Global reconstruction reduces the uncertainty of oceanic nitrous oxide emissions and reveals a vigorous seasonal cycle

Assessment of the global budget of the greenhouse gas nitrous oxide (N2O) is limited by poor knowledge of the oceanic N2O flux to the atmosphere, of which the magnitude, spatial distribution, and temporal variability remain highly uncertain. Here, we reconstruct climatological N2O emissions from the ocean by training a supervised learning algorithm with over 158,000 N2O measurements from the surface ocean-the largest synthesis to date. The reconstruction captures observed latitudinal gradients and coastal hot spots of N2O flux and reveals a vigorous global seasonal cycle. We estimate an annual mean N2O flux of 4.2 +/- 1.0 Tg N.y(-1), 64% of which occurs in the tropics, and 20% in coastal upwelling systems that occupy less than 3% of the ocean area. This N2O flux ranges from a low of 3.3 +/- 1.3 Tg N.y(-1) in the boreal spring to a high of 5.5 +/- 2.0 Tg N.y(-1) in the boreal summer. Much of the seasonal variations in global N2O emissions can be traced to seasonal upwelling in the tropical ocean and winter mixing in the Southern Ocean. The dominant contribution to seasonality by productive, low-oxygen tropical upwelling systems (>75%) suggests a sensitivity of the global N2O flux to El Nino-Southern Oscillation and anthropogenic stratification of the low latitude ocean. This ocean flux estimate is consistent with the range adopted by the Intergovernmental Panel on Climate Change, but reduces its uncertainty by more than fivefold, enabling more precise determination of other terms in the atmospheric N2O budget.