Drivers of increasing coastal CO2 uptake identified by a global model with seamless integration of coastal marine carbon dynamics

I will present the first global ocean-biogeochemistry model with a seamless integration of coastal carbon dynamics, ICON-Coast, and provide insights from recent simulations on the drivers of the increasing CO2 uptake efficiency of the coastal ocean. Based on the unstructured triangular grid of the model, we globally apply a mesh refinement in the coastal ocean to better resolve complex circulation features as well as ocean-shelf exchange. Moreover, we incorporate tidal currents including bottom drag effects, and extended the model's biogeochemistry component to account for key shelf-specific carbon transformation processes. In this way the model encompasses all coastal areas around the globe within a single, consistent ocean-biogeochemistry model, thus naturally accounting for two-way coupling of ocean-shelf feedback mechanisms at the global scale. First hindcast simulations over the 20^th century indicate that the increasing CO2 uptake efficiency of the coastal ocean is mainly driven by the rising pCO2 in the atmosphere (40%), climate-induced changes in the circulation (40%) and increasing historical nutrient loads from rivers (20%). While river inputs caused a significant boost in organic carbon sequestration by enhanced biological productivity, this mainly induced a shift in the resource utilization, from dissolved inorganic carbon delivered by the open ocean towards absorbed CO2 from the atmosphere. Thus the comparatively weak riverine impact on the CO2 uptake at the sea surface is mediated by an enhanced advective export of organic carbon, this way further intensifying the carbonation of the open ocean.