Nitrous oxide emissions 1999 to 2009 from a global atmospheric inversion

N2O surface fluxes were estimated for 1999 to 2009 using a time-dependent Bayesian inversion technique. Observations were drawn from 5 different networks, incorporating 59 surface sites and a number of ship-based measurement series. To avoid biases in the inverted fluxes, the data were adjusted to a common scale and scale offsets were included in the optimization problem. The fluxes were calculated at the same resolution as the transport model (3.75 longitude × 2.5 latitude) and at monthly time resolution. Over the 11-year period, the global total N2O source varied from 17.5 to 20.1 Tg a-1 N. Tropical and subtropical land regions were found to consistently have the highest N2O emissions, in particular in South Asia (20 ± 3% of global total), South America (13 ± 4%) and Africa (19 ± 3%), while emissions from temperate regions were smaller: Europe (6 ± 1%) and North America (7 ± 2%). A significant multi-annual trend in N2O emissions (0.045 Tg a-2 N) from South Asia was found and confirms inventory estimates of this trend. Considerable interannual variability in the global N2O source was observed (0.8 Tg a-1 N, 1 standard deviation, SD) and was largely driven by variability in tropical and subtropical soil fluxes, in particular in South America (0.3 Tg a-1 N, 1 SD) and Africa (0.3 Tg a-1 N, 1 SD). Notable variability was also found for N2O fluxes in the tropical and southern oceans (0.15 and 0.2 Tg a-1 N, 1 SD, respectively). Interannual variability in the N2O source shows some correlation with the El Niño-Southern Oscillation (ENSO), where El Niño conditions are associated with lower N2O fluxes from soils and from the ocean and vice versa for La Niña conditions.