Key remaining capacity-building gaps............................... 332 References......................................................... 333 Chapter 7F: Estuaries andɸdeltas............................................. 339 Keynote points ..................................................... 341 1. Introduction..................................................... 341 2. Documented changes in the state of estuaries and deltas.............. 342 3. Consequences of the changes for human communities, economies andɸwell-being................................................... 344 4. Key region-speciƼc changes and consequences...................... 345 5. Outlook ........................................................ 346 6. Key remaining knowledge and capacity-building gaps................. 347 References......................................................... 348 Chapter 7G: Seagrass meadows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 Keynote points ..................................................... 355 1. Introduction..................................................... 355 2. Socioeconomic consequences .................................... 356 3. Region-speciƼc changes.......................................... 357 4. Outlook ........................................................ 358 5. Key remaining knowledge gaps .................................... 358 6. Key remaining capacity-building gaps............................... 359 References ........................................................ 362 Chapter 7H: Mangroves .................................................... 365 Keynote points ..................................................... 367 1. Introduction..................................................... 367 2. Documented change in state of mangroves between 2010 and 2020 .... 368 3. Consequences of the changes for human communities, economies andɸwell-being................................................... 370 4. Key region-speciƼc changes and consequences...................... 372 5. Outlook ........................................................ 373 6. Key remaining knowledge and capacity-building gaps................. 373 References......................................................... 374 Ŵŵŵŵxv Contents Page 2. Offshore hydrocarbon exploration, production and decommissioning . . . 285 3. Economic, social, and environmental aspects of offshore hydrocarbon exploration, production and decommissioning ....................... 288 4. Key knowledge and capacity-building gaps .......................... 290 5. Role of the offshore hydrocarbon industry in facilitating the marine renewable energy industry ........................................ 291 6. Conclusion ..................................................... 292 References ........................................................ 293 Chapter 2: Trends in inputs of anthropoKenic noise into the marine environment ..... 297 Keynote points ..................................................... 299 1. Introduction..................................................... 299 2. Description of the environmental status............................. 300 3. Description of economic and social consequences and other economic or social changes ............................................... 308 4. Key region-speciƼc changes and consequences...................... 308 5. Outlook ........................................................ 310 6. Key remaining knowledge gaps .................................... 312 7. Key remaining capacity-building gaps............................... 313 References ........................................................ 313 Chapter 21: Developments in renewable enerKy sources ........................... 321 Keynote points ..................................................... 323 1. Introduction..................................................... 323 2. State of marine renewable energy at the global level .................. 324 3. Potential environmental impacts of marine renewable energy development ................................................... 329 4. Socioeconomic beneƼts and impacts from marine renewable energy deployment..................................................... 332 5. Key remaining knowledge and capacity-building gaps................. 333 6. Anticipated future trends.......................................... 335 References......................................................... 336 Chapter 22: Invasive species ................................................... 343 Keynote points ..................................................... 345 1. Introduction .................................................... 345 2. Documented baseline and changes in non-indigenous species ......... 347 3. Consequences for human communities, economies andɸwell-being ..... 348 4. Key region-speciƼc baselines, changes and consequences............. 350 5. Outlook ........................................................ 354 6. Other .......................................................... 356 References......................................................... 