Editing IPCC:AR6/WGII/Chapter-2 (section)

==== 2.4.4.5 Observed Changes in Primary Productivity ====

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===== 2.4.4.5.1 Observed changes in terrestrial primary productivity =====

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The difference between photosynthesis by plants (gross primary productivity, GPP) and plant energy use through respiration is the net growth of plants (NPP), which removes CO 2 from the atmosphere and mitigates emissions from deforestation and other LUCs ( [[#2.4.4.4|Section 2.4.4.4]] ). Global terrestrial NPP has exceeded emissions due to land use since the early 2000s, making terrestrial ecosystems a net carbon sink ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ).

Global terrestrial NPP increased by 6% from 1982 to 1999 through increased temperature and increased solar radiation in the Amazon from decreased cloud cover ( [[#Nemani--2003|Nemani et al., 2003]] ), and then decreased 1% from 2000 to 2009, because of extensive droughts in the Southern Hemisphere ( [[#Zhao--2010|Zhao and Running, 2010]] ). From 1999 to 2015, increased aridity caused extensive declines in the NDVI globally, particularly semiarid ecosystems ( [[#Huang--2016|Huang et al., 2016]] ), indicating widespread decreases in NPP ( [[#Yuan--2019|Yuan et al., 2019]] ).

Global terrestrial GPP increased 2% from 1951 to 2010 and continued increasing at least until 2016, with increased atmospheric CO 2 showing a greater influence than natural factors ( [[#Li--2017|Li et al., 2017]] ; [[#Fernandez-Martinez--2019|Fernandez-Martinez et al., 2019]] ; [[#Liu--2019a|Liu et al., 2019a]] ; [[#Cai--2020|Cai and Prentice, 2020]] ; [[#Melnikova--2020|Melnikova and Sasai, 2020]] ). Global forest area increased 7% from 1982 to 2016, mainly from forest plantations and regrowth in boreal and temperate forests in Asia and Europe ( [[#Song--2018|Song et al., 2018]] ); regrowth in secondary forests &gt;20 years old, mainly in boreal, temperate and subtropical regions, generated a net removal of 7.7 Gt yr -1 CO 2 from the atmosphere from 2001 to 2019 ( [[#Harris--2021|Harris et al., 2021]] ). Vegetation growth that exceeds the modelled CO 2 fertilisation, gaps in field data and incomplete knowledge of plant mortality and soil carbon responses introduce uncertainties into quantifying the magnitude of CO 2 fertilisation ( [[#Walker--2021|Walker et al., 2021]] ). A combination of CO 2 fertilisation of global vegetation and secondary forest regrowth has increased global vegetation productivity ( ''medium evidence'' , ''medium agreement'' ).

The relative increase in GPP per unit of increased atmospheric CO 2 declined from 1982 to 2015, indicating a weakening of any CO 2 fertilisation effect ( [[#Wang--2020c|Wang et al., 2020c]] ). Increased growth from CO 2 fertilisation has begun to shorten the lifespan of trees due to a trade-off between growth rate and longevity, based on analyses of tree rings of 110 species around the world ( [[#Brienen--2020|Brienen et al., 2020]] ). Furthermore, water availability controls the magnitude of NPP ( [[#Beer--2010|Beer et al., 2010]] ; [[#Jung--2017|Jung et al., 2017]] ; [[#Yu--2017|Yu et al., 2017]] ), including water from precipitation ( [[#Beer--2010|Beer et al., 2010]] ), soil moisture ( [[#Stocker--2019|Stocker et al., 2019]] ), groundwater storage ( [[#Humphrey--2018|Humphrey et al., 2018]] ; [[#Madani--2020a|Madani et al., 2020a]] ) and atmospheric vapour ( [[#Novick--2016|Novick et al., 2016]] ; [[#Madani--2020b|Madani et al., 2020b]] ). Drought stress reduced NPP across tropical forests from 2000 to 2015 ( [[#Zhang--2019b|Zhang et al., 2019b]] ) and GPP in the tropics from 1982 to 2016 ( [[#Madani--2020b|Madani et al., 2020b]] ). Drought stress has also reduced GPP in some semiarid and arid lands ( [[#Huang--2016|Huang et al., 2016]] ; [[#Liu--2019a|Liu et al., 2019a]] ). In addition, nitrogen and phosphorus constrain CO 2 fertilisation ( [[#Terrer--2019|Terrer et al., 2019]] ), although phosphorus limitation of tropical tree growth is species-specific ( [[#Alvarez-Clare--2013|Alvarez-Clare et al., 2013]] ; [[#Thompson--2019|Thompson et al., 2019]] ). NPP has decreased during some time periods and in some regions where drought stress has exerted a greater influence than increased atmospheric CO 2 ( ''medium evidence'' , ''high agreement'' ).

