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

===== 3.4.3.4.2 Projected changes =====

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Based on an ensemble of CMIP5 ESMs, SROCC projected declines in global zooplankton biomass by 2100 dependent on emission scenario ( ''low confidence'' ) (Table 3.23). The new CMIP6 ESM ensemble projects a decline in global zooplankton biomass by −3.9 ± 8.2% ( ''very likely range'' ) and −9.0 ± 8.9% in the period 2081–2100 relative to 1995–2014 under SSP1-2.6 and SSP5-8.5, respectively (Figure 3.21d; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ), thus reinforcing the SROCC assessment albeit with greater inter-model uncertainties.

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[[File:17b6a979e794ad7f282345cd4bdd0579 IPCC_AR6_WGII_Figure_3_021.png]]

'''Figure 3.21 |''' '''Projected change in marine biomass.''' Simulated global biomass changes of (a,b,c) surface phytoplankton, (d,e,f) zooplankton, (g,h,i) animals and (j,k,l) seafloor benthos. In (a,d,g,j), the multi-model mean (solid lines) and ''very likely range'' (envelope) over 2000–2100 relative to 1995–2014, for SSP1-2.6 and SSP5-8.5. Spatial patterns of simulated change by 2090–2099 are calculated relative to 1995–2014 for (b,e,h,k) SSP1-2.6 and (c,f,i,l) SSP5-8.5. Confidence intervals can be affected by the number of models available for the Coupled Model Intercomparison Project 6 (CMIP6) scenarios and for different variables. Only one model was available for panel (j), so no confidence interval is calculated. For panels (a–f), the ensemble projections of global changes in phytoplankton and zooplankton biomasses updated based on Kwiatkowski et al. (2019) include, under SSP1-2.6 and SSP5-8.5, respectively, a total of nine and ten CMIP6 Earth system models (ESMs). For panels (b,c,e,f), unhatched areas represent regions where at least 80% of models agree on the sign of biomass anomaly. For panels (g,h,i), the ensemble projections of global changes in total animal biomass updated based on [[#Tittensor--2021|Tittensor et al. (2021)]] include six to nine published global fisheries and marine ecosystem models from the Fisheries and Marine Ecosystem Model Intercomparison Project ( [[#Tittensor--2018|Tittensor et al., 2018]] ; [[#Tittensor--2021|Tittensor et al., 2021]] ), forced with standardised outputs from two CMIP6 ESMs. For panels (j,k,l), globally integrated changes in total seafloor biomass have been updated based on [[#Yool--2017|Yool et al. (2017)]] with one benthic model ( [[#Kelly-Gerreyn--2014|Kelly-Gerreyn et al., 2014]] ) forced with the CMIP6 ESM UKESM-1.

Using an ensemble of global-scale marine ecosystem and fisheries models (Fish-MIP) ( [[#Tittensor--2018|Tittensor et al., 2018]] ) with the CMIP5 ESM ensemble, SROCC concludes that projected ocean warming and decreased phytoplankton production and biomass will reduce global marine animal biomass during the 21st century ( ''medium confidence'' ). The simulated declines (with ''very likely range'' ) are −3.5 ± 4.8% and −14.0 ± 14.6% under RCP2.6 and RCP8.5, respectively, by 2080–2099 relative to 1995–2014 (SROCC [[IPCC:Wg2:Chapter:Chapter-5#5.2|Section 5.2.3]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Lotze--2019|Lotze et al., 2019]] ) [[#footnote-001|6]] . Updated Fish-MIP simulations with CMIP6 (Figure 3.21g,h,i) confirm the projected decline in total marine animal biomass in the 21st century ( [[#Tittensor--2021|Tittensor et al., 2021]] ). The simulated declines (with ''very likely range'' ) are −5.7 ± 4.1% and −15.5 ± 8.5% under SSP1-2.6 and SSP5-8.5, respectively, by 2080–2099 relative to 1995–2014 (Figure 3.21g), showing greater declines and lower inter-model uncertainties ( [[#Tittensor--2021|Tittensor et al., 2021]] ). These declines result from combined warming and decreased primary production (with ''low confidence'' in future changes in primary production; [[#3.4.3.5|Section 3.4.3.5]] ) and are amplified at each trophic level within all ESM and marine ecosystem model projections across all scenarios ( ''medium confidence'' ) ( [[#Kwiatkowski--2019|Kwiatkowski et al., 2019]] ; [[#Lotze--2019|Lotze et al., 2019]] ; [[#Tittensor--2021|Tittensor et al., 2021]] ). However, there is ''limited evidence'' about how underlying food-web mechanisms amplify the climate signal from primary producers to higher trophic levels, and several putative mechanisms have been proposed ( [[#3.4.4|Section 3.4.4.2.2]] ; [[#Chust--2014a|Chust et al., 2014a]] ; [[#Stock--2014|Stock et al., 2014]] ; [[#Kwiatkowski--2019|Kwiatkowski et al., 2019]] ; [[#Lotze--2019|Lotze et al., 2019]] ; [[#Heneghan--2021|Heneghan et al., 2021]] ). As assessed in SROCC, the biomass projections contain considerable regional variation with declines in tropical to temperate regions and strong increases in total animal biomass are projected in polar regions under high-emission scenarios, with climate-change effects that are spatially similar but less pronounced under lower-emission scenarios (Figure 3.21b,c,e,f,h,i; [[#Tai--2019|Tai et al., 2019]] ; [[#Tittensor--2021|Tittensor et al., 2021]] ).

