Abstract
Oceanic emissions represent a highly uncertain term in the natural atmospheric methane (CH4) budget, due to the sparse sampling of dissolved CH4 in the marine environment. Here we overcome this limitation by training machine-learning models to map the surface distribution of methane disequilibrium (∆CH4). Our approach yields a global diffusive CH4 flux of 2–6TgCH4yr−1 from the ocean to the atmosphere, after propagating uncertainties in ∆CH4 and gas transfer velocity. Combined with constraints on bubble-driven ebullitive fluxes, we place total oceanic CH4 emissions between 6–12TgCH4yr−1, narrowing the range adopted by recent atmospheric budgets (5–25TgCH4yr−1) by a factor of three. The global flux is dominated by shallow near-shore environments, where CH4 released from the seafloor can escape to the atmosphere before oxidation. In the open ocean, our models reveal a significant relationship between ∆CH4 and primary production that is consistent with hypothesized pathways of in situ methane production during organic matter cycling.
Highlights
(CH4) budget, due to the sparse sampling of dissolved CH4 in the marine environment
Diffusive air–sea gas fluxes can be estimated from their ocean–atmosphere disequilibrium using gas transfer theory[15]
We employed a correlation analysis to determine which of our set of physical and biogeochemical predictor variables most closely approximates the ensemble-mean distribution of ΔCH4 mapped by our machine learning models (Supplementary Table 2 and Fig. 6)
Summary
(CH4) budget, due to the sparse sampling of dissolved CH4 in the marine environment. Here we overcome this limitation by training machine-learning models to map the surface distribution of methane disequilibrium (ΔCH4). Future anthropogenic impacts on the atmospheric CH4 budget are not restricted to direct emissions (e.g. during agriculture and energy production), but will include climatedriven perturbation of the natural CH4 cycle[3] This motivates recent efforts to place strong baseline constraints on natural CH4 sources and understand their environmental sensitivity[4]. Novel methanogenesis pathways have been identified that may produce CH4 in situ in the surface ocean mixed layer, providing a more direct conduit to atmosphere[12,13,14] Both diffusive and ebullitive CH4 emissions remain uncertain due to sparse data constraints and the crude extrapolation methods used to upscale their rates[4], limiting our understanding of the ocean’s leverage over atmospheric CH4. We provide a new robust estimate for the global diffusive flux and combine it with upper and lower bounds on ebullition rates, narrowing the uncertainty range for the total oceanic methane source
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