356 xviŵŵŵ World Ocean Assessment II:ŴVolume II Page Chapter 2: Developments in the e\ploration for and use of marine Kenetic resources . 363 Keynote points ..................................................... 365 1. Introduction..................................................... 365 2. Trends between 2010 and 2020.................................... 366 3. Economic and social consequences and changes .................... 370 4. Key region-speciƼc developments in knowledge and their consequences 371 5. Capacity-building gaps ........................................... 371 6. Methodological challenges and future trends ........................ 373 7. Marine genetic resources and the Sustainable Development Goals...... 374 References......................................................... 376 Chapter 24: 1arine hydratesɸƁ a potentially emerKinK issue ........................ 381 Keynote points ..................................................... 383 1. Introduction..................................................... 383 2. What are marine hydrates? ........................................ 383 3. Potential risks from marine methane hydrates ....................... 386 4. Marine hydrates as a source of energy.............................. 388 5. Key knowledge and capacity-building gaps .......................... 390 6. Outlook ........................................................ 390 References......................................................... 390 Chapter 25: Cumulative effects................................................. 395 Keynote points ..................................................... 397 1. Introduction..................................................... 397 2. Cumulative effects assessments................................... 398 3. Regional applications of cumulative effects assessments on the marine environment: distribution and approaches ................. 402 4. Outlook ........................................................ 406 References......................................................... 413 Part si\: Trends in manaKement approaches to the marine environment ............. 421 Chapter 2: Developments in marine spatial planninK.............................. 423 Keynote points ..................................................... 425 1. Introduction..................................................... 425 2. Types of marine spatial planning ................................... 426 3. Marine spatial planning: a step-by-step approach toward ecosystem-based management.................................... 427 4. Tools for marine spatial planning................................... 428 5. Progress in implementing marine spatial planning .................... 430 References......................................................... 436 Ŵŵŵŵxvii Contents Page Chapter 27: Developments in manaKement approaches............................ 441 Keynote points ..................................................... 443 1. Introduction..................................................... 443 2. Management approaches......................................... 444 3. Advances in ocean management approaches........................ 448 4. Management tools to support mitigation of and adaptation to climate change, including building resilience................................ 458 5. Key region-speciƼc issues ........................................ 460 6. Capacity-building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 7. Gaps and future perspectives...................................... 462 8. Outlook ........................................................ 463 References......................................................... 465 Chapter 28: Developmentsɸin the understandinK of overall beneƼts from the ocean to humans ..................................................... 471 Keynote points ..................................................... 473 1. Introduction..................................................... 473 2. BeneƼts and their distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477 3. DisbeneƼts to humans............................................ 478 4. Threats to ocean ecosystem services............................... 479 5. Safeguarding ocean beneƼts through regional and international cooperation and improved implementation of international law as reƽected in the United Nations Convention on the Law of the Sea ....... 480 References......................................................... 483 Anne\es ................................................................. 487 Anne\ I: OriKinal members of the writinK teams approved by the &ureau ............. 489 Anne\ II: Peer reviewers nominated for each chapter .............................. 