Extremes, Abrupt Changes SPM6 and Managing Risks Coordinating Lead Authors Matthew Collins (UK), Michael Sutherland (Trinidad and Tobago) Lead Authors Laurens Bouwer (Netherlands), So-Min Cheong (Republic of Korea), Thomas Frölicher (Switzerland), Hélène Jacot Des Combes (Fiji), Mathew Koll Roxy (India), Iñigo Losada (Spain), Kathleen McInnes (Australia), Beate Ratter (Germany), Evelia Rivera-Arriaga (Mexico), Raden Dwi Susanto (Indonesia), Didier Swingedouw (France), Lourdes Tibig (Philippines) Contributing Authors Pepijn Bakker (Netherlands), C. Mark Eakin (USA), Kerry Emanuel (USA), Michael Grose (Australia), Mark Hemer (Australia), Laura Jackson (UK), Andreas Kääb (Norway), Jules Kajtar (UK), Thomas Knutson (USA), Charlotte Laufkötter (Switzerland), Ilan Noy (New Zealand), Mark Payne (Denmark), Roshanka Ranasinghe (Netherlands), Giovanni Sgubin (Italy), Mary-Louise Timmermans (USA) Review Editors Amjad Abdulla (Maldives), Marcelino Hernádez González (Cuba), Carol Turley (UK) Chapter Scientist Jules Kajtar (UK) This chapter should be cited as: Collins M., M. Sutherland, L. Bouwer, S.-M. Cheong, T. Frölicher, H. Jacot Des Combes, M. Koll Roxy, I. Losada, K. McInnes, B. Ratter, E. Rivera-Arriaga, R.D. Susanto, D. Swingedouw, and L. Tibig, 2019: Extremes, Abrupt Changes and Managing Risk. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. In press. 589 Chapter 6 Extremes, Abrupt Changes and Managing Risks Table of contents Executive Summary ..................................................................................... 591 6.7 Risks of Abrupt Change in Ocean Circulation and Potential Consequences ................................................... 618 6.1 Introduction ........................................................................................... 594 6.7.1 Key Processes and Feedbacks, Observations, Detection and Attribution, Projections .................. 618 6.1.1 Definitions of Principal Terms ....................................... 594 6.7.2 Impacts on Climate, Natural 6.2 Climate Change influences on Abrupt and Human Systems ......................................................... 621 Changes, Irreversibility, Tipping Points 6.7.3 Risk Management and Adaptation ........................... 623 and Extreme Events ........................................................................ 595 6.8 Compound Events and Cascading Impacts ................. 624 6.2.1 Introduction ............................................................................. 595 6.2.2 Recent Anomalous Extreme Climate 6.8.1 Concepts .................................................................................... 624 Events and their Causes ................................................... 596 6.8.2 Multiple Hazards .................................................................. 624 6.8.3 Cascading Impacts on Ecosystems .......................... 624 6.3 Changes in Tracks, Intensity, and Frequency 6.8.4 Cascading Impacts on Social Systems .................... 625 of Tropical and Extratropical Cyclones and 6.8.5 Risk Management and Adaptation, Associated Sea Surface Dynamics ..................................... 601 Sustainable and Resilient Pathways ......................... 625 6.3.1 Changes in Storms and Associated 6.8.6 Global Impact of Tipping Points .................................. 626 Sea Surface Dynamics ....................................................... 601 Box 6.1: Multiple Hazards, Compound 6.3.2 Impacts ....................................................................................... 605 Risk and Cascading Impacts .................................... 626 6.3.3 Risk Management and Adaptation ........................... 606 6.9 Governance and Policy Options, 6.4 Marine Heatwaves and their Implications .................. 606 Risk Management, Including Disaster 6.4.1 Observations and Key Processes, Risk Reduction and Enhancing Resilience .................... 629 Detection and Attribution, Projections .................. 607 6.9.1 Decision Making for Abrupt Change 6.4.2 Impacts on Natural, Physical and Extreme Events .......................................................... 629 and Human Systems ......................................................... 610 6.9.2 Transformative Governance and 6.4.3 Risk Management and Adaptation, Integrating Disaster Risk Reduction Monitoring and Early Warning Systems ................. 611 and Climate Change Adaptation ................................ 630 6.5 Extreme ENSO Events and Other Modes 6.10 Knowledge Gaps ............................................................................... 633 of Interannual Climate Variability ....................................... 