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

==== 2.3.3.1 Atmospheric trends ====

<div id="section-2-3-3-1-atmospheric-trends-block-1"></div>

The atmospheric abundance of N <sub>2</sub> O has increased since 1750, from a pre-industrial concentration of 270 ppbv to 330 ppbv in 2017 (high agreement, robust evidence) (US National Oceanographic and Atmospheric Agency, Earth Systems Research Laboratory)

(Figure 2.10). The rate of increase has also increased, from approximately 0.15 ppbv yr <sup>–1</sup> 100 years ago, to 0.85 ppbv yr <sup>–1</sup> over the period 2001–2015 (Wells et al. 2018 <sup>[[#fn:r661|661]]</sup> ). Atmospheric N <sub>2</sub> O isotopic composition (14/15N) was relatively constant during the pre-industrial period (Prokopiou et al. 2018 <sup>[[#fn:r662|662]]</sup> ) and shows a decrease in the δ15N as the N <sub>2</sub> O mixing ratio in the atmosphere has increased between 1940 and 2005. This recent decrease indicates that terrestrial sources are the primary driver of increasing trends and marine sources contribute around 25% (Snider et al. 2015 <sup>[[#fn:r663|663]]</sup> ). Microbial denitrification and nitrification processes are responsible for more than 80% of total global N <sub>2</sub> O emissions, which includes natural soils, agriculture and oceans, with the remainder coming from non-biological sources such as biomass burning and fossil-fuel combustion (Fowler et al. 2015 <sup>[[#fn:r664|664]]</sup> ). The isotopic trend also indicates a shift from denitrification to nitrification as the primary source of N <sub>2</sub> O as a result of the use of synthetic nitrogen fertiliser (high evidence, high agreement) (Park et al. 2012 <sup>[[#fn:r665|665]]</sup> ; Toyoda et al. 2013 <sup>[[#fn:r666|666]]</sup> ; Snider et al. 2015 <sup>[[#fn:r667|667]]</sup> ; Prokopiou et al. 2018 <sup>[[#fn:r668|668]]</sup> ).

The three independent sources of N <sub>2</sub> O emissions estimates from agriculture at global, regional and national levels are: USEPA, EDGAR and FAOSTAT (USEPA 2013 <sup>[[#fn:r669|669]]</sup> ; Tubiello et al. 2015 <sup>[[#fn:r670|670]]</sup> ; Janssens-Maenhout et al. 2017a <sup>[[#fn:r671|671]]</sup> ). EDGAR and FAOSTAT have temporal resolution beyond 2005 and these databases compare well with national inventory data (Figure 2.10). USEPA has historical estimates through 2005 and projections thereafter. The independent data use IPCC methods, with Tier 1 emission factors and national reporting of activity data. Tier 2 approaches are also available based on top-down and bottom-up approaches. Recent estimates using inversion modelling and process models estimate total annual global N <sub>2</sub> O emissions of 16.1–18.7 (bottom-up) and 15.9–17.7 TgN (top-down), demonstrating relatively close agreement (Thompson et al. 2014 <sup>[[#fn:r672|672]]</sup> ). Agriculture is the largest source and has increased with extensification and intensification. Recent modelling estimates of terrestrial sources show a higher emissions range that is slightly more constrained than what was reported in AR5: approximately 9 (7–11) TgN <sub>2</sub> O-N yr <sup>–1</sup> (Saikawa et al. 2014 <sup>[[#fn:r673|673]]</sup> ; Tian et al. 2016 <sup>[[#fn:r674|674]]</sup> ) compared to 6.6 (3.3–9.0) TgN <sub>2</sub> O-N yr <sup>–1</sup> (Ciais et al. 2013a <sup>[[#fn:r675|675]]</sup> ). Estimates of marine N <sub>2</sub> O emissions are between 2.5 and 4.6 TgN <sub>2</sub> O-N yr <sup>–1</sup> (Buitenhuis et al., 2018 <sup>[[#fn:r676|676]]</sup> ; Saikawa et al., 2014 <sup>[[#fn:r677|677]]</sup> ).

