UC Irvine

Nitrous oxide (N₂O), like carbon dioxide, is a long-lived greenhouse gas that accumulates in the atmosphere. Over the past 150 years, increasing atmospheric N₂O concentrations have contributed to stratospheric ozone depletion¹ and climate change², with the current rate of increase estimated at 2 per cent per decade. Existing national inventories do not provide a full picture of N₂O emissions, owing to their omission of natural sources and limitations in methodology for attributing anthropogenic sources. Here we present a global N₂O inventory that incorporates both natural and anthropogenic sources and accounts for the interaction between nitrogen additions and the biochemical processes that control N₂O emissions. We use bottom-up (inventory, statistical extrapolation of flux measurements, process-based land and ocean modelling) and top-down (atmospheric inversion) approaches to provide a comprehensive quantification of global N₂O sources and sinks resulting from 21 natural and human sectors between 1980 and 2016. Global N₂O emissions were 17.0 (minimum-maximum estimates: 12.2-23.5) teragrams of nitrogen per year (bottom-up) and 16.9 (15.9-17.7) teragrams of nitrogen per year (top-down) between 2007 and 2016. Global human-induced emissions, which are dominated by nitrogen additions to croplands, increased by 30% over the past four decades to 7.3 (4.2-11.4) teragrams of nitrogen per year. This increase was mainly responsible for the growth in the atmospheric burden. Our findings point to growing N₂O emissions in emerging economies-particularly Brazil, China and India. Analysis of process-based model estimates reveals an emerging N₂O-climate feedback resulting from interactions between nitrogen additions and climate change. The recent growth in N₂O emissions exceeds some of the highest projected emission scenarios^3,4, underscoring the urgency to mitigate N₂O emissions.