World Ocean Assessment II, Chapter 05, Trends in the physical and chemical state of the ocean

The Second World Ocean Assessment WORLD OCEAN ASSESSMENT II Volume I The Second World Ocean Assessment WORLD OCEAN ASSESSMENT II Volume I 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 iii Foreword by the Secretary-General The past year has presented unprecedented challenges. The coronavirus disease (COVID-19) pandemic has disrupted lives and livelihoods and exposed our societies’ fragility. Sadly, the pandemic is not the only crisis that humanity faces. Climate change and biodiversity loss continue unabated, threatening sustainable development and our viability as a species. These challenges are particularly evident when we look at the state of our planet’s life support system, the ocean. In 2015, the Ƽrst World Ocean Assessment warned that many areas of the ocean had been seriously degraded, the greatest threat to the ocean being the failure to deal with the many pressures caused by human activities. The message in the second World Ocean Assessment is that the situation has not improved, with the many beneƼts that the ocean provides at risk. The Assessment advises that, to ensure sustainability, we must work together to improve integrated ocean management, including through joint research, capacity development and the sharing of data, information and technology. The ocean plays a crucial role in the achievement of the Sustainable Development Goals and the livelihoods of billions of people. We urgently need to change how we interact with it. The forthcoming United Nations Decade of Ocean Science for Sustainable Development and the United Nations Decade on Ecosystem Restoration provide opportunities for us to understand more and to reverse the damage that has already been done. The information in the second Assessment can assist in this process, as well as inform relevant intergovernmental conferences scheduled for 2021. I urge leaders and all stakeholders to heed the warnings in the Assessment as we work to conserve and sustainably manage our planet’s marine environment. Let us foster not only a green but also a blue recovery from the COVID-19 pandemic. AǞǘǞǣǝ GǗǘȨǚǚȨǙ v Summary In its resolutions 57/141 and 58/240, the General Assembly decided to establish a regular process under the United Nations for global reporting and assessment of the state of the marine environment, including socioeconomic aspects, both current and foreseeable, building on existing regional assessments. In its resolution 71/257, the Assembly recalled that the scope of the Ƽrst cycle of the Regular Process focused on establishing a baseline and decided that the scope of the second cycle would extend to evaluating trends and identifying gaps. The programme of work for the period 2017–2020 of the second cycle of the Regular Process includes the preparation by the Group of Experts of the Regular Process for Global Reporting and Assessment of the State of the Marine Environment, including Socioeconomic Aspects, of the second World Ocean Assessment, building on the baselines established by the First Global Integrated Marine Assessment (Ƽrst World Ocean Assessment). In its resolution 72/73, the Assembly decided that the Group of Experts should proceed on the basis of a single comprehensive assessment. The present document was prepared by the Group of Experts in accordance with those decisions. Disclaimer The present document is a product of the Group of Experts of the Regular Process for Global Reporting and Assessment of the State of the Marine Environment, including Socioeconomic Aspects, which is responsible for the contents of the publication. The members of the Group of Experts and the pool of experts who participated in the writing of the second World Ocean Assessment contributed in their personal capacity. The members of the Group and the pool are not representatives of any Government or any other authority or organization. The designations employed, including geographical names, and the presentation of the materials in the present publication, including the citations, maps and bibliography, do not imply the expression of any opinion whatsoever on the part of the United Nations concerning the names and legal status of any country, territory, city or area or of its authorities or concerning the delimitation of its frontiers or boundaries and do not imply oƾcial endorsement or acceptance by the United Nations. Information contained in the present publication emanating from actions and decisions taken by States does not imply oƾcial endorsement, acceptance or recognition by the United Nations of such actions and decisions, and such information is included without prejudice to the position of any State Member of the United Nations. vii Preface The goal for the General Assembly in creating the Regular Process for the Global Reporting and Assessment of the State of the Marine Environment, including Socioeconomic Aspects, was to ensure a comprehensive overview of the ocean and the relationships between the ocean and humans, covering all environmental, social and economic aspects. Such an overview would serve as a background to the many decisions that must be taken in that Ƽeld at the international, national and local levels in pursuit of sustainable development. The Ƽrst World Ocean Assessment was completed in 2015 and represents a major step towards that goal. Inevitably, with such an ambitious goal, not only were some aspects not fully covered in the Ƽrst output of the Regular Process, but also, as time passed, the assessment that was made up to 2015 needed to be updated. The General Assembly therefore provided for further global integrated marine assessments to record developments from the baseline provided by the Ƽrst Assessment and, where possible, to show trends. In 2016, it decided that a second comprehensive assessment should be prepared by the end of 2020. The present volume contains the second World Ocean Assessment. It provides more information on aspects of the ocean and its relationships with humans, including separate assessments of the abyssal plains and marine hydrates, and brings together in speciƼc chapters matters that were addressed in different sections of the Ƽrst Assessment, such as the state of Ƽsh species and marine infrastructure. As with the Ƽrst Assessment, the production of the present Assessment has been a major task, relying essentially on voluntary efforts of hundreds of experts in many Ƽelds, with support from the regular budget of the United Nations. As before, it has been a privilege for the Group of Experts of the Regular Process for Global Reporting and Assessment of the State of the Marine Environment, including Socioeconomic Aspects, to organize, contribute to and Ƽnalize the Assessment. Crucial support has again been provided by the Secretariat, including the Division for Ocean Affairs and the Law of the Sea, several international organizations and a number of States Members of the United Nations, as detailed in chapter 2. The Group of Experts is grateful to all those people and institutions but, under the terms of reference and working methods endorsed by the General Assembly, is ultimately responsible for the Ƽnal text. The bulk of the text was written before the outbreak of the