The R2O-MIP: The riverine influence on the global ocean CO2 flux

The influence of rivers on the air-sea CO₂ flux is of major importance for quantifying the global ocean sink of anthropogenic CO₂. Past approaches have identified that rivers induce a source of CO2 in the ocean using budgeting, process based models and indirect reconstructions, but major uncertainties still remain in our understanding of its magnitude and distribution. The River2Ocean Model Intercomparison Project (R2O-MIP) aims to further our understanding on the role of rivers in the carbon budget using state-of-the-art riverine data and coastal transformation rates, coupled to current Global Ocean Biogeochemical Models (GOBMs). Tier 1 of the MIP focusses on pre-industrial river-induced CO2 flux, while Tier 2 aims to tackle anthropogenic perturbations of riverine carbon and nutrients.

Here, we present novel synthesis datasets to serve as riverine inputs of carbon and nitrogen to the ocean, which amount in a transfer of 1 PgCyr^-1 and 41 TgNyr^-1 to the ocean. Our global review of field studies reveals that the residence time of terrestrial dissolved organic carbon (tDOC) in the ocean varies from a few weeks (tropics) to around a decade (Arctic), and scales exponentially with sea surface temperature, with a Q10 value of 5. In addition, it is assumed that around 10 % of tDOC is strongly refractory, having residence times of over 30 years in the ocean. Long-lasting estimates suggest that terrestrial particulate organic carbon is subject to a burial efficiency of 44 % in the coastal ocean, which we will use for filtering the riverine input dataset due to model limitations in representing burial and the coastal ocean. After filtering, the riverine data will be added to GOBMs at river mouths over seasonally-varying timescales. Ensuing impacts on the CO2 flux will be investigated at the global and regional scales, having potential implications for assessments of the global carbon budget.