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

===== 3.4.3.2.2 Projected changes =====

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The CMIP6 ESM ensembles project that, by 2100, 18.8 ± 19.0% (mean ± ''very likely'' range) and 38.9 ± 9.4% of the ocean surface will ''very likely'' undergo a change of 20 d or more (advance or delay) in the start of the phytoplankton growth period under SSP1-2.6 and SSP5-8.5, respectively (Figure 3.17a,b) ( ''low confidence'' due to the dependence with the projected changes in phytoplankton biomass, the trends of which are reported with ''low confidence'' ) ( [[#3.4.3.4|Section 3.4.3.4]] ; SROCC [[IPCC:Wg2:Chapter:Chapter-5#5.2|Section 5.2.3]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ). Phytoplankton growth is projected to begin later in the Northern Hemisphere subtropics, and earlier at high latitudes in some regions around the Antarctic Peninsula, and over large areas in the Northern Hemisphere ( ''low to medium confidence'' as there are improved constraints from historical variability in this region and consistency with CMIP5-based-studies results) ( [[#Henson--2018b|Henson et al., 2018b]] ; [[#Asch--2019|Asch et al., 2019]] ). There is ''high agreement'' in model projections that the start of the phytoplankton growth period will ''very likely'' advance in the Arctic Ocean under a high-emission scenario for CMIP5 and CMIP6 (Figure 3.17b; [[#Henson--2018b|Henson et al., 2018b]] ; [[#Asch--2019|Asch et al., 2019]] ; [[#Tedesco--2019|Tedesco et al., 2019]] ; [[#Lannuzel--2020|Lannuzel et al., 2020]] ). The CMIP6 ensemble projections further show limited changes in phenology across most of the Southern Ocean but large regional variations in the tropics (Figure 3.17). Overall, the regional patterns are qualitatively similar under SSP1-2.6 and SSP5-8.5 but with greater magnitude and larger areas under SSP5-8.5 ( ''low confidence'' ).

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'''Figure 3.17 |''' '''Projected phytoplankton phenology.''' (a,c) Spatial patterns and (b,d) density distributions of projected change in phytoplankton phenology by 2100 under Shared Socioeconomic Pathway (SSP)1-2.6 and SSP5-8.5, respectively. Difference in the start of the phytoplankton growth period is calculated as 2090–2099 minus 1996–2013. Negative (positive) values indicate earlier (later) start of the phytoplankton growth period by 2100. The ensemble projections of global changes in phytoplankton phenology include, under SSP1-2.6 and SSP5-8.5, respectively, a total of five Coupled Model Intercomparison Project 6 Earth system models containing coupled ocean biogeochemical models that cover a wide range of complexity ( [[#Kwiatkowski--2019|Kwiatkowski et al., 2019]] ). (The phenology calculations are based on [[#Racault--2017|Racault et al., 2017]] , using updated data.)

At latitudes &gt;40°N, temperature-linked phenology of fish reproduction with high geographic fidelity to spawning grounds (geographic spawners) is projected to change at double the rate of that for phytoplankton, which will ''likely'' cause phenological mismatches resulting in increased risk of starvation for fish larvae ( ''medium to high confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-2#2.3|Section 2.3.4.2.3]] ; [[#Asch--2019|Asch et al., 2019]] ; [[#Durant--2019|Durant et al., 2019]] ; [[#Régnier--2019|Régnier et al., 2019]] ; [[#Gulev--2021|Gulev et al., 2021]] ; [[#Laurel--2021|Laurel et al., 2021]] ). Furthermore, under RCP8.5, trophic mismatch events exceeding ±30 days ( [[#Asch--2019|Asch et al., 2019]] ) leading to fish-recruitment failure are expected to increase tenfold for geographic spawners across much of the North Atlantic, North Pacific and Arctic Ocean basins ( ''low confidence'' ) ( [[#Neuheimer--2018|Neuheimer et al., 2018]] ). In contrast, temporal mismatches between fish that relocate spawning grounds in response to environmental variations (environmental spawners) and phytoplankton blooms are projected to remain shorter and less varied, suggesting that across ocean basins, range shifts by environmental spawners may increase their resilience. Nevertheless, this compensation mechanism might fail at locations where phytoplankton bloom phenology is not controlled by temperature-driven water-column stratification, leading to a possible sixfold local increase in extreme mismatches under climate change ( [[#Asch--2019|Asch et al., 2019]] ).

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