Nitrous oxide (N2O) is the third most important long-lived GHG and an important stratospheric ozone depleting substance. Agricultural practices and the use of N-fertilizers have greatly enhanced emissions of N2O. Here, we present estimates of N2O emissions determined from three global atmospheric inversion frameworks during the period 1998–2016. We find that global N2O emissions increased substantially from 2009 and at a faster rate than estimated by the IPCC emission factor approach. The regions of East Asia and South America made the largest contributions to the global increase. From the inversion-based emissions, we estimate a global emission factor of 2.3 ± 0.6%, which is significantly larger than the IPCC Tier-1 default for combined direct and indirect emissions of 1.375%. The larger emission factor and accelerating emission increase found from the inversions suggest that N2O emission may have a nonlinear response at global and regional scales with high levels of N-input.
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We acknowledge the people and institutions who provided atmospheric observations of N2O that were used in the inversions or for validation, namely: E. Dlugokencky, G. Dutton, C. Sweeney (NOAA); J. Mühle (UCSD), P. Krummel, P. Fraser, L. P. Steele, R. Wang (CSIRO); S. O’Doherty, D. Young (Bristol University); Y. Tohjima, T. Machida (NIES); T. Laurila, J. Hatakka, T. Aalto (FMI); J. Moncrieff (University of Edinburgh); and H. Matsueda, Y. Sawa (MRI-JMA). AGAGE is supported principally by NASA (USA) grants to MIT and SIO, and also by BEIS (UK) and NOAA (USA) grants to Bristol University, CSIRO and BoM (Australia); FOEN grants to Empa (Switzerland), NILU (Norway), SNU (Korea), CMA (China), NIES (Japan) and Urbino University (Italy). We thank W. Feng (NCAS Leeds) for TOMCAT model support. L.L. is supported by MINEC-Spain and European Commission ERDF Ramón y Cajal grant (RYC-2016-20269), Programa Propio from UPM, and acknowledges the Comunidad de Madrid (Spain) and structural funds 2014-2020 (ERDF and ESF), project AGRISOST-CM S2018/ BAA-4330. R.L.T. acknowledges financial support from VERIFY (grant no. 76810) funded by the European Commission under the H2020 programme, H.T. acknowledges support from OUC–AU Joint Center. P.K.P. is partly supported by the Environment Research and Technology Development Fund (no. 2-1802) of the Ministry of the Environment, Japan. The authors are grateful to G. Billen and J. Garnier for useful comments, and to the Food and Agriculture Organization of the United Nations.