Editing IPCC:AR6/SROCC/Chapter-6 (section)

== 6.1 Introduction ==

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This chapter assesses extremes and abrupt or irreversible changes in the ocean and cryosphere in a changing climate, to identify regional hot spots, cascading effects, their impacts on human and natural systems, and sustainable and resilient risk management strategies. While not comprehensive in terms of discussing all such phenomena, it addresses a number of issues that are prominent in both the policy area and in the scientific literature. Further information may also be found in Chapters 2 to 4 for other aspects of the ocean and cryosphere.

Building on the Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX; IPCC, 2012 <sup>[[#fn:r1|1]]</sup> ), IPCC 5th Assessment Report (AR5; IPCC, 2013 <sup>[[#fn:r2|2]]</sup> ; IPCC, 2014 <sup>[[#fn:r3|3]]</sup> ) assessments and the Special Report on Global Warming of 1.5 '''°''' C (SR15; IPCC, 2018 <sup>[[#fn:r4|4]]</sup> ), for each of the topics addressed, we provide an assessment of:

* Key processes and feedbacks, observations, detection and attribution, projections;
* Impacts on human and natural systems;
* Monitoring and early warning systems;
* Risk management and adaptation, sustainable and resilient pathways.

The chapter is organised in terms of the space- and time-scales of different phenomena. We move from small-scale TCs, which last for days to weeks, to the global-scale AMOC, which has time scales of decades to centuries. A common risk framework is adopted, based on that used in AR5 and introduced in Chapter 1, Section 1.5 and Cross-Chapter Box 1 in Chapter 1 (Figure 6.1).

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'''Figure 6.1'''

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'''Figure 6.1 | Framework used in this chapter (see discussion in Chapter 1). Singular or multiple climate drivers can lead to extreme hazards and associated cascading impacts, which combined with non-climatic drivers affect exposure and vulnerability, leading to compound risks. Extremes discussed are tropical cyclones (TCs) and extratropical cyclones (ETCs) and associated sea surface dynamics (Section […]'''
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Figure 6.1 | Framework used in this chapter (see discussion in Chapter 1). Singular or multiple climate drivers can lead to extreme hazards and associated cascading impacts, which combined with non-climatic drivers affect exposure and vulnerability, leading to compound risks. Extremes discussed are tropical cyclones (TCs) and extratropical cyclones (ETCs) and associated sea surface dynamics (Section 6.3); marine heatwaves (MHWs) (Section 6.4), extreme El Niño and La Niña events (Section 6.5); and extreme oceanic decadal variability (Section 6.6). Examples of abrupt events, irreversibility and tipping points discussed are the Atlantic Meridional Overturning Circulation (AMOC) and subpolar gyre (SPG) system (Section 6.7). Section 6.2 also collects examples of such events from the rest of the Special Report on the Oceans and Cryosphere in a Changing Climate (SROCC) and compiles examples of events whose occurrence or severity has been linked to climate change. Cascading impacts and compound events are discussed in Section 6.8 and three examples are given in Box 6.1. Section 6.9 discusses risk management, climate resilience pathways, transformative governance adaptation and mitigation required to address societal and environmental risks.

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While much of what is discussed within the chapter concerns the ocean, we also summarise abrupt events in the cryosphere in Section 6.2, drawing information from Chapters 2 to 4, where the main assessment of those phenomena may be found.

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=== 6.1.1 Definitions of Principal Terms ===

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In discussing concepts such as abrupt changes, irreversibility, tipping points and extreme events it is important to define precisely what is meant by those terms. The following definitions are therefore adopted (based on either AR5, Special Report on Global Warming of 1.5°C (SR15) or Special Report on Climate Change and Land (SRCCL) Glossaries):

'''Abrupt climate change:''' A large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems.

'''Extreme weather/climate event:''' An extreme event is an event that is rare at a particular place and time of year. Definitions of ‘rare’ vary, but an extreme event would normally be as rare as or rarer than the 10th or 90th percentile of a probability density function estimated from observations. By definition, 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).

'''Irreversibility:''' A perturbed state of a dynamical system is defined as irreversible on a given timescale, if the recovery timescale from this state due to natural processes is significantly longer than the time it takes for the system to reach this perturbed state. In the context of this report, the recovery time scale of interest is hundreds to thousands of years.

'''Tipping point:''' A level of change in system properties beyond which a system reorganises, often in a nonlinear manner, and does not return to the initial state even if the drivers of the change are abated. For the climate system, the term refers to a critical threshold when global or regional climate changes from one stable state to another stable state. Tipping points are also used when referring to impact; the term can imply that an impact tipping point is (about to be) reached in a natural or human system.

These above four terms generally refer to aspects of the physical climate system. Here we extend their definitions to natural and human systems. For example, there may be gradual physical climate change which causes an irreversible change in an ecosystem. An adaptation tipping point could be reached when an adaptation option no longer remains effective. There may be a tipping point within a governance structure.

We also introduce two new key terms relevant for discussing risk-related concepts:

'''Compound events''' refer to the combination of multiple drivers and/or hazards that contribute to societal or environmental risks.

'''Cascading impacts''' from extreme weather/climate events occur when an extreme hazard generates a sequence of secondary events in natural and human systems that result in physical, natural, social or economic disruption, whereby the resulting impact is significantly larger than the initial impact. Cascading impacts are complex and multi-dimensional, and are associated more with the magnitude of vulnerability than with that of the hazard.

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