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

==== 5.2.3.3 Emissions from Ocean, Inland Water Bodies and Estuaries ====

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Since AR5 (WGI, Section 6.4.3), new estimates of the global ocean N <sub>2</sub> O source derived from ocean biogeochemistry models are 3.4 (2.5–4.3) TgN yr <sup>–1</sup> for the period 2007–2016 (Figure 5.16; [[#Manizza--2012|Manizza et al., 2012]] ; [[#Suntharalingam--2012|Suntharalingam et al., 2012]] ; [[#Martinez-Rey--2015|Martinez-Rey et al., 2015]] ; [[#Landolfi--2017|Landolfi et al., 2017]] ; [[#Buitenhuis--2018|Buitenhuis et al., 2018]] ; [[#Tian--2020|Tian et al., 2020]] ). This is slightly lower than climatological estimates from empirically based methods and surface ocean data syntheses ( [[#Bianchi--2012|Bianchi et al., 2012]] ; S. [[#Yang--2020|]] [[#Yang--2020|Yang et al., 2020]] ). Nitrous oxide processes in coastal upwelling zones continue to be poorly represented in global estimates of marine N <sub>2</sub> O emissions ( [[#Kock--2016|Kock et al., 2016]] ), but may account for an additional 0.2–0.6 TgN yr <sup>–1</sup> of the global ocean source ( [[#Seitzinger--2000|Seitzinger et al., 2000]] ; [[#Nevison--2004|Nevison et al., 2004]] ).

In the oxic ocean (&gt;97% of ocean volume), nitrification is believed to be the primary N <sub>2</sub> O source ( [[#Freing--2012|Freing et al., 2012]] ). In sub-oxic ocean zones ( [[#5.3|Section 5.3]] ), where denitrification prevails, higher N <sub>2</sub> O yields and turnover rates make these regions potentially significant sources of N <sub>2</sub> O ( [[#Arévalo-Martínez--2015|Arévalo-Martínez et al., 2015]] ; [[#Babbin--2015|Babbin et al., 2015]] ; [[#Ji--2015|Ji et al., 2015]] ). The relative proportion of ocean N <sub>2</sub> O from oxygen-minimum zones is highly uncertain ( [[#Zamora--2012|Zamora et al., 2012]] ). Estimates derived from in situ sampling, particularly in the eastern tropical Pacific, suggest significant fluxes from these regions, and potentially account for up to 50% of the global ocean source ( [[#Codispoti--2010|Codispoti, 2010]] ; [[#Arévalo-Martínez--2015|Arévalo-Martínez et al., 2015]] ; [[#Babbin--2015|Babbin et al., 2015]] ). However, recent global-scale analyses estimate lower contributions (4–7%, [[#Battaglia--2018b|Battaglia and Joos, 2018b]] ; [[#Buitenhuis--2018|Buitenhuis et al., 2018]] ). Further investigation is required to reconcile these estimates and provide improved constraints on the N <sub>2</sub> O source from low-oxygen zones.

Atmospheric deposition of anthropogenic N on oceans can stimulate marine productivity and influence ocean emissions of N <sub>2</sub> O. New ocean model analyses since AR5 (WGI, 6.4.3), suggest a relatively modest global potential impact of 0.01–0.32 TgN yr <sup>–1</sup> (pre-industrial to present-day) equivalent to 0.5–3.3% of the global ocean N <sub>2</sub> O source ( [[#Suntharalingam--2012|Suntharalingam et al., 2012]] ; [[#Jickells--2017|Jickells et al., 2017]] ; [[#Landolfi--2017|Landolfi et al., 2017]] ). However, larger proportionate impacts are predicted in nitrogen-limited coastal and inland waters downwind of continental pollution outflow, such as the Northern Indian Ocean ( [[#Jickells--2017|Jickells et al., 2017]] ; [[#Suntharalingam--2019|Suntharalingam et al., 2019]] ).

Inland waters and estuaries are generally sources of N <sub>2</sub> O as a result of nitrification and denitrification of dissolved inorganic nitrogen, however, they can serve as N <sub>2</sub> O sinks in specific conditions ( [[#Webb--2019|Webb et al., 2019]] ). Since AR5 (WGI, 6.4.3), improved emissions factors, including their spatio-temporal scaling, and consideration of transport within the aquatic system allows for better constraint of these emissions ( [[#Murray--2015|Murray et al., 2015]] ; [[#Hu--2016|Hu et al., 2016]] ; [[#Lauerwald--2019|Lauerwald et al., 2019]] ; [[#Maavara--2019|Maavara et al., 2019]] ; [[#Kortelainen--2020|Kortelainen et al., 2020]] ; [[#Yao--2020|Yao et al., 2020]] ). Despite uncertainties because of the side effects of canals and reservoirs on nutrient cycling, these advances permit attribution of a fraction of inland water N <sub>2</sub> O emissions to anthropogenic sources ( [[#Tian--2020|Tian et al., 2020]] ), which contributes to the increased anthropogenic share of the global N <sub>2</sub> O source in this report compared to AR5 ( [[#Ciais--2013|Ciais et al., 2013]] ). As an indirect consequence of agricultural nitrogen use and waste-water treatment, the anthropogenic emissions from inland waters have increased by about a quarter (0.1 TgN yr <sup>–1</sup> ) between the 1980s and 2007–2016 ( [[#Tian--2020|Tian et al., 2020]] ).

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