Global ocean methane emissions dominated by shallow coastal waters

Area of Science:

• * Atmospheric chemistry and biogeochemical cycles.
• * Marine science and oceanography.
• * Climate change research.

Background:

• * Oceanic emissions of methane (CH4) are a significant but uncertain component of the global atmospheric CH4 budget.
• * Previous estimates are limited by sparse sampling of dissolved CH4 in marine environments.
• * Accurate quantification of oceanic CH4 flux is crucial for understanding the natural CH4 budget and climate feedbacks.

Purpose of the Study:

• * To overcome limitations of sparse sampling in estimating oceanic CH4 emissions.
• * To develop a machine-learning approach for mapping surface CH4 disequilibrium (∆CH4).
• * To refine estimates of global oceanic CH4 flux and identify key emission regions.

Main Methods:

• * Training machine-learning models to map the surface distribution of methane disequilibrium (∆CH4) using available data.
• * Propagating uncertainties in ∆CH4 and gas transfer velocity to calculate diffusive CH4 flux.
• * Integrating diffusive flux estimates with constraints on bubble-driven ebullitive fluxes for total oceanic CH4 emissions.

Main Results:

• * A global diffusive CH4 flux of 2-6 TgCH4yr-1 from the ocean to the atmosphere was determined.
• * Total oceanic CH4 emissions were estimated to be between 6-12 TgCH4yr-1, reducing the uncertainty range by a factor of three.
• * Shallow near-shore environments were identified as dominant contributors to the global flux.
• * A significant relationship between ∆CH4 and primary production was observed in the open ocean.

Conclusions:

• * The study provides a more constrained estimate of oceanic CH4 emissions, improving the accuracy of the global atmospheric CH4 budget.
• * Shallow coastal areas are critical sources of atmospheric CH4, highlighting the importance of seafloor CH4 release before oxidation.
• * The link between CH4 disequilibrium and primary production in the open ocean supports hypotheses of in situ CH4 production during organic matter cycling.