497 55 Chapter 9 Pressures from changes in climate and atmosphere Contributors: 'EVPSW+EVGME7SXSPIEHQIQFIVERHGSRZIRIV(IRMWI&VIMXFYVK1SRMGE'EQTMPPSW4EXVMGME'EWXMPPS-&VMGIRS7EREI'LMFEGS-PIEHQIQFIV1EXXLI['SPPMRW+ERM\)WRESPE /EVIR)ZERWGS-PIEHQIQFIV0SYMWI&*MVXL8LSQEW*VʯPMGLIV.EWSR1,EPP-7TIRGIV(EZMH Halpern, Karen L. Hunter, Gabriel Ibarra, Sung-Yong Kim, Roxy M. Koll, Kathleen McInnes, Jon Saen^, 'a 8hanh :u co-leaH member, &ess ;arH anH 8ymon >ielinsOi co-leaH member. Ŵŵŵŵ57 Chapter 9: Pressures from changes in climate and atmosphere Keynote points xExtreme climate events. Marine heatwaves and tropical cyclones are shown to be increasing in severity owing to human activities and are having an impact on nature and human societies. Extreme El Niño events have been observed but, because they occur infreUuently, a human inƽuence has not been detected. All three phenomena are projected to increase in the future, with the severity of impacts also increasing, but such increases can be reduced by climate change mitigation efforts. xSea level rise. The alarming observed pace of sea level rise, combined with increasing storminess and coastal urbanization, has resulted in the ampliƼed susceptibility of coastal cities to erosion and ƽooding and increased the need for substantial investments in hard infrastructure and the restoration of natural barriers, such as reefs. x3cean aciHiƼcation anH Heox]Kenation. The accelerated increase of anthropogenic CO2 in the atmosphere is creating an increase in the acidiƼcation and deoxygenation of the ocean. Under such conditions, both in nature and in the laboratory, marine organisms that support ecosystems and human livelihoods and nutrition typically respond poorly. Marine habitats experience a loss of diversity, many long-lived organisms die and a few resilient species proliferate. Less serious damage to life-supporting ecosystems would be possible under lower-emission scenarios. x3tLer TL]sical anH cLemical TroTerties. Changes in ocean temperature and salinity induced by climate change and human activities are affecting marine ecosystems by changing the distribution of marine species, decreasing the ecological value of coastal ecosystems and changing marine primary production. Human well-being and the economy are consequently affected. 1. Introduction The Ƽrst part of the present chapter is based on three topics in the context of extreme climate events related to the ocean, namely, marine heatwaves, extreme El Niño Southern Oscillation events and tropical cyclones. Both physical aspects of the impact of climate change on the phenomena and potential impacts on natural and human systems are considered. The conclusions are based on a much more detailed assessment that can be found in chapter 6 of the Special Report on Oceans and Cryosphere in a Changing Climate of the Intergovernmental Panel on Climate Change 29. An extreme event is one that is rare at a particular place and time of year. (eƼnitions of “rare” vary, but an extreme event is normally as rare as, or rarer than, the tenth or ninetieth percentile of a probability estimated from observations. By deƼnition, the characteristics of what is called an extreme event may vary from place to place in an absolute sense. When a pattern of extreme weather persists for some time, such as a season, it may be classed as an extreme climate event, especially if it yields an average or total that is itself extreme (e.g., high temperature, drought or total rainfall over a season. The second part of the chapter expands upon pressures from changes in ocean physical and chemical properties. Projected sea temperature increases of up to 1.5°C over pre-industrial levels by 2050 will continue to drive latitudinal abundance shifts in marine species, including 58ŵŵŵ World Ocean Assessment II:Ŵ:olume II those of importance for coastal livelihoods. Many large coastal cities are located in deltaic settings and are vulnerable to ƽoods because of their proximity to rivers and the sea, general low elevations and land subsidence (Nicholls and others, 200. Carbon dioxide emissions and global warming are also causing ocean acidiƼcation and deoxygenation. Those changes have consequences for the people who depend on healthy marine ecosystems worldwide. At the time of the Ƽrst World Ocean Assessment (United Nations, 201, the chemistry of ocean acidiƼcation was well understood, yet the consequences for ecosystems and society were poorly known. The effects of declining oxygen on nutrient cycles and Ƽsh stocks were predicted to worsen, especially when climate change-driven oxygen depletion combines with coastal eutrophication. Reduced biodiversity and declines in Ƽsh populations were linked to falling oxygen levels across the worldƅs oceans. New information is provided on marine organism and ecosystem responses to ocean acidiƼcation and deoxygenation and related capacity-building. In the present chapter, in conjunction with chapter 5, the climate change aspects of the present Assessment are developed. The present chapter expands on the pressures on marine ecosystems and human populations of some of the physical and chemical changes caused by climate change. Some related aspects are also covered in chapter 7K and chapter 15. 