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

==== 5.2.3.5 N <sub>2</sub> O Budget ====

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The synthesis of bottom-up estimates of N <sub>2</sub> O sources (Sections 5.2.3.2–5.2.3.4 and Figure 5.17) yields a global source of 17.0 (12.2 to 23.5) TgN yr <sup>–1</sup> for the years 2007–2016 (Table 5.3). This estimate is comparable to AR5, but the uncertainty range has been reduced primarily due to improved estimates of ocean and anthropogenic N <sub>2</sub> O sources. Since AR5 (WGI, Section 6.4.3), improved capacity to estimate N <sub>2</sub> O sources from atmospheric N <sub>2</sub> O measurements by inverting models of atmospheric transport provides a new and independent constraint for the global N <sub>2</sub> O budget ( [[#Saikawa--2014|Saikawa et al., 2014]] ; [[#Thompson--2019|Thompson et al., 2019]] ; [[#Tian--2020|Tian et al., 2020]] ). The decadal mean source derived from these inversions is remarkably consistent with the bottom-up global N <sub>2</sub> O budget for the same period, however, the split between land and ocean sources based on atmospheric inversions is less constrained, yielding a smaller land source of 11.3 (10.2 to 13.2) TgN yr <sup>–1</sup> and a larger ocean source of 5.7 (3.4 to 7.2) TgN yr <sup>–1</sup> , respectively, compared to bottom-up estimates.

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[[File:f57d2c2590e8bcd70a228730cd6cefc3 IPCC_AR6_WGI_Figure_5_17.png]]

'''Figure 5.17 |''' '''Global nitrous oxide (N''' <sub>2</sub> '''O) budget (2007–2016).''' Values and data sources as in Table 5.3. The atmospheric stock is calculated from mean N <sub>2</sub> O concentration, multiplying a factor of 4.79 ± 0.05 Tg ppb <sup>–1</sup> ( [[#Prather--2012|Prather et al., 2012]] ). Pool sizes for the other reservoirs are largely unknown. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

Supported by multiple studies and extensive observational evidence (Sections 5.2.3.2–5.2.3.4 and Figure 5.17), anthropogenic emissions contributed about 40% (7.3; uncertainty range: 4.2 to 11.4 TgN yr <sup>–1</sup> ) to the total N <sub>2</sub> O source in 2007–2016 ( ''high confidence'' ). This estimate is larger than in AR5 (WGI, 6.4.3) due to a larger estimated effect of nitrogen deposition on soil N <sub>2</sub> O emissions and the explicit consideration of the role of anthropogenic nitrogen in determining inland water and estuary emissions.

Based on bottom-up estimates, anthropogenic emissions from agricultural nitrogen use, industry and other indirect effects have increased by 1.7 (1.0 to 2.7) TgN yr <sup>–1</sup> between the decades 1980–1989 and 2007–2016, and are the primary cause of the increase in the total N <sub>2</sub> O source ( ''high confidence'' ). Atmospheric inversions indicate that changes in surface emissions, rather than in the atmospheric transport or sink of N <sub>2</sub> O, are the cause for the increased atmospheric growth rate of N <sub>2</sub> O ( ''robust evidence, high agreement'' ) ( [[#Thompson--2019|Thompson et al., 2019]] ). However, the increase of 1.6 (1.4 to 1.7) TgN yr <sup>–1</sup> in global emissions between 2000–2005 and 2010–2015 based on atmospheric inversions is somewhat larger than bottom-up estimates over the same period, primarily because of differences in the estimates of land-based emissions.

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'''Table 5.3 |''' '''Global N''' <sub>2</sub> '''O budget (units TgN y''' '''r''' –1 ''') averaged over the 1980s, 1990s, 2000s as well as the recent decade starting in 2007''' . Uncertainties represent the assessed range of source/sink estimates. All numbers are reproduced from [[#Tian--2020|Tian et al. (2020)]] based on a compilation of inventories, bottom-up models, as well as atmospheric inversions. For detailed information on datasets, see Data Table 5.SM.6.

{| class="wikitable"
|-
! colspan="2"|
! AR6 1980–1989

(TgN yr <sup>–1</sup> )

! AR6 1990–1999

(TgN yr <sup>–1</sup> )

! AR6 2000–2009

(TgN yr <sup>–1</sup> )

! AR6 (2007–2016)

(TgN yr <sup>–1</sup> )

! AR5 (2006–2011)