To conclude, N <sub>2</sub> O is continuing to accumulate in the atmosphere at an increasingly higher rate (very high confidence), driven primarily by increases in manure production and synthetic nitrogen fertiliser use from the mid-20th century onwards (high confidence). Findings since AR5 have constrained regional and global estimates of annual N <sub>2</sub> O emissions and improved our understanding of the spatio-temporal dynamics of N <sub>2</sub> O emissions, including soil rewetting and freeze-thaw cycles which are important determinants of total annual emission fluxes in some regions (medium confidence).

<div id="section-2-3-3-1-atmospheric-trends-block-2"></div>

<span id="figure-2.10"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 2.10'''

<span id="globally-averaged-atmospheric-n2o-mixing-ratios-since-1984.-data-source-noaaesrl-global-monitoring-division-www.esrl.noaa.govgmdhatscombinedn2o.html."></span>
<!-- IMG CAPTION -->
'''Globally averaged atmospheric N2O mixing ratios since 1984. Data source: NOAA/ESRL Global Monitoring Division (www.esrl.noaa.gov/gmd/hats/combined/N2O.html).'''
<!-- IMG FILE -->
[[File:7682712740be97b970691aad4a5a9f5f Figure-2.10-1024x599.jpg]]

Globally averaged atmospheric N <sub>2</sub> O mixing ratios since 1984. Data source: NOAA/ESRL Global Monitoring Division ( [[IPCC:Srccl:Chapter:Chapter-2:Www.esrl.noaa.gov:Gmd:Hats:Combined:N2o.html|www.esrl.noaa.gov/gmd/hats/combined/N <sub>2</sub> O.html]] ).

<!-- END IMG -->
<div id="section-2-3-3-1-atmospheric-trends-block-3"></div>

<span id="figure-2.11"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 2.11'''

<span id="average-agricultural-n2o-emissions-estimates-from-1990.-sub-sectorial-agricultural-emissions-are-based-on-the-emissions-database-for-global-atmospheric-research-edgar-v4.3.2-janssens-maenhout-et-al.-2017a-faostat-tubiello-et-al.-2013-and-national-ghgi-data-grassi-et-al.-2018.-ghgi-data-are-aggregate-values-for-the-sector.-note-that-edgar-data-are-complete-only-through"></span>
<!-- IMG CAPTION -->
'''Average agricultural N2O emissions estimates from 1990. Sub-sectorial agricultural emissions are based on the Emissions Database for Global Atmospheric Research (EDGAR v4.3.2; Janssens-Maenhout et al. 2017a); FAOSTAT (Tubiello et al. 2013) and National GHGI data (Grassi et al. 2018). GHGI data are aggregate values for the sector. Note that EDGAR data are complete only through […]'''
<!-- IMG FILE -->
[[File:42cde1d261437e74b8cd743bfb1e2db2 Figure-2.11-1024x577.jpg]]

Average agricultural N <sub>2</sub> O emissions estimates from 1990. Sub-sectorial agricultural emissions are based on the Emissions Database for Global Atmospheric Research (EDGAR v4.3.2 <sup>[[#fn:r2128|2128]]</sup> ; Janssens-Maenhout et al. 2017a <sup>[[#fn:r2129|2129]]</sup> ); FAOSTAT (Tubiello et al. 2013 <sup>[[#fn:r2130|2130]]</sup> ) and National GHGI data (Grassi et al. 2018 <sup>[[#fn:r2131|2131]]</sup> ). GHGI data are aggregate values for the sector. Note that EDGAR data are complete only through 2012; the EDGAR data in the right-hand panel represent the three years 2010–2012 and are presented for comparison.

<!-- END IMG -->
<div id="section-2-3-3-2-land-use-effects"></div>

<span id="land-use-effects-1"></span>