2. Climate pressures: extreme climate events and pressures from changes in ocean physical and chemical properties 2.1. Extreme climate events Marine heatwaves are periods of extremely high ocean temperatures that persist for days to months, that can extend up to thousands of km and can penetrate multiple hundreds of m into the deep ocean (Hobday and others, 2016. Over the past two decades, marine heatwaves have had a negative impact on marine organisms and ecosystems in all ocean basins, including critical foundation species such as corals, seagrasses and kelps (Hughes and others, 201 Smale and others, 2019. Satellite observations reveal that marine heatwaves doubled in frequency between 1982 and 2016, and that they have also become longer lasting and more intense and extensive (Frölicher and others, 2018 Oliver and others, 2018. Between 2006 and 2015, 8 to 90ɸperɸcent of all globally occurring marine heatwaves were attributable to the temperature increase since the period 1850Ɓ1900 (Frölicher and others, 2018. Marine heatwaves will further increase in frequency, duration, spatial extent and intensity under future global warming (Frölicher and others, 2018 (armaraki and others, 2019, pushing some marine organisms, Ƽsh stocks and ecosystems beyond the limits of their resilience, with cascading impacts on economies and societies (Smale and others, 2019. Globally, the frequency of marine heatwaves is very likely to increase by a factor of about 50 times by the period 2081–2100 under the high-emission Representative Concentration Pathway (RCP 8.5 scenario and by a factor of about 20 times under the low-emission RCP 2.6 scenario (:an :uuren and others, 2011, relative to the reference period 1850–1900. Such future trends in marine heatwave frequency can largely be explained by increases in mean ocean temperature. The largest changes in the frequency of marine heatwaves are projected for the Arctic Ocean and the tropical oceans (Ƽgure I Intergovernmental Panel on Climate Change (IPCC, 2019, chap. 6, Ƽgure 6.. 59 Chapter 9: Pressures from changes in climate and atmosphere Figure I 0SGEXMSRWSJI\XVIQIIZIRXW[MXLERMHIRXMƼIHPMROXSGPMQEXIGLERKIGEYWIHF]LYQER activities Cyclone Extreme rainfall Drought Marine heatwave Tidal ƽooding Wave-induced ƽooding Cold or snowstorm Compound event: drought, low sea levels Compound event: drought, rainfall, MHW Compound event: multiple cyclones Sea-ice minimum Source:Figureɸadapted from IPCC, 2019, Ƽgure 6.2. Limiting global warming would reduce the risk of impacts of marine heatwaves, but critical thresholds for some ecosystems (e.g., kelp forestsandcoralreefswill be reached even at relatively low levels of future global warming(Kingandothers,2017.Earlywarning systems, producing skilful forecasts of marine heatwaves, can further help to reduce vulnerabilities in Ƽshing, tourism and conservation, but are yet unproven on a large scale (Payne and others, 2017Tommasi and others, 2017. One of the best data-rich examples of the impact of a marine heatwave on well-managed ƼsheriesisoftheGulfofAlaskaintheNorth PaciƼc. A prolonged warm ocean event weakened benthic ocean and surface mixing, in turn disrupting trophies, invertebrate and forage Ƽsh populations, and decimated the PaciƼccod Ƽshery, triggering a series of repeating mass marine mammal and seabird die-offs that had a ripple effect through coastal economies. The El Niño Southern Oscillation is a coupled atmosphere-ocean phenomenon, identiƼed by an oscillation between warm and cold ocean temperatures in the tropical central eastern PaciƼcOceanandanassociatedƽuctuation in the global-scale tropical and subtropical surface pressure patterns. Typically, it has a preferred timescale of about two to seven years. It is often measured by the surface pressure anomaly difference between Tahiti, French Polynesia, and Darwin, Australia, and/or the sea surface temperatures in the central and eastern equatorial PaciƼc(RasmussenandCarpenter, 1982.Ithasclimaticeffectsthroughout the PaciƼcregionandinmanyotherpartsof 60ŵŵŵ World Ocean Assessment II:Ŵ:olume II the world through global teleconnections. The warm phase of the Oscillation is called El Niño and the cold phase is called La Niña. The strongest El Niño and La Niña events since the pre-industrialɸera have occurred during the past 50 years, and that variability is unusually high when compared with average variability during the last millennium (Cobb and others, 201 Santoso and others, 2017. There have been three occurrences of extreme El Niño events during the modern observational period (1982/8, 1997/98, 2015/16, all characterized by pronounced rainfall in the normally dry equatorial East PaciƼc. There have been two occurrences of extreme La Niña (1988/89, 1998/99.ɸ Extreme El Niño and La Niña events are likely to occur more frequently with global warming and are likely to intensify existing impacts, with drier or wetter responses in several regions across the globe, even at relatively low levels of future global warming (Cai and others, 2014; Cai and others, 2015; Power and Delage, 2018. Sustained long-term monitoring and improved forecasts can be used in managing the risks of extreme El Niño and La Niña events associated with human health, agriculture, Ƽsheries, coral reefs, aquaculture, wildƼre, drought and ƽood management (LƅHeureux and others, 2017. A tropical cyclone is the general term for a strong, cyclonic-scale disturbance that originates over the tropical ocean. Based on one-minute maximum sustained wind speed, the cyclonic disturbances are categorized into tropical depressions (Ƶ 17 m/s, tropical storms (18–2 m/s and