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

=== 5.8.3 Projected Impacts ===

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There is ''medium confidence'' that climate change will reduce global fisheries’ productivity ( [[IPCC:Wg2:Chapter:Chapter-3#3.4.4|Section 3.4.4.2.3]] ), with more significant reductions in tropical and subtropical regions and gains in the poleward areas ( [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Oremus--2020|Oremus et al., 2020]] ). Through an ensemble of marine ecosystem models and Earth System Models, mean global animal biomass in the ocean has been estimated to decrease by 5% under the RCP2.6 emissions scenario and 17% under RCP8.5 by 2100, with an average decline of 5% for every 1°C of warming ( [[#Lotze--2019|Lotze et al., 2019]] ), affecting food provision, revenue distribution, and potentially hindering the rebuilding of depleted fish stocks ( [[#Britten--2017|Britten et al., 2017]] ). The projected declining rates result in a 5.3–7% estimated global decrease in marine fish catch potential by 2050 ( [[#Cheung--2019|Cheung et al., 2019]] ), particularly accentuated in tropical marine ecosystems and affecting many low-income countries ( [[#Barange--2018|Barange and Cochrane, 2018]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; Cross-Chapter Box MOVING PLATE this chapter). Projections indicate that by 2060 the number of exclusive economic zones (EEZs) with new transboundary stocks will increase to 46 under strong mitigation RCP2.6, and up to 60 EEZs under the RCP8.5 GHG emissions scenario ( [[#Pinsky--2018|Pinsky et al., 2018]] ). Similarly, by combining six intercompared marine ecosystem models, [[#Bryndum-Buchholz--2019|Bryndum-Buchholz et al. (2019)]] projected that under the RCP8.5 scenario a total marine animal biomass decline of 15–30% would occur in the North and South Atlantic and Pacific and the Indian Ocean by 2100. In contrast, polar ocean basins would experience a 20–80% increase. In the eastern Bering Sea, simulations based on RCP8.5 predict declines of pollock (&gt;70%) and cod (&gt;35%) stocks by the end of the century ( [[#Holsman--2020|Holsman et al., 2020]] ). Temperate tunas (albacore, Atlantic bluefin and southern bluefin) and the tropical bigeye tuna are expected to decline in the tropics and shift poleward by the end of the century under RCP8.5, while skipjack and yellowfin tunas are projected to increase abundance in tropical areas of the eastern Pacific but decrease in the equatorial western Pacific ( ''medium confidence'' ) ( [[#Erauskin-Extramiana--2019|Erauskin-Extramiana et al., 2019]] ). In the western and central Pacific, redistribution of tropical tuna due to climate change is projected to affect license revenues from purse seine fishing and shift more fishing into high seas areas ( [[#Bell--2018|Bell et al., 2018]] ; Table 15.5). For the east Atlantic, observational evidence indicates that not only will tuna distribution change with temperature anomalies, but also fishing effort distribution ( [[#Rubio--2020a|Rubio et al., 2020a]] ). There is ''medium confidence'' that climate change will create new fishing opportunities when exploited fish stocks shift their distribution into new fishing regions in enclosed seas, such as the Mediterranean and the Black Sea ( [[#Hidalgo--2018|Hidalgo et al., 2018]] ; [[#Pinsky--2018|Pinsky et al., 2018]] ). However, in general, where land barriers constrain the latitudinal shifts, the expected impacts of climate change are population declines and reduced productivity ( ''high confidence'' ) ( [[#Oxenford--2018|Oxenford and Monnereau, 2018]] ). Besides direct impacts on the abundance of fisheries-targeted species, climate-change-induced proliferation of invasive species could also affect fisheries’ productivity ( ''low confidence'' ) ( [[#Mellin--2016|Mellin et al., 2016]] ; [[#Goldsmith--2019|Goldsmith et al., 2019]] ).

Shifting marine fisheries will affect national economies ( ''high confidence'' ) ( [[#Bindoff--2019|Bindoff et al., 2019]] ). It has been suggested that, without government subsides, fishing is already non-profitable in 54% of the international waters ( [[#Sala--2018|Sala et al., 2018]] ). Projections are that fishing maximum revenue potential from landed catches will decrease further by 10.4% (±4.2%) by 2050 relative to 2000 under RCP8.5, close to 35% greater than the decrease projected for the global maximum catch potential (7.7±4.4%); ( [[#Lam--2016|Lam et al., 2016]] ). The global revenue potential loss for that period ranges from USD 6 to 15 billion (depending on the model), but impacts may be amplified at the regional scale for fisheries-dependent and low-income countries. The maximum revenue potential percentage decrease in the EEZ under RCP8.5 is estimated to be over 2.3 times larger than that of the high seas ( [[#Lam--2016|Lam et al., 2016]] ). Ocean acidification is also expected to drive large global economic impacts ( ''medium confidence'' ) ( [[#Cooley--2015|Cooley et al., 2015]] ; [[#Fernandes--2017|Fernandes et al., 2017]] ; [[#Macko--2017|Macko et al., 2017]] ; [[#Hansel--2020|Hansel et al., 2020]] ), and there is ''high confidence'' that the integrated economic consequences of all interacting climate change-related factors would result in even larger losses. Changes in the frequency and intensity of extreme events will also alter marine ecosystems and productivity. Marine heatwaves can lead to severe and persistent impacts, from mass mortality of benthic communities to decline in fisheries catch ( [[#IPCC--2021|IPCC, 2021]] , Box 9.2). These events have ''very likely'' doubled in frequency between 1982 and 2016 and have also become more intense and longer ( [[#Smale--2019|Smale et al., 2019]] ; [[#Laufkotter--2020|Laufkotter et al., 2020]] ); for all future scenarios Earth System Models project even more frequent, intense and longer-lasting marine heatwaves ( [[#Eyring--2021|Eyring et al., 2021]] ; [[#IPCC--2021|IPCC, 2021]] , Box 9.2).

In addition to temperature and water availability stress, climate change will bring new water quality challenges in freshwater systems, including increased dissolved organic carbon and toxic metal loads ( ''high confidence'' ) ( [[#Chen--2016|Chen et al., 2016]] ). [[#Harrod--2018a|Harrod et al. (2018a)]] found that the two major inland fishery producers (China and India) will face significant stress in the future, a large group of countries that produce around 60% of total yield is projected to face medium stress, and a small group of 17 countries has the least severe repercussions ( ''medium confidence'' ). Climate warming may enhance northward colonisation of water bodies of commercial freshwater species in the Arctic, where there are few ecological competitors ( ''medium confidence'' ) ( [[#Campana--2020|Campana et al., 2020]] ) but at the same time may also accentuate the age-truncation effect of harvesting, elevating the population’s vulnerability to environmental perturbations ( [[#Smalås--2019|Smalås et al., 2019]] ). Detailed information on many of the most important inland fisheries is limited.

In terms of food safety, major concerns linked to climate change include the continued trend of increasing HABs, and the quantity of pollutants reaching aquatic systems (Box 3.3; [[#5.11|Section 5.11]] ).

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