Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.2.3.2 Anthropogenic N <sub>2</sub> O Emissions ====

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The AR5 (WGI, Section 6.4.3) and SRCCL ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3|Section 2.3.3]] ) concluded that agriculture is the largest anthropogenic source of N <sub>2</sub> O emissions. Since SRCCL (2.3.3), a new synthesis of inventory-based and modelling studies shows that the widespread use of synthetic fertilizers and manure on cropland and pasture, manure management and aquaculture resulted in 3.8 (2.5–5.8) TgN yr <sup>–1</sup> (average 2007–2016) ( ''robust evidence, high agreement'' ) (Table 5.3; [[#Winiwarter--2018|Winiwarter et al., 2018]] ; [[#FAO--2019|FAO, 2019]] ; [[#Janssens-Maenhout--2019|Janssens-Maenhout et al., 2019]] ; [[#Tian--2020|Tian et al., 2020]] ). Observations from field-measurements ( [[#Song--2018|Song et al., 2018]] ), inventories ( [[#Wang--2020|Wang et al., 2020]] ) and atmospheric inversions ( [[#Thompson--2019|Thompson et al., 2019]] ) further corroborate the assessment of SRCCL that there is a non-linear relationship between N <sub>2</sub> O emissions and nitrogen input, implying an increasing fraction of fertilizer lost as N <sub>2</sub> O with larger fertilizer excess ( ''medium evidence'' , ''high agreement'' ). Several studies using complementary methods indicate that agricultural N <sub>2</sub> O emissions have increased by more than 45% since the 1980s ( ''high confidence'' ) (Figure 5.16 and Table 5.3; [[#Davidson--2009|Davidson, 2009]] ; [[#Winiwarter--2018|Winiwarter et al., 2018]] ; [[#Janssens-Maenhout--2019|Janssens-Maenhout et al., 2019]] ; [[#Tian--2020|Tian et al., 2020]] ), mainly due to the increased use of nitrogen fertilizer and manure. N <sub>2</sub> O emissions from aquaculture are among the fastest rising contributors of N <sub>2</sub> O emissions, but their overall magnitude is still small in the overall N <sub>2</sub> O budget ( [[#Tian--2020|Tian et al., 2020]] ).

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[[File:6f0779b71e8fa59d513466d8418a459c IPCC_AR6_WGI_Figure_5_16.png]]

'''Figure 5.16 |''' '''Decadal mean nitrous oxide (N''' <sub>2</sub> '''O) emissions for 2007–2016 and its change since 1850 based on process-model projections''' . The total effect, including that from anthropogenic nitrogen additions (atmospheric deposition, manure addition, fertilizer use and land-use), is evaluated against the background flux driven by changes in atmospheric carbon dioxide (CO <sub>2</sub> ) concentration, and climate change. Fluxes are derived from the N <sub>2</sub> O model intercomparison project ensemble of terrestrial biosphere models ( [[#Tian--2019|Tian et al., 2019]] ) and three ocean biogeochemical models ( [[#Landolfi--2017|Landolfi et al., 2017]] ; [[#Battaglia--2018a|Battaglia and Joos, 2018a]] ; [[#Buitenhuis--2018|Buitenhuis et al., 2018]] ). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

The principal non-agricultural anthropogenic sources of N <sub>2</sub> O are industry, specifically chemical processing, wastewater, and the combustion of fossil fuels (Table 5.3). Industrial emissions of N <sub>2</sub> O mainly due to nitric and adipic acid production have decreased in North America and Europe since the widespread installation of abatement technologies in the 1990s (Pérez-Ram '''ί''' rez et al., 2003; [[#Lee--2011|Lee et al., 2011]] ; [[#Janssens-Maenhout--2019|Janssens-Maenhout et al., 2019]] ). There is still considerable uncertainty in industrial emissions from other regions of the world with contrasting trends between inventories ( [[#Thompson--2019|Thompson et al., 2019]] ). Globally, industrial emissions and emissions from fossil fuel combustion by stationary sources, such as power plants, as well as smaller emissions from mobile sources (e.g., road transport and aviation) have remained nearly constant between the 1980s and 2007–2016 ( ''medium evidence'' , ''medium agreement'' ) ( [[#Winiwarter--2018|Winiwarter et al., 2018]] ; [[#Janssens-Maenhout--2019|Janssens-Maenhout et al., 2019]] ; [[#Tian--2020|Tian et al., 2020]] ). Wastewater N <sub>2</sub> O emissions, including those from domestic and industrial sources, have increased from 0.2 (0.1–0.3) TgN yr <sup>–1</sup> to 0.35 (0.2–0.5) TgN yr <sup>–1</sup> between the 1980s and 2007–2016 ( [[#Tian--2020|Tian et al., 2020]] ).

Biomass burning from crop residue burning, grassland, savannah and forest fires, as well as biomass burnt in household stoves, releases N <sub>2</sub> O during the combustion of organic matter. Updated inventories since AR5 (WGI, Section 6.4.3) result in a lower range of the decadal mean emissions of 0.6 (0.5–0.8) TgN yr <sup>–1</sup> ( [[#van%20der%20Werf--2017|van der Werf et al., 2017]] ; [[#Tian--2020|Tian et al., 2020]] ). The attribution of grassland, savannah or forest fires to natural or anthropogenic origins is uncertain, preventing a separation of the biomass burning source into natural and anthropogenic.

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