SROCC assessed that reduced food supply to the deep sea will drive a reduction in abyssal seafloor biota by 2100 for RCP8.5 (Table 3.23). Simulations from one size-resolved benthic biomass model coupled to an ocean-biogeochemistry model forced with the CMIP5 ESM HadGEM2-ES ( [[#Yool--2017|Yool et al., 2017]] ) project a decline in the globally integrated total seafloor biomass of −1.1 and −17.6% by 2100 under RCP2.6 and RCP8.5, respectively ( ''limited evidence, high agreement'' ). In waters shallower than 100 m, total benthic biomass is projected to increase by 3.2% on average by 2100 under RCP8.5, primarily driven by warming-increased growth rates ( [[#Yool--2013|Yool et al., 2013]] ), while at depths &gt;2000 m (representing 83% of the ocean seafloor), declines of −32% arise from climate-driven decreases in surface primary production and POC flux to the seafloor ( [[#Yool--2013|Yool et al., 2013]] ; [[#Kelly-Gerreyn--2014|Kelly-Gerreyn et al., 2014]] ; [[#Yool--2015|Yool et al., 2015]] ; [[#Yool--2017|Yool et al., 2017]] ). These patterns are qualitatively similar under RCP2.6, except in the Pacific and Indian Ocean basins, where some increased total seafloor biomass is projected ( [[#Yool--2013|Yool et al., 2013]] ). Updated simulations with the same benthic biomass model ( [[#Kelly-Gerreyn--2014|Kelly-Gerreyn et al., 2014]] ) forced with the CMIP6 ESM UKESM-1 project declines in total seafloor biomass of −9.8 and −13.0% by 2081–2100 relative to 1995–2014 for SSP1-2.6 and SSP5-8.5, respectively (Figure 3.21j,k,l). These projected changes in benthic biomass are based on ''limited evidence'' . Development of ensemble projections forced with a range of ESMs and a benthic model that considers the ecological roles of temperature ( [[#Hunt--2006|Hunt and Roy, 2006]] ; [[#Reuman--2014|Reuman et al., 2014]] ), oxygen ( [[#Mosch--2012|Mosch et al., 2012]] ) and ocean acidification ( [[#Andersson--2011|Andersson et al., 2011]] ) will provide opportunities to better quantify uncertainty in projected declines in total seafloor biomass under climate change.

Overall, ocean warming and decreased phytoplankton production and biomass will drive a global decline in biomass for zooplankton ( ''low confidence'' ), marine animals ( ''medium confidence'' ) and seafloor benthos ( ''low confidence'' ), with regional differences in the direction and magnitude of changes ( ''high confidence'' ). There is increasing evidence that responses will amplify throughout the food web and at ocean depths, with relatively modest changes in surface primary producers translating into substantial changes at higher trophic levels and for deep-water benthic communities ( ''medium confidence'' ).

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