NASA - National Aeronautics and Space Administration

N2 O 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 N2 O source varied from 17.5 to 20.1 Tg a−1 N. Tropical and subtropical land regions were found to consistently have the highest N2 O 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 N2 O 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 N2 O 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 N2 O fluxes in the tropical and southern oceans (0.15 and 0.2 Tg a−1 N, 1 SD, respectively). Interannual variability in the N2 O source shows some correlation with the El Niño–Southern Oscillation (ENSO), where El Niño conditions are associated with lower N2 O fluxes from soils and from the ocean and vice versa for La Niña conditions.