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

=== 5.8.1 Observed Impacts ===

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Ocean systems are already facing significant impacts of climate change. At the ocean surface, temperature has on average increased by 0.88 [0.68–1.01] °C from 1850–1900 to 2011–2020 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] ). Marine heatwaves have increased in frequency over the 20th century, with an approximate doubling since the 1980s ( ''high confidence'' ), and their intensity and duration have also increased ( ''medium confidence'' ) ( [[#IPCC--2021|IPCC, 2021]] , Box 9.2). In the Northeast Pacific, for example, an intense and long-lasting marine heatwave during 2013–2015 bridged to the strong 2015–2016 El Niño ( [[#Tseng--2017|Tseng et al., 2017]] ) resulted in over 5 years of warmer-than-normal temperatures affecting the migration, distribution and abundance of several marine species, including fisheries resources ( [[#Cornwall--2019|Cornwall, 2019]] ; [[#Jiménez-Quiroz--2019|Jiménez-Quiroz et al., 2019]] ). The surface open ocean pH has declined globally over the last 40 years by 0.003–0.026 pH per decade ( ''virtually certain'' ), and a decline in the ocean interior pH has been observed in all ocean basins over the past two to three decades ( ''high confidence'' ) ( [[#Gulev--2021|Gulev et al., 2021]] ). The ocean is losing dissolved oxygen ( ''very likely'' ) in the range of 0.5–3.3% between 1970 and 2010 for the 0–1000 m depth stratum ( [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Canadell--2021|Canadell et al., 2021]] ), salt content is being redistributed ( ''very likely'' ) ( [[#Liu--2019a|Liu et al., 2019a]] ; [[#Gulev--2021|Gulev et al., 2021]] ) and vertical stratification is increasing ( ''virtually certain'' ) ( [[#HLPE--2017a|HLPE, 2017a]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). There is ''high confidence'' that all these new physical, chemical and biological conditions affect marine organisms’ physiology, distribution and ecology, with an overall shift in biomass and species composition affecting ecosystem structure and function (Chapter 3). Under climate change, freshwater ecosystems are highly exposed to eutrophication, species invasion and rising temperatures ( [[#Lynch--2016|Lynch et al., 2016]] ; [[#Hassan--2020|Hassan et al., 2020]] ). Major threats to wetland fisheries include water stress, sedimentation, weed proliferation, sea level rise and loss of wetland connectivity ( [[#Naskar--2018|Naskar et al., 2018]] ).

Changes in aquatic ecosystems directly affect humans by altering livelihood, cultural identity and sense of self, and seafood provision, quality and safety. The state of marine fishery resources has continued to decline, with the proportion of fish stocks at biologically unsustainable levels of exploitation increasing from 10% in 1974 to 34.2% in 2017 ( [[#FAO--2020d|FAO, 2020d]] ). There is ''medium confidence'' that fisheries production declines in different world regions can be partly attributed to climate change, along with overfishing and other socioeconomic factors. It has been estimated that, from 1930 to 2010, the amount of fish that can be sustainably harvested from several marine fish populations has decreased by 4.1% globally due to ocean warming, with some regions (East Asian Marginal Seas, the North Sea, the Iberian Coast and the Celtic-Biscay Shelf), experiencing losses of 15–35% ( [[#Free--2019|Free et al., 2019]] ). There is regional variation such as redistribution of fishing grounds, due to climate-induced fish species migrations (Cross-Chapter Box MOVING PLATE this chapter). In Tanzania, for example, most small-scale fishers (75%) have reported shifting fishing grounds from nearshore to offshore areas during the last decade, due to perceived combined effects of overfishing and environmental impacts ( [[#Silas--2020|Silas et al., 2020]] ). Observed impacts in some inland aquatic systems indicate substantial productivity reductions ( ''medium confidence'' ). For example, sustained warming in Lake Tanganyika during the last ∼ 150 years has affected the biological productivity by strengthening and shallowing stratification of the water column ( [[#Cohen--2016|Cohen et al., 2016]] ). Still, over 60% of the published reports on directly observed impacts of climate change on freshwater biota are on salmonids in North America and Europe, highlighting significant literature gaps for other fish species and regions ( [[#Myers--2017a|Myers et al., 2017a]] ).

There is ''low confidence'' in climate change affecting the nutritious value of seafood. Contrasting evidence suggests that ocean warming and acidification could be altering the nutritional quality of commercial mollusks, primarily by reducing healthy fatty acids content ( [[#Tate--2017|Tate et al., 2017]] ; Ab [[#Lah--2018|Lah et al., 2018]] ; [[#Lemasson--2019|Lemasson et al., 2019]] ), but Coleman (2019) found no significant changes in a widely distributed coastal fish species.

In terms of food safety, there is ''high confidence'' that climate change increases the trends in seafood consumption related illnesses due to biological agents such as algae-produced toxins, ciguatera and ''Vibrio'' (Cross-Chapter Box ILLNESS in Chapter 2, Sections 5.11 and 5.12). Increased surface water warming changes the occurrence, intensity, species composition and toxicity of marine and freshwater algae and bacteria, and expansion to areas where they had not been reported before ( [[#Botana--2016|Botana, 2016]] ; [[#McCabe--2016|McCabe et al., 2016]] ; [[#Griffith--2019|Griffith et al., 2019]] ). There is ''limited evidence'' suggesting that risks linked to the bioaccumulation of chemicals are also of concern, such as neurotoxic methylmercury (MeHg) and heavy metals, due to water quality and trophic changes induced by climate change ( [[#Shi--2016|Shi et al., 2016]] ; [[#Schartup--2019|Schartup et al., 2019]] ).

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