| Summary: | Methane is a potent greenhouse gas, but a complete accounting of global methane sources and sinks is still ongoing. Sediments beneath glaciers and ice sheets harbour carbon reserves that, under certain conditions, can be converted to methane. However, the formation and release of such methane is an unquantified component of the arctic methane budget. Lamarche-Gagnon et al. present direct measurements of dissolved methane in proglacial discharge from a land-terminating glacier of the Greenland Ice Sheet. This proglacial discharge was supersaturated with methane and had diffusive methane fluxes similar to other terrestrial rivers for the duration of the summer measurement period. Their results suggest that variability in proglacial discharge is associated with methane release from subglacial environments, implicating both the form and evolution of the subglacial hydrologic system as a newly discovered control in the arctic methane cycle.The study by Lamarche-Gagnon et al. is an important example of how the cryosphere can interact with the surrounding Earth system in unexpected and potentially significant ways. Characterizing the ability of subglacial sediments to convert and store methane and the ability of the subglacial hydrologic system to export this methane to the atmosphere, through both modeling and observational studies, are critical steps in improving our knowledge of the sources and sinks of arctic methane and better constraining their future changes.
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