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Limits on marine carbon dioxide removal potential set by coastal air-sea gas exchange rates | Communications Earth & Environment
Marine carbon dioxide removal is gaining interest as a group of techniques to mitigate rising atmospheric carbon dioxide levels and combat climate change. Crucial to these techniques is knowledge about the gas transfer velocity, which controls air-sea fluxes. Using the helium-3/sulfur hexafluoride dual-tracer technique, we show that gas transfer velocities are lower in a fjord (Hvalfjörður, Iceland), a potential site for marine carbon dioxide removal research, compared to the open ocean at similar wind speeds. Tracer data indicate a four-day residence time within the fjord, leading to quick export of surface water to the North Atlantic Ocean, where open-ocean gas exchange dominates. Consequently, local gas exchange has only a limited effect on overall carbon dioxide uptake, assuming exported water stays in contact with the atmosphere. However, in coastal environments with longer residence times and lower gas transfer velocities, or with rapid subduction of exported surface waters, assuming open-ocean gas exchange would drastically overestimate carbon dioxide uptake potential. Local coastal gas exchange is significantly lower than open-ocean gas exchange, which could impact marine carbon dioxide removal estimates where coastal waters have long residence times or exported surface waters are rapidly subducting, according to an in-situ tracer experiment from an Icelandic fjord.
