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

===== 3.4.3.5.2 Projected changes in primary production =====

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Across 10 CMIP5 and 13 CMIP6 ESM ensembles, global mean NPP is projected to decline by 2080–2099 relative to 2006–2015, under all RCPs and SSPs ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). However, under comparable radiative forcing, the CMIP6 multi-model mean projections of primary production declines (mean ± SD: −0.56 ± 4.12% under SSP1-2.6, and −3.00 ± 9.10% under SSP5-8.5) are less than those of previous CMIP5 models (3.42 ± 2.47% under RCP2.6, and 8.54 ± 5.88% under RCP8.5) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.4.4.2|Section 5.4.4.2]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ; [[#Canadell--2021|Canadell et al., 2021]] ). The inter-model uncertainty associated with CMIP6 NPP projections is larger than in CMIP5, and it is consistently larger than the scenario uncertainty. For each SSP across the CMIP6 ensemble, individual models project both increases and decreases in global primary production, reflecting a diverse suite of bottom-up and top-down ecological processes, which are variously parameterised across models ( [[#Laufkötter--2015|Laufkötter et al., 2015]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ). Furthermore, accurate simulation of many of the biogeochemical tracers upon which NPP depends (e.g., the distribution of iron; [[#Tagliabue--2016|Tagliabue et al., 2016]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ) remains a significant and ongoing challenge to ESMs ( ''high confidence'' ) ( [[#Séférian--2020|Séférian et al., 2020]] ).

Regionally, multi-model mean changes in primary production show generally similar patterns of large declines in the North Atlantic and the western equatorial Pacific, while in the high latitudes, primary production consistently increases in CMIP5 and CMIP6 by 2100 (Cross-Chapter Paper 6; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). In the Indian Ocean and subtropical North Pacific, which were regions of consistent NPP decline in CMIP5 projections ( [[#Bopp--2013|Bopp et al., 2013]] ), the regional declines are reduced in magnitude, less spatially extensive and are typically less robust in CMIP6. Further assessment of simultaneous changes in processes such as nutrient advection, nitrogen fixation, the microbial loop and top-down grazing pressure (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.4.4.2|Section 5.4.4.2]] ; [[#Laufkötter--2015|Laufkötter et al., 2015]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Canadell--2021|Canadell et al., 2021]] ) are required to fully understand the regional primary production response in CMIP6 ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). Given the regional variations in the estimates of primary production changes and the uncertainty in the representation of the dominant drivers, there remains ''low confidence'' in the projected global decline in NPP.

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