coronavirus disease (COVID-19) pandemic. Some mention of the effects of that pandemic has been included (for example, in the sections of chapter 8A dealing with Ƽsheries, shipping and tourism), but the full implications of the pandemic on human interactions with the ocean are still being worked out and will need to be explored fully in the third cycle of the Regular Process. Nevertheless, the ocean and the services that it provides will have an important role in the recovery from the pandemic. It is hoped that the information in the present Assessment will help with that process. As with the Ƽrst Assessment, the present document contains no policy analysis or recommendations, in line with the guidance endorsed by the General Assembly. It is therefore for national Governments and competent international authorities to decide what action should be taken in the light of the assessments under the Regular Process. RȨǞǣǙǝǞ RǗǕȳ and AǠȳǞ Sǣǟȱǝȱǡ Joint Coordinators of the Group of Experts of the Regular Process JǚǞ SȱǤǟǣȰǘ Member of the Group of Experts of the Regular Process, assisting the Joint Coordinators Ŵŵŵŵxv Contents Page Chapter 7I: Salt marshes ................................................... 381 Keynote points ..................................................... 383 1. Introduction..................................................... 383 2. Description of the environmental changes between 2010 and 2020...... 385 3. Consequences of the changes for human communities, economies andɸwell-being................................................... 386 4. Key region-speciƼc changes and consequences...................... 386 5. Outlook ........................................................ 387 6. Key remaining knowledge gaps .................................... 388 7. Key remaining capacity-building gaps............................... 389 References......................................................... 389 Chapter 7J: Continental slopes and submarine canyons......................... 395 Keynote points ..................................................... 397 1. Introduction..................................................... 397 2. Developments in understanding of slopes and canyons ............... 399 3. Ecosystem services and beneƼts on slopes and in canyons ............ 404 4. Human impacts ................................................. 405 5. Key remaining knowledge gaps .................................... 406 6. Key remaining capacity-building gaps............................... 407 References ........................................................ 408 Chapter 7K: High-latitude ice................................................ 421 Keynote points ..................................................... 423 1. Introduction..................................................... 423 2. Description of the environmental changes between 2010 and 2020...... 424 3. Economic and social consequences................................ 428 4. Outlook ........................................................ 430 5. Key remaining knowledge and capacity-building gaps................. 431 References......................................................... 431 Chapter 7L: Seamounts andɸpinnacles........................................ 437 Keynote points ..................................................... 439 1. Introduction..................................................... 439 2. Description of changes in knowledge between 2010 and 2020.......... 440 3. Description of economic and social changes......................... 441 4. Key region-speciƼc research in recent years ......................... 442 5. Outlook ........................................................ 444 6. Key remaining knowledge gaps .................................... 444 7. Key remaining capacity-building gaps............................... 445 References......................................................... 446 xviŵŵŵ World Ocean Assessment II:ŴVolume I Page Chapter 7M: Abyssal plains ................................................. 453 Keynote points ..................................................... 455 1. Introduction .................................................... 455 2. Shifting baselines and documenting status and change in abyssal biodiversity ..................................................... 456 3. Major natural and anthropogenic pressures.......................... 464 4. Consequences of the changes on human communities, economies andɸwell-being................................................... 465 5. Outlook ........................................................ 468 6. Key remaining knowledge gaps .................................... 468 References......................................................... 469 Chapter 7N: Open ocean.................................................... 477 Keynote points ..................................................... 479 1. Introduction..................................................... 479 2. Environmental changes in the open ocean since 2010................. 481 3. Consequences of the changes for human communities, economies andɸwell-being................................................... 484 4. Key region-speciƼc changes and consequences...................... 486 5. Outlook ........................................................ 487 6. Key remaining knowledge gaps .................................... 488 7. Key remaining capacity-building gaps............................... 488 References......................................................... 488 Chapter 7O: Ridges, plateaux and trenches.................................... 495 Keynote points ..................................................... 497 1. Introduction and summary of the Ƽrst World Ocean Assessment ........ 497 2. Description of the environmental changes between 2010 and 2020 ..... 499 3. Description of economic and social changes between 2010 and 2020 . . . 502 4. Key region-speciƼc changes and consequences...................... 505 5. Outlook ........................................................ 506 6. Key remaining knowledge gaps .................................... 507 7. Key remaining capacity-building gaps............................... 507 References ........................................................ 508 Chapter 7P: Hydrothermal vents and coldɸseeps ............................... 513 Keynote points ..................................................... 515 1. Introduction..................................................... 515 2. Environmental changes since the Ƽrst World Ocean Assessment ........ 518 3. Economic and social consequences................................ 519 4. Key region-speciƼc changes and consequences...................... 521 Ŵŵŵŵxvii Contents Page 5. Outlook ........................................................ 523 6. Key remaining knowledge gaps .................................... 523 7. Key remaining capacity-building gaps .............................. 524 References......................................................... 524 Chapter 7Q: Sargasso Sea .................................................. 531 Keynote points ..................................................... 533 1. Introduction..................................................... 