tropical cyclones (ƶɸ m/s, category 1 to category 5 (Knutson and others, 2010. A tropical cyclone is called a hurricane, typhoon or cyclone, depending on geographic location. Anthropogenic climate change has increased precipitation, winds and extreme sea level events associated with a number of observed tropical cyclones. For example, studies have shown that the rainfall intensity of tropical cyclone (Hurricane Harvey increased by at least 8ɸperɸcent (8–19ɸperɸcent owing to climate change (Risser and Wehner, 2017; Van Oldenborgh and others, 2017. Anthropogenic climate change may have contributed to a poleward migration of maximum tropical cyclone intensity in the western North PaciƼc in recent decades related to anthropogenically forced tropical expansion (Sharmila and Walsh, 2018. There is emerging evidence of a number of regional changes in tropical cyclone behaviour, such as an increase in the annual global proportion of category 4 or 5 tropical cyclones in recent decades, extremely severe tropical cyclones occurring in the Arabian Sea, cyclones making landfall in East and South-East Asia, an increase in frequency of moderately large storm surge events in the United States since 1923 and a decrease in frequency of severe tropical cyclones making landfall in eastern Australia since the late 1800s. There is low conƼdence that they represent detectable anthropogenic signals. Extreme wave heights, which contribute to extreme sea level events, coastal erosion and ƽooding, have increased in the Southern Ocean and the North Atlantic Ocean by about 1.0 cm per year and 0.8 cm per year over the period 1985–2018 (Young and Ribal, 2019. An increase in the average intensity of tropical cyclones, and the associated average precipitation rates, is projected for a 2°C global temperature rise, although there isɸlow conƼdenceɸin future frequency changes at the global scale (Yamada and others, 2017. Rising sea levels will contribute to higher extreme sea levels associated with tropical cyclones in the future (Garner and others, 2017.ɸProjections suggest that the proportion of category 4 and 5 tropical cyclones will increase (Knutson and others, 2015; Park and others, 2017.ɸSuch changes will affect storm surge frequency and intensity, as well as coastal infrastructure and mortality. Investment in disaster risk reduction, ƽood management (ecosystem and engineered and early warning systems decreases Ŵŵŵŵ61 Chapter 9: Pressures from changes in climate and atmosphere economic loss from tropical cyclones that occur near coasts and islands. However, such investments may be hindered by limited local capacities (e.g., ageing infrastructure and other non-climatic factors that, for example, can lead to increased losses and mortality from extreme winds and storm surges in developing countries despite adaptation efforts. There is emerging evidence of increasing risks for locations affected by unprecedented storm trajectories. Management of risk from such changing storm trajectories and intensity proves challenging because of the diƾculties of early warning and its receptivity by affected populations. 2.2. Sea level rise and cities Cities located along coastlines and in archipelagic and island States are becoming increasingly susceptible to erosion and sea level rise (De Sherbinin and others, 2007; Hanson and others, 2011; Takagi and others, 2016. Many comprise large areas of reclaimed land (the gain of land from the sea, wetlands or other water bodies, which is retained and protected from erosion by hard engineered structures, such as sea walls and rock armouring (Sengupta and others, 2018. It is likely that many of such engineered coastlines will need to be adapted and upgraded to keep pace with rising sea levels. In highly urbanized environments that are often already heavily degraded, hard engineered structures are often the only option available and are considered to be successful options (Hallegatte and others, 2013; Hinkel and others, 2014, but there are a wide range of broader negative impacts of land reclamation and those structures on the surrounding environment (Dafforn and others, 2015. Globally, many regions (especially cities are claiming that more than 50ɸperɸcent of their coastlines are armoured (e.g., Chapman, 2003; Burt and others, 2013, and that number will likely rise in the future in response to burgeoning economies, coastal populations and urbanization (e.g., see plans for the reclamation of the entire coastlines of two Malaysian states in Chee and others, 2017. As an alternative to hard engineered coastal defences, construction of which is complex and expensive, where possible, natural coastal ecosystems such as mangroves and salt marshes should be used as natural barriers or combined with hard infrastructure using hybrid approaches (Temmerman and others, 2013. The use of such ecosystems can not only protect the land but also provide valuable ecosystem functions and services. As hard engineered coastal defences may be considered an effective short-term solution to coastal ƽooding, more investment will be needed owing to observed increasing storminess and sea level rise (Mendelsohn and others, 2012; Vitousek and