Atmospheric carbon emissions from benthic trawling depend on water depth and ocean circulation

Through its vastness, resilience and biogeochemical complexity, the ocean offers humanity some of the largest potential natural pathways for removing carbon dioxide from the atmosphere while avoiding new sources of anthropogenic emissions. In proposing a network of new marine protected areas in service of global ocean conservation, Sala et al. describe a potentially large climate benefit of such a strategy: a reduction or elimination in carbon emissions that can result from the resuspension and subsequent remineralization of organic matter in ocean sediments following benthic trawling and dredging. Sala et al. estimated that as much as 1.47 Pg CO2 yr-1 — equivalent to 4% of all global CO2 emissions in 2020 — could be produced globally in the first year disturbance associated with benthic trawling, with CO2 “emissions” from trawling activity declining after roughly a decade to 0.58 Pg CO2 yr-1.However, using the published output from a series of experiments in an inverse ocean circulation model to account explicitly for the timescale of atmospheric ventilation, we show that the Sala et al. estimate of global CO2 efflux in the first year after trawling must be discounted by at least 33%, yielding an adjusted flux of 0.99 Pg CO2 yr-1, if one aims to ultimately estimate emissions to the atmosphere. Similarly, the cumulative CO2 emissions attributable under the Sala et al. model to a continuous pattern of global benthic trawling activity over a 30-year timescale must be discounted by at least 12% (compare 19.6 and 17.2 Pg CO2).