| Summary: | Effective mitigation for N _2 O emissions, now the third most important anthropogenic greenhouse gas and the largest remaining anthropogenic source of stratospheric ozone depleting substances, requires understanding of the sources and how they may increase this century. Here we update estimates and their uncertainties for current anthropogenic and natural N _2 O emissions and for emissions scenarios to 2050. Although major uncertainties remain, ‘bottom-up’ inventories and ‘top-down’ atmospheric modeling yield estimates that are in broad agreement. Global natural N _2 O emissions are most likely between 10 and 12 Tg N _2 O-N yr ^−1 . Net anthropogenic N _2 O emissions are now about 5.3 Tg N _2 O-N yr ^−1 . Gross anthropogenic emissions by sector are 66% from agriculture, 15% from energy and transport sectors, 11% from biomass burning, and 8% from other sources. A decrease in natural emissions from tropical soils due to deforestation reduces gross anthropogenic emissions by about 14%. Business-as-usual emission scenarios project almost a doubling of anthropogenic N _2 O emissions by 2050. In contrast, concerted mitigation scenarios project an average decline of 22% relative to 2005, which would lead to a near stabilization of atmospheric concentration of N _2 O at about 350 ppb. The impact of growing demand for biofuels on future projections of N _2 O emissions is highly uncertain; N _2 O emissions from second and third generation biofuels could remain trivial or could become the most significant source to date. It will not be possible to completely eliminate anthropogenic N _2 O emissions from agriculture, but better matching of crop N needs and N supply offers significant opportunities for emission reductions.
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