Abstract
Anthropogenic and natural wetlands are key contributors to global emissions of methane (CH4), a potent greenhouse gas influencing Earth's climate. Wetland CH4 emissions are typically attributed to anaerobic microbial respiration; however, biotic sources alone do not account for the full extent of CH4 emissions observed in wetlands, leaving a substantial gap in understanding wetland CH4 emission sources. Here, we reveal that abiotic sunlight-driven processes efficiently drive CH4 production in wetlands with shallow water table. This is facilitated by rapid photochemical reactive oxygen species (ROS) generation at the sunlit sediment-water interface, which oxidize abundant methyl donors like dimethyl sulfoxide to form CH4, supporting a substantial photochemical contribution to CH4 formation. Globally, photochemical CH4 production was estimated at 1.86 ± 0.35, 1.59 ± 0.24 and 0.012 ± 0.003 Tg CH4 yr-1 for cropland, terrestrial, and coastal wetlands, contributing to 2.1-2.5% and 0.6-8.0% of CH4 emission in fresh water and coastal wetland system, respectively. With thawing permafrost and extended summer seasons, wetlands in polar and alpine regions emerge as hotspots for sunlight-driven CH4 emission. These findings highlight photochemical processes as a previously overlooked component of wetland CH4 cycling.