533 2. Change of state ................................................. 534 3. Institutional arrangements ........................................ 537 4. Consequences of changes ........................................ 538 5. Outlook ........................................................ 539 References......................................................... 540 Volume II Chapter 8: Trends in the state of human society in relation to the ocean.............. 1 Chapter 8A: Coastal communities andɸmaritime industries ...................... 3 Keynote points ..................................................... 5 1. Introduction..................................................... 5 2. Coastal communities............................................. 6 3. Capture Ƽsheries, shellƼsh harvesting and aquaculture . . . . . . . . . . . . . . . . 9 4. Shipping........................................................ 10 5. Seabed mining .................................................. 14 6. Offshore hydrocarbons ........................................... 15 7. Tourism and recreation ........................................... 15 8. Marine genetic resources ......................................... 20 9. Marine renewable energy ......................................... 21 10. Desalinization................................................... 21 11. Salt production.................................................. 22 12. Key knowledge and capacity-building gaps .......................... 23 13. Outlook ........................................................ 24 References......................................................... 24 Chapter 8B: Human health as affected by the ocean ............................ 31 Keynote points ..................................................... 33 1. Introduction..................................................... 33 2. General aspects of the relationship between human health and the ocean 33 3. Health of coastal communities relative to inland communities.......... 39 4. Effects of exposure to contaminated seawater ....................... 39 xviiiŵŵŵ World Ocean Assessment II:ŴVolume I Page 5. Problems for human health posed by food from the sea ............... 42 6. Key remaining knowledge and capacity-building gaps................. 44 7. Outlook ........................................................ 45 References......................................................... 45 Part Ƽve: Trends in pressures onɸthe marine environment .......................... 53 Chapter : Pressures from chanKes in climate and atmosphere ..................... 55 Keynote points ..................................................... 57 1. Introduction..................................................... 57 2. Climate pressures: extreme climate events and pressures from changes in ocean physical and chemical properties........................... 58 3. Capacity-building: Global Ocean AcidiƼcation Observing Network and Global Ocean Oxygen Network .................................... 67 4. Summary....................................................... 68 References......................................................... 69 Chapter 1: ChanKes in nutrient inputs to the marine environment................... 77 Keynote points ..................................................... 79 1. Introduction..................................................... 79 2. Situation reported in the Ƽrst World Ocean Assessment ............... 81 3. Global-scale patterns and trends................................... 82 4. Patterns and trends within regions ................................. 85 5. Outlook ........................................................ 91 References......................................................... 92 Chapter 11: ChanKes in liUuid and atmospheric inputs to the marine environment from land includinK throuKh Kroundwater ships and offshore installations............... 101 Keynote points ..................................................... 103 1. Introduction..................................................... 104 2. Situation recorded in the Ƽrst World Ocean Assessment ............... 104 3. Persistent organic pollutants, including run-off from the use of agricultural pesticides............................................ 105 4. Metals ......................................................... 112 5. Radioactive substances .......................................... 122 6. Pharmaceuticals and personal care products ........................ 127 7. Atmospheric pollutants (nitrogen oxides, sulfur oxides) ............... 131 8. Hydrocarbons from terrestrial sources, ships and offshore installations, including arrangements for response to spills and discharges .......... 132 9. Other substances used on, and discharged from, offshore installations . . 134 10. Relationship to the Sustainable Development Goals .................. 135 11. Key remaining knowledge gaps .................................... 136 Ŵŵŵŵxix Contents Page 12. Key remaining capacity-building gaps............................... 138 References......................................................... 139 Chapter 12: ChanKes in inputs and distribution of solid waste other than dredKed material in the marine environment............................................. 151 Keynote points ..................................................... 153 1. Activities resulting in marine debris, including plastics, abandoned Ƽshing gear, microparticles and nanoparticles, and estimates of sources from land, ships and offshore installations ............................... 153 2. Dumping at sea, including garbage from ships and sewage sludge . . . . . . 171 References......................................................... 177 Chapter 1: ChanKes in erosion and sedimentation................................ 185 Keynote points ..................................................... 187 1. Introduction..................................................... 187 2. Changes in state of coastal erosion and sedimentation................ 188 3. Consequences of the changes for human communities, economies andɸwell-being................................................... 192 4. Key region-speciƼc changes and consequences...................... 193 5. Outlook ........................................................ 195 6. Key remaining knowledge and capacity-building gaps ................ 195 References......................................................... 196 Chapter 14: ChanKes in coastal and marine infrastructure.......................... 201 Keynote points ..................................................... 203 1. Introduction..................................................... 203 2. Documented changes in the state of marine and coastal infrastructures . 204 3. Consequences of changes for human communities, economies andɸwell-being................................................... 207 4. Key region-speciƼc changes and consequences...................... 207 5. Outlook ........................................................ 210 6. Key remaining knowledge and capacity-building gaps................. 211 References......................................................... 211 Chapter 1: ChanKes in capture Ƽsheries and harvestinK of wild marine invertebrates.. 