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===== 2.4.4.5.2 Observed changes in freshwater ecosystem productivity =====

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Temperature affects primary productivity by moderating phytoplankton growth rates, ice cover, thermal stratification and the length of growing seasons ( [[#Rühland--2015|Rühland et al., 2015]] ; [[#Richardson--2018|Richardson et al., 2018]] ). Global warming has reinforced eutrophication, especially cyanobacteria blooms ( [[#Wagner--2009|Wagner and Adrian, 2009]] ; [[#Kosten--2012|Kosten et al., 2012]] ; [[#O’Neil--2012|O’Neil et al., 2012]] ; [[#De%20Senerpont%20Domis--2013|De Senerpont Domis et al., 2013]] ; [[#Adrian--2016|Adrian et al., 2016]] ; [[#Visser--2016|Visser et al., 2016]] ; [[#Huisman--2018|Huisman et al., 2018]] ) ( ''very high confidence'' ) ''.'' Conversely '','' warming can reduce cyanobacteria in hypertrophic lakes ( [[#Richardson--2019|Richardson et al., 2019]] ). Freshwater cyanobacteria may benefit directly from elevated CO 2 concentrations ( [[#Visser--2016|Visser et al., 2016]] ; [[#Ji--2017|Ji et al., 2017]] ; [[#Huisman--2018|Huisman et al., 2018]] ; [[#Richardson--2019|Richardson et al., 2019]] ).

Macrophyte growth in freshwaters is likely to increase with rising water temperatures, atmospheric CO 2 and precipitation ( ''robust evidence'' , ''high agreement'' ) ( [[#Dhir--2015|Dhir, 2015]] ; [[#Hossain--2016|Hossain et al., 2016]] ; [[#Short--2016|Short et al., 2016]] ; [[#Reitsema--2018|Reitsema et al., 2018]] ). Nonetheless, primary productivity in rivers is variable and unpredictable ( [[#Bernhardt--2018|Bernhardt et al., 2018]] ) because seasonal variations in temperature and light are uncorrelated, frequent high-flow events reduce biomass of autotrophs and droughts can strand and desiccate autotrophs.

In large, nutrient-poor lakes, warming-induced prolonged thermal stratification can reduce primary production ( ''medium confidence'' ) ( [[#Kraemer--2017|Kraemer et al., 2017]] ). Warming may reduce phytoplankton concentrations when temperature-induced increases in consumption of phytoplankton outpace increases in phytoplankton production ( [[#De%20Senerpont%20Domis--2013|De Senerpont Domis et al., 2013]] ). These decreases in productivity may be under-recognised responses to climate change.

Summary: There is ''robust'' evidence of an increase in primary production along with warming trends. However, increases or declines of algae cannot entirely be attributed to climate change; they are lake-specific and modulated through weather conditions, lake morphology, salinity, land use and restoration and biotic interactions ( ''medium confidence'' ) ( [[#O’Beirne--2017|O’Beirne et al., 2017]] ; [[#Velthuis--2017|Velthuis et al., 2017]] ; [[#Rusak--2018|Rusak et al., 2018]] ; [[#Ho--2019|Ho et al., 2019]] ).

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