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

ABSTRACT

Assessment of the global budget of the greenhouse gas nitrous oxide ([Formula see text]O) is limited by poor knowledge of the oceanic [Formula see text]O flux to the atmosphere, of which the magnitude, spatial distribution, and temporal variability remain highly uncertain. Here, we reconstruct climatological [Formula see text]O emissions from the ocean by training a supervised learning algorithm with over 158,000 [Formula see text]O measurements from the surface ocean-the largest synthesis to date. The reconstruction captures observed latitudinal gradients and coastal hot spots of [Formula see text]O flux and reveals a vigorous global seasonal cycle. We estimate an annual mean [Formula see text]O flux of 4.2 ± 1.0 Tg N[Formula see text], 64% of which occurs in the tropics, and 20% in coastal upwelling systems that occupy less than 3% of the ocean area. This [Formula see text]O flux ranges from a low of 3.3 ± 1.3 Tg N[Formula see text] in the boreal spring to a high of 5.5 ± 2.0 Tg N[Formula see text] in the boreal summer. Much of the seasonal variations in global [Formula see text]O 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 [Formula see text]O flux to El Niño-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 [Formula see text]O budget.