Editing IPCC:AR6/WGII/Chapter-3 (section)

===== 3.4.3.1.3 Projected changes in species range shifts =====

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Continued changes in the biogeography of marine predators and prey are anticipated under future climate change, with climate velocity in the epipelagic zone during 2050–2100 under RCP8.5 projected to be sevenfold faster than that during 1955–2005 ( ''medium confidence'' ) (Figure 3.4; [[#Brito-Morales--2020|Brito-Morales et al., 2020]] ). This has substantial ecological implications, as projections suggest near elimination of overlaps between the distributions of certain predator–prey pairs in the northeast Atlantic Ocean when their current joint distributions (1989–2014) are compared with those projected (2037–2062) under RCP8.5 ( [[#Sadykova--2020|Sadykova et al., 2020]] ).

Deepening of epipelagic isotherms is projected to accelerate over 2006–2100 to rates of 8.5 m per decade under RCP4.5 and 32 m per decade under RCP8.5 ( [[#Jorda--2020|Jorda et al., 2020]] ). Although vertical redistribution of thermal niches is three to four orders of magnitude slower than horizontal displacement, maximum depth limits imposed by the seafloor and photic layer (both of which are projected to be reached in this century) will ''likely'' vertically compress suitable habitat for most marine organisms ( ''medium confidence'' ) ( [[#Dueri--2014|Dueri et al., 2014]] ; [[#Jorda--2020|Jorda et al., 2020]] ).

Projections from coupled biogeochemical and ecosystem models suggest a general decline in mesopelagic biomass ( [[#Lefort--2015|Lefort et al., 2015]] ), although this may vary among ocean basins. The volume of OMZs have been expanding at many locations ( ''high confidence'' ), and the oxygen content of the subsurface ocean is projected to decline to historically unprecedented conditions over the 21st century ( ''medium confidence'' ) ( [[#3.2.3|Section 3.2.3.2]] ; WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.3.2]] ; [[#Canadell--2021|Canadell et al., 2021]] ) at a rate of 10–15 µM per decade in OMZs ( [[#3.2.3|Section 3.2.3.2]] ; [[#Breitburg--2018|Breitburg et al., 2018]] ). Oxygen availability and the effects of ocean acidification (Sections 3.3, 3.4.2) on zooplankton might become a dominant constraint in the upper ocean’s metabolic index, which is projected to decrease globally by 20% by 2100 ( [[#Deutsch--2015|Deutsch et al., 2015]] ; [[#Steinberg--2017|Steinberg and Landry, 2017]] ). In addition, extremely rapid acceleration of climate velocities projected in the mesopelagic under all emissions scenarios suggest that species in this ocean stratum will be even more exposed to future warming than species in the epipelagic (Figure 3.4; [[#Brito-Morales--2020|Brito-Morales et al., 2020]] ). But projections also suggest that warming-related increases in trophic efficiency lead to a 17% increase in the biomass of the deep-scattering layer (zooplankton and fish in the mesopelagic) by 2100 (low ''confidence'' ) ( [[#Bindoff--2019a|Bindoff et al., 2019a]] ). Observational studies appear to show that mesopelagic fishes adapted to warm water increased in abundance and distribution in the California Current associated with warming and the expansion of OMZ ( [[#Koslow--2019|Koslow et al., 2019]] ), suggesting that some mesopelagic fish stocks might be resilient to a changing climate ( ''medium confidence'' ).

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