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 (N2O) is limited by poor knowledge of the oceanicN2O flux to the atmosphere, of which the magnitude, spatial distribution, and temporal variability remain highly uncertain. Here, we reconstruct climatologicalN2O emissions from the ocean by training a supervised learning algorithm with over 158,000N2O measurements from the surface ocean—the largest synthesis to date. The reconstruction captures observed latitudinal gradients and coastal hot spots ofN2O flux and reveals a vigorous global seasonal cycle. We estimate an annual meanN2O 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. ThisN2O flux ranges from a low of 3.3 ± 1.3 Tg N⋅y−1in the boreal spring to a high of 5.5 ± 2.0 Tg N⋅y−1in the boreal summer. Much of the seasonal variations in globalN2O 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 globalN2O 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 atmosphericN2O budget.

On the natural spatio-temporal heterogeneity of South Pacific nitrous oxide

“…The seasonal cycle, which includes terrestrial sources and is heavily influenced by seasonal shifts in the intertropical convergence zone, peaks in Austral winter (January/February) with minima 0.50 ± 0.13 ppb (se) lower in summer (July/August). The rapid N 2 O cycling rates inherent to the highly productive OMZs cause these regions to respond quickly to time-dependent shifts in the underlying biogeochemistry and physical structure of the water column 18 , 24 .…”

“…In contrast, the shallower and steeper subsurface N 2 O maxima in the OMZs combine to enable rapid diffusive mixing into the surface ocean and establish these regions as hotspots of N 2 O efflux to the atmosphere. Critically, in OMZs, N 2 O exhibits low residence times and rapid turnover rates, permitting high variability, both over seasonal and interannual scales 18 , 23 , 24 and as the OMZs expand into the future 25 , 26 . This feedback among OMZ size, N 2 O emissions, and climate emphasizes the importance of quantifying the N 2 O production of these regions.…”

Ideas and perspectives: A strategic assessment of methane and nitrous oxide measurements in the marine environment

“…4c), thereby fulfilling one of the key goals of ME-MENTO (Bange et al, 2009). Net global open ocean emissions of N 2 O are now similarly estimated at 3-5 Tg N yr −1 by both Yang et al (2020) and the Global Nitrous Oxide Project (Tian et al, 2020). In comparison, net global ocean CH 4 emissions from machine-learning mapping were estimated at 6-12 Tg CH 4 yr −1 (Weber et al, 2019), compared to 9-22 Tg CH 4 yr −1 in the most up-to-date CH 4 synthesis (Saunois et al, 2020).…”

“…Machine-learning mapping also recently identified the various contributions of physical and biogeochemical predictor variables for CH 4 (e.g., depth, primary production; Weber et al, 2019;Fig. 4b) and N 2 O distributions (e.g., chlorophyll, sea surface temperature, apparent oxygen utilization, and mixed-layer depth; Yang et al, 2020;Fig. 4a).…”

“…Towards the coastline, CH 4 supersaturation increases by orders of magnitude ( Fig. 5b), reflecting terrestrial inputs (e.g., river and groundwater), increased organic matter loading , and CH 4 diffusion and ebullition from shallow Yang et al, 2020) and (b) air-sea CH 4 disequilibrium mapped using an artificial neural network model (adapted from Weber et al, 2019). For consistency with the original publications, the air-sea disequilibrium is shown in different units for N 2 O (partial pressure) and CH 4 (concentration).…”