(TgN yr <sup>–1</sup> )

|-
! colspan="7"| '''B''' '''ottom-up Budget'''

|-
| colspan="7"| '''Anthro''' '''pogenic sources'''

|-
|
| Fossil fuel combustion and Industry

| 0.9 (0.8 to 1.1)

| 0.9 (0.9 to 1.0)

| 1.0 (0.8 to 1.0)

| 1.0 (0.8 to 1.1)

| 0.7 (0.2 to 1.8)

|-
|
| Agriculture (incl. aquaculture)

| 2.6 (1.8 to 4.1)

| 3.0 (2.1 to 4.8)

| 3.4 (2.3 to 5.2)

| 3.8 (2.5 to 5.8)

| 4.1 (1.7 to 4.8)

|-
|
| Biomass and biofuel burning

| 0.7 (0.7 to 0.7)

| 0.7 (0.6 to 0.8)

| 0.6 (0.6 to 0.6)

| 0.6 (0.5 to 0.8)

| 0.7 (0.2 to 1.0)

|-
|
| Wastewater

| 0.2 (0.1 to 0.3)

| 0.3 (0.2 to 0.4)

| 0.3 (0.2 to 0.4)

| 0.4 (0.2 to 0.5)

| 0.2 (0.1 to 0.3)

|-
|
| Inland water, estuaries, coastal zones

| 0.4 (0.2 to 0.5)

| 0.4 (0.2 to 0.5)

| 0.4 (0.2 to 0.6)

| 0.5 (0.2 to 0.7)

|
|-
|
| Atmospheric nitrogen deposition on ocean

| 0.1 (0.1 to 0.2)

| 0.1 (0.1 to 0.2)

| 0.1 (0.1 to 0.2)

| 0.1 (0.1 to 0.2)

| 0.2 (0.1 to 0.4)

|-
|
| Atmospheric nitrogen deposition on land

| 0.6 (0.3 to 1.2)

| 0.7 (0.4 to 1.4)

| 0.7 (0.4 to 1.3)

| 0.8 (0.4 to 1.4)

| 0.4 (0.3 to 0.9)

|-
|
| Other indirect effects from CO <sub>2</sub> , climate and land-use change

| 0.1 (–0.4 to 0.7)

| 0.1 (–0.5 to 0.7)

| 0.2 (–0.4 to 0.9)

| 0.2 (–0.6 to 1.1)

|
|-
|
| '''Tota''' '''l anthropogenic'''

| '''5.6 (3.6 to 8.7)'''

| '''6.2 (3.9 to 9.6)'''

| '''6.7 (4.1 to 10.3)'''

| '''7.3 (4.2 to 11.4)'''

| '''6.3 (2.6 to 9.2)'''

|-
| colspan="7"| '''Natural so''' '''urces and sinks'''

|-
|
| Rivers, estuaries, and coastal zones

| 0.3 (0.3 to 0.4)

| 0.3 (0.3 to 0.4)

| 0.3 (0.3 to 0.4)

| 0.3 (0.3 to 0.4)

| 0.6 (0.1 to 2.9)

|-
|
| Open oceans

| 3.6 (3.0 to 4.4)

| 3.5 (2.8 to 4.4)

| 3.5 (2.7 to 4.3)

| 3.4 (2.5 to 4.3)

| 3.8 (1.8 to 9.4)

|-
|
| Soils under natural vegetation

| 5.6 (4.9 to 6.6)

| 5.6 (4.9 to 6.5)

| 5.6 (5.0 to 6.5)

| 5.6 (4.9 to 6.5)

| 6.6 (3.3 to 9.0)

|-
|
| Atmospheric chemistry

| 0.4 (0.2 to 1.2)

| 0.4 (0.2 to 1.2)

| 0.4 (0.2 to 1.2)

| 0.4 (0.2 to 1.2)

| 0.6 (0.3 to 1.2)

|-
|
| Surface sink

| –0.01 (–0.3 to 0)

| –0.01 (–0.3 to 0)

| –0.01 (–0.3 to 0)

| –0.01 (–0.3 to 0)

| –0.01 (–1 to 0)

|-
|
| '''Total natural'''

| '''9.9 (8.5''' – '''12.2)'''

| '''9.8 (8.3–12.1)'''

| '''9.8 (8.2''' – '''12.0)'''

| '''9.7 (8.0''' – '''12.0)'''

| '''11.6 (5.5–23.5)'''

|-
| colspan="2"| '''Total b''' '''ottom-up source'''

| '''15.5 (12.1 to 20.9)'''

| '''15.9 (12.2 to 21.7)'''

| '''16.4 (12.3 to 22.4)'''

| '''17.0 (12.2 to 23.5)'''

| '''17.9 (8.1 to 30.7)'''

|-
| colspan="2"| '''Obser''' '''ved growth rate'''

|
| '''3.7 (3.7 to 3.7)'''

| '''4.5 (4.3 to 4.6)'''

| '''3.6 (3.5 to 3.8)'''

|-
| colspan="2"| '''Inferred str''' '''atospheric sink'''

|
| '''12.9 (12.2-13.5)'''

| '''13.1 (12.4–13.6)'''

| '''14.3 (4.3 to 28.7)'''

|-
| colspan="7"| '''Atmosp''' '''heric inversion'''

|-
|
| Atmospheric loss

|
| 12.1 (11.4 to 13.3)

| 12.4 (11.7 to 13.3)

|
|-
|
| Total source

|
| 15.9 (15.1 to 16.9)

| 16.9 (15.9 to 17.7)

|
|-
|
| Imbalance

|
| 3.6 (2.2 to 5.7)

| 4.2 (2.4 to 6.4)

|
|}

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