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Assessment of the global budget of the greenhouse gas nitrous oxide (${\mathrm{N}}_2$O) is limited by poor knowledge of the oceanic${\mathrm{N}}_2$O flux to the atmosphere, of which the magnitude, spatial distribution, and temporal variability remain highly uncertain. Here, we reconstruct climatological${\mathrm{N}}_2$O emissions from the ocean by training a supervised learning algorithm with over 158,000${\mathrm{N}}_2$O measurements from the surface ocean—the largest synthesis to date. The reconstruction captures observed latitudinal gradients and coastal hot spots of${\mathrm{N}}_2$O flux and reveals a vigorous global seasonal cycle. We estimate an annual mean${\mathrm{N}}_2$O flux of 4.2 ± 1.0 Tg N$\cdot {\mathrm{y}}^{-1}$, 64% of which occurs in the tropics, and 20% in coastal upwelling systems that occupy less than 3% of the ocean area. This${\mathrm{N}}_2$O flux ranges from a low of 3.3 ± 1.3 Tg N$\cdot {\mathrm{y}}^{-1}$in the boreal spring to a high of 5.5 ± 2.0 Tg N$\cdot {\mathrm{y}}^{-1}$in the boreal summer. Much of the seasonal variations in global${\mathrm{N}}_2$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${\mathrm{N}}_2$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${\mathrm{N}}_2$O budget.

 

Abstract. In the current era of rapid climate change, accuratecharacterization of climate-relevant gas dynamics – namely production,consumption, and net emissions – is required for all biomes, especially thoseecosystems most susceptible to the impact of change. Marine environmentsinclude regions that act as net sources or sinks for numerous climate-activetrace gases including methane (CH4) and nitrous oxide (N2O). Thetemporal and spatial distributions of CH4 and N2O are controlledby the interaction of complex biogeochemical and physical processes. Toevaluate and quantify how these mechanisms affect marine CH4 andN2O cycling requires a combination of traditional scientificdisciplines including oceanography, microbiology, and numerical modeling.Fundamental to these efforts is ensuring that the datasets produced byindependent scientists are comparable and interoperable. Equally critical istransparent communication within the research community about the technicalimprovements required to increase our collective understanding of marineCH4 and N2O. A workshop sponsored by Ocean Carbon and Biogeochemistry (OCB)was organized to enhance dialogue and collaborations pertaining tomarine CH4 and N2O. Here, we summarize the outcomes from theworkshop to describe the challenges and opportunities for near-futureCH4 and N2O research in the marine environment. 

Abstract. Large-scale climatic forcing is impactingoceanic biogeochemical cycles and is expected to influence the water-columndistribution of trace gases, including methane and nitrous oxide. Our abilityas a scientific community to evaluate changes in the water-column inventoriesof methane and nitrous oxide depends largely on our capacity to obtain robustand accurate concentration measurements that can be validated acrossdifferent laboratory groups. This study represents the first formalinternational intercomparison of oceanic methane and nitrous oxidemeasurements whereby participating laboratories received batches of seawatersamples from the subtropical Pacific Ocean and the Baltic Sea. Additionally,compressed gas standards from the same calibration scale were distributed tothe majority of participating laboratories to improve the analytical accuracyof the gas measurements. The computations used by each laboratory to derivethe dissolved gas concentrations were also evaluated for inconsistencies(e.g., pressure and temperature corrections, solubility constants). Theresults from the intercomparison and intercalibration provided invaluableinsights into methane and nitrous oxide measurements. It was observed thatanalyses of seawater samples with the lowest concentrations of methane andnitrous oxide had the lowest precisions. In comparison, while the analyticalprecision for samples with the highest concentrations of trace gases wasbetter, the variability between the different laboratories was higher:36% for methane and 27% for nitrous oxide. In addition, thecomparison of different batches of seawater samples with methane and nitrousoxide concentrations that ranged over an order of magnitude revealed theramifications of different calibration procedures for each trace gas.Finally, this study builds upon the intercomparison results to developrecommendations for improving oceanic methane and nitrous oxide measurements,with the aim of precluding future analytical discrepancies betweenlaboratories.