215 Keynote points ..................................................... 217 1. Introduction .................................................... 217 2. Catch-landing disparities, Sustainable Development Goals and small-scale Ƽsheries ......................................... 220 3. Invertebrate landings ............................................ 224 4. Levels of by-catch and side effects ................................ 225 5. Post-harvest Ƽsh losses .......................................... 225 xxŵŵŵ World Ocean Assessment II:ŴVolume I Page 6. Potential for Ƽsheries enhancement ................................ 225 7. Marine protein and oils in agriculture and aquaculture................. 225 8. Illegal, unreported or unregulated Ƽshing ............................ 226 9. Outlook ........................................................ 227 10. Key knowledge gaps ............................................. 228 11. Key capacity-building gaps........................................ 228 References......................................................... 228 Chapter 1: ChanKes in aUuaculture............................................. 235 Keynote points ..................................................... 237 1. Current status and major improvements ............................ 237 2. Aquaculture and the environment .................................. 240 3. Aquaculture and society .......................................... 241 4. Key remaining knowledge gaps .................................... 241 5. Key remaining capacity-building gaps............................... 242 6. Outlook ........................................................ 243 References......................................................... 244 Chapter 17: ChanKes in seaweed harvestinK andɸuse .............................. 247 Keynote points ..................................................... 249 1. Introduction..................................................... 249 2. Documented changes in the state of seaweed production and uses (2012–2017) .................................................... 250 3. Consequences of changes in seaweed harvesting and use for communities, economies andɸwell-being ............................ 253 4. Key region-speciƼc changes and consequences...................... 253 5. Outlook ........................................................ 254 6. Key remaining knowledge and capacity-building gaps................. 254 References......................................................... 255 Chapter 18: ChanKes in seabed mininK .......................................... 257 Keynote points ..................................................... 259 1. Introduction..................................................... 259 2. Changes in scale and signiƼcance of sea ƽoor mining ................ 262 3. Environmental aspects ........................................... 270 4. Economic and social impacts...................................... 273 5. Capacity-building needs .......................................... 276 References......................................................... 277 Chapter 1: ChanKes in hydrocarbon e\ploration and e\traction .................... 281 Keynote points ..................................................... 283 1. Introduction .................................................... 283 Ŵŵŵŵxxi 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 xxiiŵŵŵ World Ocean Assessment II:ŴVolume I 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 Ŵŵŵŵxxiii 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 83 Chapter 5 Trends in the physical and chemical state ofthe ocean Contributors: Carlos Garcia-Soto (convener and lead member), Levke Caesar, Anny Cazenave, Lijing Cheng, Alicia Cheripka, Paul Durack, Karen Evans (co-lead member), David Halpern, Libby Jewett, Sung Yong Kim, Guancheng Li, Ignatius Rigor, Sunke Schmidtko, Juying Wang (co-lead member) and Tymon Zielinski (co-lead member). 90 World Ocean Assessment II:Volume I Figure IV 8renH of tLe strenKtL of tLe %tlantiG meriHional overturninK GirGulation in observations sinGe 00 from various Tro\ies 2 1 0 -1 -2 -3 12 11 10 9 34 33 32 31 30 29 3 2 1 0 -1 -2 -3 3.3 3.1 2.9 2.7 2.5 Caesar and others, 2018: global sea surface data Rahmstorf and others, 2015: terrestrial proxies Thornalley and others, 2018: subsurface temperatures Sherwood and others, 2011: deep-sea coral data Thornalley and others, 2018: sortable silt data Smeed and others, 2018: RAPID project measurements Thibodeau and others, 2018: benthic foraminifera core data Temperature anomaly (K) 1.25 1.00 0.75 0.50 0.25 0.00 -0.25 -0.50 1.50 190018001700160015001400 2000 Year Normalized temperature proxy Bulk 15 N (‰) 56JPC ss (μm) Atlantic meridional overturning circulation anomaly (Sv) 18 O CR02-23 (‰ Vienna Pee Dee Belemnite) The gure shows the long-term evolution of the sea surface and land temperatures in the North Atlantic region (light blue (Caesar and others, 2018), navy (Rahmstorf and others, 2015), blue (Thornalley and others, 2018)), data from deep-sea cores (dark green (Sherwood and others, 2011), light green (Thornalley and others, 2018), orange (Thibodeau and others, 2018)) and the linear trend of in situ circulation monitoring by the RAPID project (tan (Smeed and others, 2018)). Figure V 8renH of tLe strenKtL of tLe %tlantiG meriHional overturninK GirGulation in observations Atlantic meridional overturning circulation anomaly (Sv) 0 -2 -4 2 200019801960194019201880 2020 Year 1900 Sea surface temperature proxy RAPID project Meteorological Oce Global Seasonal Forecasting System, version 5 Satellite altimetry and cable The gure shows the long-term (20-year locally weighted scatterplot smoothing ltering; blue, navy and green lines are annual values) sea surface temperature proxy (tan), the quadratic trend of an ocean reanalysis product (Meteorological Oce Global Seasonal Forecasting System, version 5; Jackson and others, 2016), a reconstruction from satellite altimetry and cable measurements (Frackja-Williams, 2015) and the linear trend of in situ circulation monitoring by the RAPID project. Source: Caesar and others, 2018. The impacts of the changes in the ocean circulation system vary. The Atlantic meridional overturning circulation is crucial for meridional heat transport and therefore strongly inuences the climate in the North Atlantic region. Its slowdown can reduce ocean carbon uptake (Zickfeld and others, 2008) and will enhance sea level rise along the east coast of the United States of America (Goddard and others, 2015). The stronger North Pacic subtropical gyre, however, leads to regional sea level rise in the western tropical North Pacic Ocean (Timmermann and others, 2010). These are the dynamic responses of sea level height to changes in ocean circulation. The poleward displacement of the western boundary currents leads to warming in regions previously unaffected by those warm and strong currents. The consequent thermal expansion will cause a rise in sea level in adjacent coastal areas, such as in the Southern Ocean and the Indian Ocean (Alory and others, 2007; Gille, 2008). Other possible impacts that need further investigation include changes in marine ecosystems and primary production, given 91 Chapter 5: Trends in the physical and chemical state of the ocean that currents transport nutrients, and effects on weather systems, such as the occurrence of heatwaves, droughts or ooding, because ocean circulation has a considerable impact on atmospheric circulation, and with that precipitation, patterns (Duchez and others, 2016). 