Editing IPCC:AR6/SROCC/Chapter-5 (section)

== 5.1 Introduction ==

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The ocean is a key component of the Earth system (Chapter 1) as it provides essential life supporting services (Inniss et al., 2017 <sup>[[#fn:r1|1]]</sup> ). For example, it stores heat trapped in the atmosphere caused by increasing concentrations of greenhouse gases, it masks and slows surface warming, it stores excess carbon dioxide and is an important component of global biogeochemical cycles. The ocean is the home to the largest continuous ecosystem, provides habitats for rich marine biodiversity, is an essential source of food and contributes to human health, livelihood and security. The ocean also supports other services to humans, for example, transport and trade, tourism, renewable energy, and cultural services such as aesthetic appeal, local and traditional knowledge and religious practices. Governance of the ocean has a unique set of challenges and opportunities compared with land systems and requiring different treatment under a changing climate.

AR5 from Working Group I (WGI) showed that there are ongoing changes to the physical and chemical state of the ocean. The AR5 WGI report (IPCC, 2013 <sup>[[#fn:r2|2]]</sup> ) concluded that (1) ‘ocean warming dominates the increased energy stored in the climate system with more than 90% of the energy accumulated since 1971’; (2) ‘the ocean has absorbed about 30% of the emitted anthropogenic carbon causing ocean acidification’ since pre-industrial times; and (3) it is ‘ ''extremely likely'' that human influence has been the dominant cause of warming since mid 20th century’.

The IPCC AR5 Working Group II (WGII) concluded that changes in the ocean such as warming, acidification and deoxygenation are affecting marine life from molecular processes to organisms and ecosystems, with major impacts on the use of marine systems by human societies (Pörtner, 2012 <sup>[[#fn:r3|3]]</sup> ). IPCC Special Report on the Impacts of Global Warming of 1.5 o C above pre-industrial levels and related global greenhouse gas emission pathways (SR15) also concluded that reducing these risks by ‘limiting warming to 1.5°C above pre-industrial levels would require transformative systemic change, integrated with sustainable development’ and that ‘adaptation needs will be lower in a 1.5°C world compared to a 2°C world.’ (de Coninck et al., 2018; Hoegh-Guldberg et al., 2018 <sup>[[#fn:r4|4]]</sup> ).

This report updates earlier assessments, evaluating new research and knowledge regarding changing ocean climate and ecosystems, risks to ecosystem services, and vulnerability of the dependent communities including governance. It also delves into changes the ocean that were beyond the scope of the previous reports. Radiation management techniques (also known as sunlight reflection methods) are excluded here. Such geo-engineering approaches are addressed in the SR15. However, natural carbon uptake and stores in the marine environment are included (Section 5.5.1).

The chapter is structured around three guiding questions:

* What are the key changes in the physical and biogeochemical properties of the ocean? (Section 5.2.2)
* How have these changes impacted key ecosystems, risks to ecosystems services and human well-being? (Section 5.2.3, 5.2.4, 5.3, 5.4)
* Are there effective pathways for adaptation and nature-based solutions to risk reduction for marine dependent communities? (Section 5.5)

This chapter covers both regional and global scales and across natural and human systems. Chapter 3 covers the polar regions, including their oceans, Chapter 4 covers sea level and its implications, and Chapter 6 covers extremes and abrupt events. This chapter uses IPCC calibrated language around scientific uncertainty, as described in Section 1.8.3. Two emissions scenarios, RCP2.6 and RCP8.5, are used for projections of climate change (see Cross-Chapter Box 1 in Chapter 1).

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