N2O Emission Trends From a Global Atmospheric Inversion

Abstract:

Nitrous oxide (N₂O) is the third most important long-lived greenhouse gas and contributes strongly to stratospheric ozone depletion through the formation of NO. Concentrations of N₂O in the atmosphere have increased by approximately 20% since the pre-industrial era owing largely to the intensification of agriculture and the use of mineral nitrogen fertilizers. Top-down methods can be used to constrain the emissions of N₂O using observations of atmospheric concentrations. Inverse modelling is a top-down approach, which relates changes in N₂O concentrations to the emissions with the help of an atmospheric transport model. Using the global inverse model, LMDz-PyVar, we estimated N₂O emissions from 1996 to 2012 (covering the period when sufficient atmospheric observations are available). Emissions were estimated monthly with a horizontal resolution of 3.25° × 1.875°.

From the inversion, we estimate a global mean emission of 17.0 ± 0.8 TgN y^-1, however, the emissions varied substantially from year-to-year. The largest inter-annual variability was located in the tropics and subtropics, where it appears to be correlated with ENSO climate variability. We did not find any significant trend in the global emission over 1996 – 2012, however, we did find important trends on continental scales. In South and East Asia, South America and Africa, N₂O emissions increased, consistent with increasing use of N-fertilizer. In contrast, in Europe and North America, N₂O emissions decreased. In Europe, this is correlated with a decrease in N-fertilizer use, while in North America the decrease is possibly due to climate variability and changes in agricultural practices.