2.3. Sea temperature and ocean heat content Sea surface temperature The global sea surface temperature analyses assessed here are derived from four published data sets (see gure VI). All data sets reveal an increase in global mean sea surface temperature since the early twentieth century. The globally averaged sea surface temperature data as calculated by a linear trend over the period 1900–2018 show an incontrovertible warming of 0.60°C±0.07°C (centennial in situ observation-based estimates of sea surface temperature, version 1, COBE1) (Ishii and others, 2005), 0.62°C±0.11°C (centennial in situ observation-based estimates of sea surface temperature, version 2, COBE2) (Hirahara and others, 2014), 0.56°C ±0.07°C (Hadley Centre sea ice and sea surface temperature data set, HadISST) (Rayner and others, 2003), 0.72°C ±0.10°C (extended reconstructed sea surface temperature, ERSST) (Huang and others, 2017)percentury (c-1), with a 90percent condence interval provided. Considering all data sets, the mean sea surface temperature rate is 0.62°C ±0.12°C c-1 over the same period. Differences between the data sets are mainly due to how each methodology treats areas with little or no data, and how each analysis accounts for changes in measurement methods. Among all data sets, the 10 warmest years on record have all occurred since 1997, with the 5 warmest years occurring since 2014. The recent decade (2009–2018) shows a much higher rate of warming than the long-term trend: 2.41°C±1.79°C (COBE1), 2.97°C±1.81°C (COBE2), 2.05°C±1.85°C (HadISST) and 2.81°C±1.98°C (ERSST) c-1. The mean rate is 2.56°C±0.68°Cc-1 in the period 2009–2018. In addition to the in situ observations, satellite-based data gave consistent changes in sea surface temperature in the period from 1981 to 2016 (Good and others, 2020; see also gureVI). Most ocean areas around the globe are warming (see gure VI.B). The broad warming over the global ocean surface is direct evidence of human inuence on the climate system (Bindoff and others, 2013). A few regions, such as the subpolar North Atlantic Ocean, have experienced cooling over the past century (often named the “cold blob” or the “North Atlantic warming hole”). A number of studies suggest that the “cold blob” indicates a weakening Atlantic meridional overturning circulation, possibly in response to increased CO2 concentrations in the atmosphere (Caesar and others, 2018). On other hand, lower warming rates have characterized the Equatorial Pacic and Eastern Tropical Pacic. In the South-East Pacic, from central Peru to northern Chile, a multidecadal surface cooling trend was detected until the late 2000s (Gutiérrez and others, 2016, and references therein), probably associated with coastal upwelling enhancement or remotely driven circulation changes (Dewitte and others, 2012). Ocean heat content Climate change from human activities is mainly due to interference with the natural ows of energy through the climate system, creating an energy imbalance caused by increased heat-trapping (greenhouse) gases (Hansen and others, 2011; Trenberth and others, 2018) in the atmosphere. More than 90percent of the energy imbalance accumulates in the ocean (Rhein and others, 2013). The heat imbalance is manifested by the increase in ocean heat content. Locally, ocean heat content (OHC) can be estimated by integrating sea temperature (T) from ocean depth z1 to z2: OHC = cÇ Ʋz1 z2Ç8dz Where Ç is the density of the seawater and Cp is the specic heat capacity of the seawater. 92 World Ocean Assessment II:Volume I Figure VI.A Global averaKe surfaGe temTerature anomalies q' annual mean 0 -0.1 -0.5 0.2 200019801960194019201880 2020 Year 19001860 0.1 0.3 -0.2 -0.6 -0.3 -0.7 -0.4 -0.8 Centennial in situ observation-based estimates of sea surface temperature, version 1 (COBE1) Centennial in situ observation-based estimates of sea surface temperature, version 2 (COBE2) Extended reconstructed sea surface temperature, version 5 (ERSST5) Group for High Resolution Sea Surface Temperature Multi-Product Ensemble, version 2 (GMPE2) Hadley Centre sea ice and sea surface temperature data set (HadISST) 0.4 Note: In situ estimates are shown from the COBE1, COBE2, ERSST5, HadISST and GMPE2 data sets Figure VI.B 7Tatial Tattern of tLe lonK-term sea surfaGe temTerature trenH q'ɸTerɸGentury from 1854 to 2018 for ERSST data 60°E 180°W 60°W 0° 30°S 60°S 60°N 30°N 0° 0.75 0.50 0.25 0.00 -0.25 -0.75 -0.50 Note: All data use a common 1981–2010 baseline. Black dot signs indicate grid boxes where trends are signicant (i.e., a trend of 0 lies outside the 90percent condence interval). The Earth’s energy imbalance and ocean heat content are the fundamental metrics for global warming (Hansen and others, 2011; Trenberth and others, 2018; Von Schuckmann and others, 2016; Cheng and others, 2018). The ocean heat content record is much less affected by internal variability in the climate system than the more commonly used sea surface temperature records, so it is better suited to detecting and attributing human inuences (Cheng and others, 2018) than other measures. Since the fth assessment report of the Intergovernmental Panel on Climate Change (Rhein and others, 2013), substantial progress has been made in improving long-term ocean heat content records, and a number of sources of uncertainty in prior measurements and analyses have been identied and are better accounted for (Abraham and others, 2013; Boyer and others, 2016; Cheng and others, 2016, 2017a; Ishii and others, 2017). At the same time, efforts have been made to improve how spatial or temporal gaps are accounted for in historical ocean temperature measurements. For example, a new spatial interpolation method was proposed (Cheng and others, 2017a), and a correction to an existing estimate was made available (Ishii and others, 2017). It is becoming clearer that many traditional gap-lling strategies introduced a conservative bias towards low-magnitude changes. Those with less bias include Cheng and others (2017a), Domingues and others (2008) and Ishii and others (2017). The three recent ocean heat content estimates based on observations show highly consistent ocean warming since the late 1950s (see gureVII). They suggest a linear rate of 0.36±0.06 Wm-2 (Ishii and others (2017) and 0.33±0.10 Wm-2 (Cheng and others, 2017a) (averaged over the Earth’s surface) in the period 1955–2018, with the mean rate of 0.34±0.08 Wm-2 among all data sets. The new estimates are collectively higher than previous estimates (Rhein and others, 2013) and more consistent with each other (Cheng and others, 2019a). The rate of ocean warming for the upper 2,000m has increased in the decades after the 1990s, with linear trends of 0.58±0.06 Wm-2 (Cheng and others, 2017a), 0.61±0.08 Wm-2 (Ishii and others, 2017) and 0.66±0.02 Wm-2 (Domingues and others, 2008; Levitus and others, 2012) between 1999 and 2018. The mean rate is 0.62±0.05 Wm-2. In the recent decade (2009–2018), the rate of ocean heat content increase is: 0.56±0.06 Wm-2 (Cheng and others, 