612 6.5.1 Key Processes and Feedbacks, Observations, References .......................................................................................................... 634 Detection and Attribution, Projections ................... 612 6.5.2 Impacts on Human and Natural Systems .............. 614 Frequently Asked Questions ................................................................ 632 6.5.3 Risk Management and Adaptation ........................... 615 FAQ 6.1 How can risks of abrupt changes in the ocean and cryosphere related 6.6 Inter-Ocean Exchanges and Global Change ................ 616 to climate change be addressed? ........... 632 6 6.6.1 Key Processes and Feedbacks, Observations, 6 Detection and Attribution, Projections ................... 616 6.6.2 Impacts on Natural and Human Systems .............. 617 590 Extremes, Abrupt Changes and Managing Risks Chapter 6 Executive Summary Both palaeoclimate and modern observations suggest that the strongest El Niño and La Niña events since the pre-industrial This chapter assesses extremes and abrupt or irreversible changes in period have occurred during the last fifty years medium( the ocean and cryosphere in a changing climate, to identify regional confidence). There have been three occurrences of extreme El hot spots, cascading effects, their impacts on human and natural Niño events during the modern observational period (1982–1983, systems, and sustainable and resilient risk management strategies. 1997–1998, 2015–2016), all characterised by pronounced rainfall It is not comprehensive in terms of the systems assessed and some in the normally dry equatorial East Pacific. There have been two information on extremes, abrupt and irreversible changes, in particular occurrences of extreme La Niña (1988–1989, 1998–1999). El for the cryosphere, may be found in other chapters. Niño and La Niña variability during the last 50 years is unusually high compared with average variability during the last millennium. {6.5, Figure 6.5} Ongoing and Emerging Changes in the Ocean and Cryosphere, and their Impacts on Ecosystems The equatorial Pacific trade wind system experienced an and Human Societies unprecedented intensification during 2001–2014, resulting in enhanced ocean heat transport from the Pacific to the Indian Anthropogenic climate change has increased observed Ocean, influencing the rate of global temperature change precipitation (medium confidence), winds (low confidence), (medium confidence). In the last two decades, total water transport and extreme sea level events (high confidence) associated from the Pacific to the Indian Ocean by the Indonesian Throughflow (ITF), with some tropical cyclones, which has increased intensity of and the Indian Ocean to Atlantic Ocean has increased (high confidence). multiple extreme events and associated cascading impacts Increased ITF has been linked to Pacific cooling trends and basin-wide (high confidence). Anthropogenic climate change may have warming trends in the Indian Ocean. Pacific sea surface temperature contributed to a poleward migration of maximum tropical cyclone (SST) cooling trends and strengthened trade winds have been linked to intensity in the western North Pacific in recent decades related to an anomalously warm tropical Atlantic. {6.6, Figure 6.7} anthropogenically-forced tropical expansion (low confidence). There is emerging evidence for an increase in the annual global Observations, both in situ (2004–2017) and based on sea proportion of Category 4 or 5 tropical cyclones in recent decades surface temperature reconstructions, indicate that the Atlantic (low confidence). {6.3, Table 6.2, Figure 6.2, Box 6.1} Meridional Overturning Circulation (AMOC) has weakened relative to 1850–1900 (medium confidence). There is insufficient Changes in Arctic sea ice have the potential to influence data to quantify the magnitude of the weakening, or to properly mid-latitude weather (medium confidence), but there is low attribute it to anthropogenic forcing due to the limited length of the confidence in the detection of this influence for specific observational record. Although attribution is currently not possible, weather types. {6.3} CMIP5 model simulations of the period 1850–2015, on average, exhibit a weakening AMOC when driven by anthropogenic forcing. Extreme wave heights, which contribute to extreme sea level {6.7, Figure 6.8} events, coastal erosion and flooding, have increased in the Southern and North Atlantic Oceans by around 1.0 cm yr–1 and Climate change is modifying multiple types of climate-related 0.8 cm yr–1 over the period 1985–2018 (medium confidence). events or hazards in terms of occurrence, intensity