Data sources:
Tian et al. 2024 Global Nitrous Oxide Budget 1980–2020. Earth System Science Data.

Importance:
Nitrous oxide (N2O) is a long-lived potent greenhouse gas and stratospheric ozone-depleting substance, which has been accumulating in the atmosphere since the pre-industrial period. The mole fraction of atmospheric N2O has increased by nearly 25% from 270 parts per billion (ppb) in 1750 to 336 ppb in 2022 (Tian et al., 2024). Understanding the dynamics of the global N2O budget and attributing changes to different sources is fundamental for tracking climate change and will contribute to the global stocktake of the Paris Agreement to track progress towards national determined contributions and the ultimate goal of achieving net-zero GHG emissions.

The Nitrous oxide budget:
The nitrous oxide (N2O) budget, as described here, refers to all sources and sinks of N2O resulting from both human activities and natural processes. Natural sources accounted for 65% of global total emissions in the 2010s. Among these, natural soils made the largest contribution (35% of global total emissions), followed by open oceans and shelves (26% of global total emissions), and other sources (including natural inland waters, estuaries, coastal vegetation, lightning, and atmospheric production, which constituted 4% of global total emissions). These natural sources have existed since before the Industrial Era and remained relatively stable. Although anthropogenic emissions only accounted for 35% of the global total emissions in the 2010s, they are the major drivers of the increasing N2O emissions over the past four decades. The largest component of anthropogenic emissions is direct agricultural emissions (from fertilizer use, manure management, and aquaculture), which accounted for 20% of total emissions. Emissions from other direct sources (including fossil fuel, industry, waste, wastewater, and biomass burning) and indirect emissions accounted for 12% and 6% of total emissions, respectively. Changes in climate, atmospheric CO2 concentration, and land cover had an overall negative effect on N2O emissions (-3% of total emissions); however, these fluxes are not well constrained, and their amounts are highly uncertain. Of all N2O emitted into the atmosphere, only about one quarter remains, contributing to climate change, while atmospheric N2O loss due to photolysis and oxidation by O(1D) accounted for around three quarters of total emissions.

Estimating the N2O budget:
The Global Carbon Project has established a consortium of more than 50 research institutions around the world to gather the observations, statistics and run global models to update and improve the N2O budget regularly (every 2-3 years).The N2O budget is estimated at the global scale using both “top-down (TD)” and “bottom-up (BU)” approaches, BU approaches include diverse data inventories, observation-driven methods and process-based models (e.g. agricultural data, emission factor-based statistics, biogeochemistry modeling). The four TD models optimally combine measurements of atmospheric N2O mixing ratios from three global observation networks with a first-guess estimate of N2O emissions (prior emission) into atmospheric inversion frameworks. Moreover, we also have estimates for the 5 source categories (“Direct agriculture emission”, “Other direct emission”, “Indirect emission”, “Perturbed fluxes from climate/CO2/land cover change”, and “natural emission”) at national scale, which are based on BU approaches.

Under the leadership of the Global Carbon Project in partnership with International Nitrogen Initiative, Tian et al. (2020) reported the first comprehensive global N2O budget using multiple BU and TD methods, with regional and sectorial details. The first global N2O budget covering the period 1980-2016 was published in Nature. The recent update (Tian et al., 2024) presents an improved and updated global N2O budget (1980-2020) and its regional attribution to 18 land regions and three ocean regions, which is published in Earth System Science Data and contributed by 58 collaborators from 55 Institutions across 15 countries.

Uncertainties:
As the number of available datasets is uneven and generally small, uncertainties of N2O fluxes are reported in brackets as a range between the minimum and maximum estimates. Perturbated N2O fluxes from climate/CO2/land cover change are the most uncertain part and the consortium of scientists is making every effort possible to improve it. In comparison, the long-term trends in N2O emissions from other anthropogenic sources are better constrained.

Future work:
As part of its scientific goals, the Global Carbon Project team will further develop more complete and better constrained N2O budgets with regular updates. For further information visit the Global Carbon Project (GCP) web page.