93 Chapter 5: Trends in the physical and chemical state of the ocean 2017a), 0.66±0.09 Wm-2 (Ishii and others, 2017) and 0.66±0.03 Wm-2 (Domingues and others, 2008; Levitus and others, 2012). The mean rate is 0.65±0.07 Wm-2. For ocean heat content, the past 10 years are the 10 warmest on record (Cheng and others, 2019a), as the heat content is less affected by natural variability. Increases in ocean heat content are observed practically throughout the global ocean, to a depth of 2,000 m (see gureVII). Some intriguing patterns emerge for long-term content change in the period 1960–2018: stronger warming in the Southern Ocean (approximately 70° south to approximately 40° south) and Atlantic Ocean (approximately 40° south to approximately 50° north) than other regions and weaker warming throughout the Pacic Ocean and Indian Ocean (approximately 30° south to approximately 60° north) (see gureVII). The long-term warming of the Southern Ocean has been identied and attributed primarily to greenhouse gases (Cheng and others, 2017a; Swart and others, 2018), driven predominantly by air-sea ux changes associated with upper-ocean overturning circulation and mixing (Swart and others, 2018). Southern Ocean warming has important consequences owing to its inuence on the southern hemisphere ice reservoir. Near-surface Southern Ocean heat content is key in limiting the seasonal development of sea ice, and warming can therefore feed back into the global climate by limiting the Earth’s albedo. In addition, ocean warming accelerates the melting of Antarctic ice shelves, threatening the stability of the Antarctic ice sheet, with global implications in terms of sea level rise (Sallée and others, 2018). Over the period 1998–2013, a slowdown in the increase of sea surface temperature and global surface temperature led to numerous assertions about a “climate hiatus” (Hartmann, 2013). The updated record until 2018 (see gureV) shows that the linear trend of sea surface temperature for the period 1998–2018 is 1.25°C±0.52°C c-1, which is greater than the linear trend during the reference period (1982– 1997) (1.00°C±0.46°C c-1). That effectively indicates the end of the slowdown in surface temperature increase with the appearance of the extreme 2015/16 El Niño event (Hu and Fedorov, 2017). In addition, it is clear that the rate of ocean heat content increase has risen since the late 1990s (see gureVII). The unabated increase in the rate of sea surface temperature and ocean heat content refute the concept of a slowdown of human-induced global warming. Figure VII.A 3bservational oGean Leat Gontent GLanKes 0 -50 100 20102000199019801960 2020 Year 19701950 50 150 -100 -150 -200 -250 200 Cheng Ishii Domingues and Levitus Note: Annual mean for the upper 2,000 m, in zettajoules (1021 joules) (Cheng and others, 2017a; Domingues and others, 2008; Levitus and others, 2012; Ishii and others, 2017). The estimate of Domingues (0–700 m) is combined with that of Levitus (700–2,000 m) to produce a 0–2,000 m time series, following the fth assessment report of the Intergovernmental Panel on Climate Change (Rhein and others, 2013). Figure VII.B STatial Tattern of lonK-term oGean Leat content trend (Wm-2 155Ɓ2018 60°E 180°W 60°W 0° 30°S 60°S 60°N 30°N 0° 1.5 1.0 0.5 0.0 -0.5 -1.5 -1.0 Note: All data use a common 1981–2010 baseline. Black dot signs indicate grid boxes where trends are signicant (i.e., a trend of 0 lies outside the 90percent condence interval). Source: Cheng and others, 2017a. 94 World Ocean Assessment II:Volume I 2.4. Salinity The studies described in the fourth and fth assessment reports of the Intergovernmental Panel on Climate Change documented spatial patterns in near-surface and subsurface salinity that represent long-term change (Bindoff and others, 2007; Rhein and others, 2013). In the rst World Ocean Assessment (United Nations, 2017), the marked long-term multidecadal changes to global ocean salinity were documented throughout the historical period. The studies noted above provide clear evidence that the near-surface, high-salinity subtropical ocean regions and the entire Atlantic basin have become more saline, and low-salinity regions, such as the West Pacic Warm Pool, and high latitude regions have become fresher when comparing the earlier historical data (from about the 1950s) with present-day salinities (e.g., Boyer and others, 2005; Hosoda and others, 2009; Durack and Wijffels, 2010; Helm and others, 2010; Skliris and others, 2014). The pattern of changes reects an amplication of climatological mean salinity and has been linked through model simulations (e.g., Durack and others, 2012, 2013; Terray and others, 2012; Vinogradova and Ponte, 2013; Durack, 2015; Levang and Schmitt, 2015; Zika and others, 2015) to indicate a coincident amplication of the atmospheric water cycle (e.g., Held and Soden, 2006). While long-term historical assessments of change are complicated by the sparse observing network extending back to the mid-twentieth century, recent assessments leverage the comprehensive global ocean coverage of Argo prole data from 2008 to the near-present. As the modern observations provide only 10 years of temporal coverage (2008 to the present), estimated changes are more strongly affected by unforced variability modes, which inuence ocean salinity regionally more than long-term estimates, but their spatial and temporal coverage allows for more accurate estimates of change. The latest Argo-only analyses have shown for the rst time that nearly all salinity anomalies in 2017 in the Atlantic between 0 and 1,500 m are positive (> 0.05 Practical Salinity Scale-78), mirroring the long-term trends noted above, with the Pacic showing a general freshening, similar to long-term trends. Since the rst Assessment, salinity retrievals from the Soil Moisture and Ocean Salinity Aquarius and Soil Moisture Active Passive satellites (e.g., Berger and others, 2002; Lagerloef and others, 2008; Tang and others, 2017) have become more prominent. While satellite salinity data are only available since 2010 and work is ongoing to intercompare and homogenize data products across satellite platforms, they are beginning to provide key insights into ocean salinity variability owing to precipitation events (e.g., Boutin and others, 2013, 2014; Drushka and others, 2016). In addition, the comparative high temporal and spatial coverage of satellite salinity, when contrasted with the in situ platforms (e.g., Argo), for the rst time provides insights into water cycle interactions with the terrestrial and oceanic water cycles, such as the Amazon outlet plume (Grodsky and others, 2014). Considering all available analyses, it is extremely likely that near-surface and subsurface salinity changes have occurred across the globe since the 1950s. A salinity pattern amplication is apparent, with fresh regions becoming fresher and salty regions becoming saltier, and is supported by all available observational studies that have considered salinity change since the advent of instrumental records. For example, high-latitude oceans have shown signicant rates of freshening. More modern assessments are currently too short to conrm consistent changes over the past decade. However, the most recent analyses suggest that consistent patterns are beginning to emerge for the Atlantic and, to a lesser degree, the upper Pacic Ocean basins. 