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

===== 3.4.3.5.1 Observed changes in primary production =====

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Analyses of satellite-derived primary production over the past two decades (1998–2018) showed generally weak and negative trends (up to −3.0%) at low and mid latitudes ( [[#Kulk--2020|Kulk et al., 2020]] ). In contrast, positive trends occurred in large areas of the South Atlantic and South Pacific Oceans, as well as in polar and coastal (upwelling) regions (up to +4.5%; Cross-Chapter Paper 6; [[#Kulk--2020|Kulk et al., 2020]] ). Data-assimilating ocean biogeochemical models estimate a global decline in primary production of 2.1% per decade in the period 1998–2015, driven by the shoaling mixed layer and decreasing nitrate concentrations ( [[#Gregg--2019|Gregg and Rousseaux, 2019]] ). This is consistent with previous assessments that identified ocean warming and increased stratification as the main drivers ( ''high confidence'' ) affecting the regional variability in primary production Bindoff et al. (2019). However, as noted in SROCC and WGI AR6 [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] (Table 3.24; [[#Gulev--2021|Gulev et al., 2021]] ), observed interannual changes in primary production on global and regional scales are nonlinear and largely influenced by natural temporal variability, providing ''low confidence'' in the trends.

'''Table 3.24 |''' Summary of previous IPCC assessments of ocean primary production and carbon export flux

{| class="wikitable"
|-
! Process

! Observed impacts

! Projected impacts

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| ''SROCC ( [[#Bindoff--2019a|Bindoff et al., 2019a]] )''

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| Open-ocean

primary production

| ‘Past open-ocean productivity trends, including those determined by satellites, [are appraised with ''low confidence'' ] due to newly identified region-specific drivers of microbial growth and the lack of corroborating ''in situ'' time series datasets.’

| ‘Net primary productivity (NPP) is ''very likely'' to decline by 4–11% by 2081–2100, relative to 2006–2015, across CMIP5 models for RCP8.5, but there is ''low confidence'' for this estimate due to the ''medium agreement'' among models and the ''limited evidence'' from observations. The tropical ocean NPP will ''very likely'' to decline by 7–16% for RCP8.5, with ''medium confidence'' as there are improved constraints from historical variability in this region.’

|-
| Open-ocean carbon export

| ‘Analyses of long-term trends in primary production and particle export production, as well as model simulations, reveal that increasing temperatures, leading to enhanced stratification and nutrient limitation, will have the greatest influence on decreasing the flux of particulate organic carbon (POC) to the deep ocean. However, different lines of evidence (including observation, modelling and experimental studies) provide ''low confidence'' on the mechanistic understanding of how climate drivers affect different components of the biological pump in the epipelagic ocean, as well as changes in the efficiency and magnitude of carbon export in the deep ocean.’

| ‘The projected changes in export production can be larger than global primary production because they are affected by both, the NPP changes, but also how shifts in food-web structure modulates the ‘transfer efficiency’ of particulate organic material, which then affects the sinking speed and lability of exported particles through the ocean interior to the sea floor.’

‘As export production is a much better understood net integral of changing net nutrient supply and can be constrained by interior ocean nutrient and oxygen levels, there is ''medium confidence'' in projections for global [export production] changes [based on CMIP5 model runs].’

|-
| ''WGI AR6 Chapters 2, 5 ( [[#Canadell--2021|Canadell et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] )''

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|-
| Open-ocean

primary production

| Global ocean marine primary production is estimated to be 47 ± 7.8 PgC yr –1 with ''low confidence'' because of the small number of recent studies and the insufficient length of the time series analysed. A small decrease in productivity is evident globally for the period 1998–2015, but regional changes are larger and of opposing signs ( ''low confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-2#2.3|Section 2.3.4.2.2]] ; [[#Gulev--2021|Gulev et al., 2021]] ).

| ‘In CMIP5 models run under RCP8.5, [POC] export flux is projected to decline by 1–12% by 2100 ( [[#Taucher--2011|Taucher and Oschlies, 2011]] ; [[#Laufkötter--2015|Laufkötter et al., 2015]] ). Similar values are predicted in 18 CMIP6 models, with declines of 2.5–21.5% (median: −14%) [...] between 1900 and 2100 under the SSP5-8.5 scenario. The mechanisms driving these changes vary widely between models due to differences in parameterisation of particle formation, remineralisation and plankton community structure’ (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.4.4.2|Section 5.4.4.2]] ; [[#Canadell--2021|Canadell et al., 2021]] ).

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