others, 2017. By 2010, the global average sea level was calculated to be 52.4ɸmm above the 1993 level and, by 2018, it had risen to 89.9 mm above the 1993 level (National Oceanic and Atmospheric Administration (NOAA, 2019. The rate of change is also increasing. For the period 1993–2018, the rate of increase was calculated at 3.2 mm per year, while for the period 2010–2018, it was calculated to be much faster, at 4.7 mm per year. Despite signiƼcant uncertainties remaining, the Intergovernmental Panel on Climate Change predicts that sea level rise will continue for centuries, even if mitigation measures are put in place. The potential widespread collapse of ice shelves could lead to a larger twenty-Ƽrst century sea level rise of up to several tenths of a metre (Church and others, 2013, which will have drastic consequences for coastal, archipelagic and small island cities, in particular those in low-lying areas. Urbanization could, however, also provide opportunities for risk reduction, given that cities are engines of economic growth and centres of innovation, political attention and private sector investments (Garschagen and Romero-Lankao, 2015. Hallegatte and others (2013 conducted a global analysis of present and future losses 68ŵŵŵ World Ocean Assessment II:ŴVolume II effective measures to mitigate the impacts of ocean acidiƼcation and deoxygenation, which may, as a result, have less serious consequences for the millions of people who are dependent on coastal protection, Ƽsheries and aquaculture in lower-emission scenarios. 4. Summary Marine heatwaves are shown to be increasing in frequency and intensity owing to climate change caused by human activities and are having a mostly negative impact on marine ecosystems. Marine heatwaves and their impacts are projected to increase in the future but those increases can be strongly limited by efforts to mitigate climate change. Forecasting systems may be employed in adapting to the effects of marine heatwaves. Extreme El Niño and La Niña events have been observed but, because they occur infrequently, a human inƽuence has not been detected. Nevertheless, models indicate an increase in the frequency of both phases of the oscillation under future scenarios of global warming. As in the case of marine heatwaves, forecasting systems, which already exist, may be employed in risk management and adaptation. While changes in the frequency and spatial distribution of tropical cyclones are hard to detect in the observational record, studies of individual cyclones have shown a human inƽuence on their intensity, in particular, the associated rainfall. Changes in intensity are projected to increase in the future, with associated impacts on storm surges and coastal infrastructure. Although all coastal cities are already facing rising sea levels, low-lying cities and developing countries that lack the ability to invest in coastal defence measures and natural barrier restoration will suffer damage and losses of a higher degree. Global population studies suggest that people are relocating to coastal areas and will continue to do so, thereby putting more people at risk economically and socially. Although cities are typically centres for innovation and investment, key examples demonstrate the diƾculty in solving such complex problems in vulnerable locations. Damage and losses are also driven by existing vulnerabilities in coastal infrastructure and may not be solely attributed to rising sea levels. Rather, increasing sea levels may exacerbate existing issues, increasing risk. The complex interactions of temperature and salinity with nutrients and chemical cycles of the ocean imply that variations in those variables owing to climate change and anthropogenic impact thus affect marine ecosystems, population, coastal communities and the related economy. Ocean warming is causing signiƼcant damage to marine ecosystems, and species are losing their habitats, forcing them to adapt or relocate to new temperatures or look for new feeding, spawning or nursery areas. Ocean acidity and the availability of suƾcient oxygen both underpin the provision of marine ecosystem services to human society. Rapid changes in ocean acidity and falling oxygen levels caused by climate change and anthropogenic CO2ɸemissions are, however, now being observed, which is changing marine habitats and ecosystems worldwide. Warming is causing oxygen levels to fall, and acidiƼcation is rapidly changing the carbonate chemistry of surface ocean waters, which together are reducing the growth and survival of many organisms and degrading ecosystem resilience. Closing knowledge gaps in ocean science by supporting capacity-building efforts that increase the understanding of how the ocean and its ecosystems are responding to changes in ocean physical and chemical properties is an important pathway to reducing the impacts of such changes and achieving Sustainable Development Goal 14. Ŵŵŵŵ69 Chapter 9: Pressures from changes in climate and atmosphere References Introduction Intergovernmental Panel on Climate Change (IPCC (2019. IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. 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