95 Chapter 5: Trends in the physical and chemical state of the ocean 2.5. Ocean acidification Global surface ocean pH has declined on average by approximately 0.1 since the Industrial Revolution (Caldeira and others, 2003), an increase in acidity of about 30percent. Ocean pH is projected to decline, approximately, by an additional 0.2–0.3 over the next century (Caldeira and others, 2003; Feely and others, 2009) unless global carbon emissions are signicantly curtailed. Those changes can be observed in extended ocean time series (see gure VIII), and the rate of change is likely to be unparalleled in at least the past 66 million years (Hönisch and others, 2012; Zeebe and others, 2016). Carbonate chemistry varies according to large-scale oceanic features, including depth, distance from continents owing to land inuence, upwelling regime, freshwater and nutrient input and latitude (Jewett and Romanou, 2017). Owing to that variability, as determined by the various characteristics, only longer-term, observational time series can detect the predicted long-term increase in acidity at individual sites on account of rising atmospheric CO2 levels. The time of emergence of the signal varies from 8 to 15 years for open ocean sites and from 16 to 41 years for coastal sites (Sutton and others, 2019), making it necessary to commit to long-term observational records, especially in the coastal zone where most commercially and culturally important marine resources reside. It has now been documented that ocean acidi- cation is making it harder for some marine organisms, such as corals, oysters and pteropods (Hoegh-Guldberg and others, 2017; Lemasson and others, 2017; Bednarsek and others, 2016; Feely and others, 2004; Orr and others, 2005), to form calcium carbonate shells and skeletons. In some cases, ocean acidication has also been shown to lower tness in some species such as coccolithophores, crabs and sea urchins (Campbell and others, 2016; Dodd and others, 2015; Riebesell and others, 2017; Munday and others, 2009). Although individual species, when tested, are vulnerable to ocean acidication in laboratory settings, how that is going to translate into changes in actual ecosystems and species populations remains unclear and mostly undocumented (McElhany, 2017). Research efforts over the past decade have begun to build understanding of how marine species, ecosystems and biogeochemical cycles may be inuenced by ocean acidication alone and in concert with other stressors, including eutrophication, warming and hypoxia (Baumann, 2019; Murray, 2019). The interaction of ocean acidication in coastal zones with coastal processes, such as upwelling of undersaturated water and land-based nutrient inuxes, has become a high priority area of research (Borgesa and Gypensb, 2010; Feely and others, 2008). Natural variability in carbonate chemistry, such as coastal upwelling and seasonal uctuations in primary productivity, is compounded by anthropogenic changes to create particularly extreme ocean acidication conditions in some regions of the global ocean (Feely and others, 2008; Cross and others, 2014). Intensive national and international efforts focused on carbonate chemistry monitoring, biological observations and biogeochemical or ecological forecast modelling over the past decade have shed light on the status and impacts of ocean acidication from the local to the global level. Gaps in the current understanding of ocean chemistry are being addressed through global monitoring capacity-building efforts, such as the Global Ocean Acidication Observing Network, increased biological impact studies and biogeochemical ecosystem modelling. 96 World Ocean Assessment II:Volume I Figure VIII Trends in surface ( 50 m ocean carbonate chemistry calculated from observations obtained at the Hawaii Ocean Time-series 4roKram in the 2orth 4aciƼc from 188 to 2018 CO2 400 375 350 325 275 425 20061994198219701958 2018 300 8.28 8.23 8.18 8.13 8.03 8.33 8.08 pH Year Mauna Loa atmospheric CO2 (ppm) ALOHA seawater pCO2 in situ (μatm) ALOHA seawater pH (in situ) 160°W 158°W 156°W 22°N 21°N 20°N 23°N 19°N Mauna Loa station ALOHA station The gure shows the linked increase in atmospheric CO2concentrations (red), seawater pCO2concentrations (green) and a corresponding decline in seawater pH (blue, secondary y-axis). Ocean chemistry data were obtained from the Hawaii Ocean Time-series Data Organization and Graphical System Source: National Oceanic and Atmospheric Administration Pacic Marine Environmental Laboratory Carbon Program. 2.6. Dissolved oxygen Since chemical analysis methods have essentially not changed (Carpenter, 1965; Wilcock and others, 1981; Knapp and others, 1991), long-term oceanic oxygen trends can be estimated fairly robustly where there is sucient data coverage. Dissolved oxygen samples are analysed by Winkler titration, which was established in 1903 and has since been used to calibrate all means of oceanic dissolved oxygen measurements. That allows a robust analysis of long-term trends in all areas with sucient data coverage. Modern Winkler titration is computer-aided, providing analysis with higher accuracy, though a bias of historic measurements could not be shown (Schmidtko and others, 2017). The postulated possible bias of 0.5percent reagent changes (Knapp and others, 1991) was tested on a global oxygen data set and found to be very unlikely, since the mapped pattern of oxygen change for a deliberately introduced bias does not match any observed pattern (Schmidtko and others, 2017). In the open ocean, most regional long-term series data show a small long-term decrease despite temporal variations on many timescales (e.g., Keeling and others, 2010). Increasing oxygen levels are found only in very limited time series (Keeling and others, 2010). Coastal changes have mostly been fuelled by riverine run-off of fertilizers, but in some cases may have been affected by larger-scale oxygen changes. They can lead to an increased occurrence of dead zones, with consequences for the regional ecology and economy (Diaz and Rosenberg, 2008). Globally, the ocean has been losing oxygen in recent decades. Both methods, comparing decadal oxygen data snapshots and local regression analyses (Schmidtko and others, 2017; Ito and others, 2017), show large-scale oxygen declines (see gures IX.A and IX.B). Despite various methods, the derived rates agree within the same water layers and given uncertainties. Deoxygenation rates vary with depth and region, resembling the manifold processes modifying the oxygen content, with isolated regions showing an increase in oxygen. The overall oxygen budget has decreased by 2percent in the past ve decades, a loss of 4.8±2.1 petamoles since 1960 (Schmidtko and others, 2017). In the upper water column, temperature-driven solubility decrease is dominating (see gure IX.C). For the period 1970–2010, the oxygen concentration in the upper 1,000 m has decreased by 0.046±0.047 mol l-1 yr-1, including a solubility change of 0.025 mol l-1 yr-1 (Schmidtko and others, 2017). Analysing shallower layers increases the solubility-related change signicantly (see 97 Chapter 5: Trends in the physical and chemical state of the ocean gure IX.C), in accordance with the