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

== 5.6 Synthesis ==

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This chapter has documented an extraordinary array of observed changes in the open ocean, deep sea and coasts. It draws on evidence from thousands of references from the literature, millions of observations and hundreds of simulations of the past and future scenarios. The ocean climate and its state, ecosystems and human systems have changed (Section 4.2.2.6, 5.2, 5.3, 5.4, Figure 5.24) and are projected to change further. The ocean is a highly connected environment allowing water and living organisms to move freely. Change is observed across physical conditions that pose hazards to ecosystems in all regions from the surface to the deepest parts (Figure 5.24). All types of human and managed systems that have been covered in this chapter have evidence of mostly negative impacts but also some positive, some very significantly, some less so (Figure 5.24). Overall the multiple lines of evidence from the literature and the assessment in this chapter’s Executive Summary point to profound and pervasive changes on regional and global scales (Figure 5.24).

The level of knowledge and confidence of the changes in the marine environment that are particularly relevant to ecosystems and human systems ranges from ''virtually certain'' to ''low confidence'' (see Figure 5.24). Many of the observed changes in some variables can be directly attributed to human influence from rising greenhouse gases and other anthropogenic forcings (Section 5.2.2 and 5.2.3). For other variables and in some systems the evidence is less direct, but the cascading of risks from changing ocean, marine ecosystems and dependent communities remains robust when considered as a whole. The observed and projected changes in the ocean systems that are covered in this chapter are consistent with our understanding of ocean chemistry and circulation, and our knowledge of the ecosystems responses. In many cases, the assessments of risk level of ecosystems for the recent past and long-term future are based on multiple lines of evidence, combining ecological and physiological knowledge (from experiments, direct observations and model projections) with the major climate drivers (e.g., Sections 5.2.5 and 5.3.4). Globally, all the marine ecosystems assessed here have elevated risk for biodiversity, ecosystem function, structure and services with increasing greenhouse gas emissions (Figure 5.16) ( ''high confidence'' ). These risks result from ocean warming, stratification, acidification, deoxygenation, SLR and associated changes as well as interactions with non-climatic human drivers. Most importantly, all the coastal ecosystems that were assessed, where linkages between natural systems and human communities are the strongest, had increased risk, and none saw a risk reduction from a warming climate ( ''high confidence'' ).

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'''Figure 5.24'''

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'''Figure SPM.2 | Synthesis of observed regional hazards and impacts in ocean (top) and high mountain and polar land regions (bottom) assessed in SROCC. For each region, physical changes, impacts on key ecosystems, and impacts on human systems and ecosystem function and services are shown. For physical changes, yellow/green refers to an increase/decrease, respectively, in […]'''
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Figure SPM.2 | Synthesis of observed regional hazards and impacts in ocean <sup>[[#fn:24|24]]</sup> (top) and high mountain and polar land regions (bottom) assessed in SROCC. For each region, physical changes, impacts on key ecosystems, and impacts on human systems and ecosystem function and services are shown. For physical changes, yellow/green refers to an increase/decrease, respectively, in amount or frequency of the measured variable. For impacts on ecosystems, human systems and ecosystems services blue or red depicts whether an observed impact is positive (beneficial) or negative (adverse), respectively, to the given system or service. Cells assigned ‘increase and decrease’ indicate that within that region, both increase and decrease of physical changes are found, but are not necessarily equal; the same holds for cells showing ‘positive and negative’ attributable impacts. For ocean regions, the confidence level refers to the confidence in attributing observed changes to changes in greenhouse gas forcing for physical changes and to climate change for ecosystem, human systems, and ecosystem services. For high mountain and polar land regions, the level of confidence in attributing physical changes and impacts at least partly to a change in the cryosphere is shown. No assessment means: not applicable, not assessed at regional scale, or the evidence is insufficient for assessment. The physical changes in the ocean are defined as: Temperature change in 0–700 m layer of the ocean except for Southern Ocean (0–2000 m) and Arctic Ocean (upper mixed layer and major inflowing branches); Oxygen in the 0–1200 m layer or oxygen minimum layer; Ocean pH as surface pH (decreasing pH corresponds to increasing ocean acidification). Ecosystems in the ocean: Coral refers to warm-water coral reefs and cold-water corals. The ‘upper water column’ category refers to epipelagic zone for all ocean regions except Polar Regions, where the impacts on some pelagic organisms in open water deeper than the upper 200 m were included. Coastal wetland includes salt marshes, mangroves and seagrasses. Kelp forests are habitats of a specific group of macroalgae. Rocky shores are coastal habitats dominated by immobile calcified organisms such as mussels and barnacles. Deep sea is seafloor ecosystems that are 3000–6000 m deep. Sea-ice associated includes ecosystems in, on and below sea ice. Habitat services refer to supporting structures and services (e.g., habitat, biodiversity, primary production). Coastal Carbon Sequestration refers to the uptake and storage of carbon by coastal blue carbon ecosystems. Ecosystems on Land: Tundra refers to tundra and alpine meadows, and includes terrestrial Antarctic ecosystems. Migration refers to an increase or decrease in net migration, not to beneficial/adverse value. Impacts on tourism refer to the operating conditions for the tourism sector. Cultural services include cultural identity, sense of home, and spiritual, intrinsic and aesthetic values, as well as contributions from glacier archaeology. The underlying information is given for land regions in tables SM2.6, SM2.7, SM2.8, SM3.8, SM3.9, and SM3.10, and for ocean regions in tables SM5.10, SM5.11, SM3.8, SM3.9, and SM3.10. {2.3.1, 2.3.2, 2.3.3, 2.3.4, 2.3.5, 2.3.6, 2.3.7, Figure 2.1, 3.2.1, 3.2.3, 3.2.4, 3.3.3, 3.4.1, 3.4.3, 3.5.2, Box 3.4, 4.2.2, 5.2.2, 5.2.3, 5.3.3, 5.4, 5.6, Figure 5.24, Box 5.3}

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The observed and projected changes in the open ocean and coastal seas have consequences on human communities and affect all aspects of well-being and have social, economic and environmental costs (Section 5.4, ''high confidence'' ). The range and diversity of impacts is striking, with varying consequences for the wider community when analysed across the key marine ecosystems services. These consequences clearly affect the capacity for human society to achieve the SDGs (e.g., Figure 5.22). The evidence of climate change in the ocean is a pervasive thread through all types of coupled human-natural systems and projections amplify these observed impacts with the least impact from lower emission scenarios.

Risk-reduction responses and their governance through adaptation at the local scale are the most common responses to climate change from ocean systems (Section 5.5.2). It is clear that there are many choices for reducing risk of climate change. Many of the actions have benefits and relatively few dis-benefits, while others have large dis-benefits and marginal effectiveness (Section 5.5.1, Figure 5.23, Table 5.7 and Table 5.8). Many of the risk reduction approaches are limited in their capacity to reduce the risks of climate change, or are at best temporary solutions, which is a significant challenge to adapting to climate change ( ''high confidence'' ). In particular, the effectiveness of the assessed risk reduction measures are minimal under high greenhouse gas emission scenarios, highlighting the critical importance of mitigation. The assessment points to the increased effectiveness and importance of a portfolio of different types of mitigation and adaptation options. Governance is also a critical element in the portfolio of options and occurs at local, national and international scales. Such responses can be more effective with the support of scientific information, local knowledge and Indigenous knowledge, and the consideration of local context and the inclusion of stakeholders.

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