heat gain in the upper water column (see gure IX.C, upper section). However, for the full ocean column, solubility-driven changes from 1970 to 2010 are small, -0.006 mol l-1 yr-1 compared with the overall oxygen loss 0.063±0.031mol l-1 yr-1. Nevertheless, temperature cannot be ruled out as the key source of such changes, through mechanisms other than solubility change. The mechanisms include stratication increase, circulation changes and thermal impacts on biogeochemical cycles (e.g., Keeling and others, 2002; Bianchi and others, 2013; Stendardo and Gruber, 2012). Figure IX.A 1ean dissolved water column o\yKen concentration 25020015010050 3000 Dissolved oxygen (μmol/kg) Figure IX.B (issolved o\yKen chanKes inɸTerɸcent per decade 420-2-4 6-6 Dissolved oxygen change (per cent/decade) Note: Solid, dotted and dashed lines indicate the presence of low oxygen (40, 80 and 120 umol l-1) at some depth within the water column. Figure IX.C :ertical distribution of o\yKen loss per decade of o\yKen chanKe Vertical distribution of oxygen loss per decade of oxygen change Expected loss from temperature-driven solubility changes Error -0.6 -0.4 -0.2 0 D ept h ( m ) 600 400 200 800 1000 0 1000 5000 4000 3000 6000 2000  Tmol O2/decade/m Figure IX.D Water column cumulative o\yKen loss owinK to solubility chanKe as a percentaKe of observed deo\yKenation 0 Depth (m) 600 400 200 800 1000 1000 5000 4000 3000 6000 2000 050 150200100 Solubility-related O2 change, percentage Note: Solubility changes above 100per cent are due to processes that increase upper-ocean oxygen content and counteract warming. Source: Schmidtko and others, 2017. 98 World Ocean Assessment II:Volume I The area of oxygen minimum zones has typically been expanding in recent decades, although there is signicant regional variability (Diaz and Rosenberg, 2008). Oxygen minimum zones have potential impacts on climate change because they emit large quantities of nitrous oxide, a potent greenhouse gas, owing to denitrication processes under anoxic conditions (e.g., Codispoti, 2010; Santoro and others, 2011). In particular, oxygen minimum zones have increased in the Pacic Ocean and the Indian Ocean. 2.7. Sea ice Sea ice in the Arctic has been one of the most iconic indicators of climate change. During the boreal winter, the areal extent of Arctic sea ice reaches a maximum area of 15.4 x 106 km2 in March and, during the boreal summer, declines to 6.4 x 106 km2 in September. Arctic sea ice areal extent is declining by -2.7±0.4percent per decade during the winter (March 1979–2019), and -12.8±2.3percent per decade during the summer (September 1979–2018) (see gure X; Feterrer and others, 2017). While the decreasing trends during the winter are more evenly distributed around the pole, the summer trends are almost twice as high in the Pacic sector of the Arctic Ocean (upper right of maps, gureX). In that area, the changes in wind related to the Arctic Oscillation have been increasingly blowing the ice away from coastal areas and into the North Atlantic (Rigor and others, 2002), leaving in its wake a much younger and thinner ice pack (Rigor and Wallace, 2004). The thickness of Arctic sea ice has decreased by at least 40percent (Rothrock and others, 1999, comparing submarine observations from 1958 to 1976 and from 1993 to 1997)), and Kwok (2018) shows that those changes persist today. The observed trends in sea ice extent (area) and thickness together indicate that the volume of Arctic sea ice has decreased by over 75percent since 1979. That estimate is coincident with many modelling studies, such as the Pan-Arctic Ice Ocean Modeling and Assimilation System (Zhang and Rothrock, 2003; Schweiger and others, 2011), which estimates that the average volume of Arctic sea ice of 11.5 x 103 km3 in September has decreased between 1979 and 2017 by -2.8103 km3 per decade, with the record minimum in total ice volume set in 2010. In Antarctica, sea ice advances to its maximum extent of 19–20 x 106 km2 in September (austral winter) and decreases to a minimum of 3.1 x 106 km2 in February (austral summer). The trends in Antarctic sea ice extent are 0.6±0.6percent per decade during the summer (February 1979–2019) and 1.1±3.7percent per decade during the winter (September 1979–2018). Net Antarctic sea ice extent showed a statistically signicant increase from 1979 to 2015. From 2016 onwards, it has been consistently below average and has set new record low values. Given that the sudden variability in Antarctic sea ice cover is largely attributed to changes in the ocean mixed layer, it is highly relevant to expand the explanation. The net overall changes in sea ice cover have varied greatly between regions. That dichotomy between Arctic and Antarctic sea ice has been attributed to limits imposed by geography. During the winter, the maximum extent of sea ice is imposed by the Antarctic circumpolar currents and the underlying bathymetry of the Southern Ocean (Nghiem and others, 2016) and, during the summer, the sea ice can only retreat to the edge of the Antarctic continent. However, gure X (C and D) shows that, regionally, the trends are more pronounced. During the summer, sea ice extent is increasing in the Weddell Sea but decreasing in the Bellingshausen Sea and the Amundsen Sea (West Antarctica), where the ice sheet is more vulnerable to ocean processes. The regional trends in sea ice extent have been related to changes in wind (and ocean currents) related to the Southern Annular Mode and the El Niño Southern Oscillation (Parkinson, 2019; and references therein). The 40-year record reveals gradual Antarctic sea ice increases followed by decreases at rates far exceeding those seen in the Arctic. 99 Chapter 5: Trends in the physical and chemical state of the ocean Since sea ice oats on the ocean, the contribution of melting sea ice to sea level rise is negligible. However, sea ice acts as a shield, keeping insolation from warming the ocean, and acts as a buttress for land ice, which terminates over the ocean, keeping warm waters and waves from the ocean from eroding the ice sheet. The loss of sea ice has made many ice sheets more vulnerable and increased the rate of sea level rise owing to the melt of the terrestrial ice sheets (e.g., Stewart and others, 2019). Figure X %rctic and %ntarctic sea ice concentration trends (per cent per decade 8% 4% 0% A. September 2018 B. March 2019 C. September 2018 D. March 2019 Russia Greenland Canada Russia Greenland Canada South America South America East Antarctica West Antarctica East Antarctica West Antarctica 20% 16% 12% -4% -8% -12% -16% -20% per decade Area not imaged by sensor Trends for the Arctic are shown in the top row, and those for the Antarctic are shown in the bottom row, for September 1979–2018 in the left column, and for March 1979–2019 in the right column. Source: National Snow and Ice Data Center, University of Colorado Boulder; Fetterer and others, 2017. 106 World Ocean Assessment II:Volume I Ishii, Masayoshi, and others (2017). Accuracy of global upper ocean heat content estimation expected from present observational data sets. Sola, vol.13, pp.163–167. Johnson, Gregory C., and others (2015). Informing deep Argo array design using Argo and full-depth hydrographic section data. 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