Zooplankton grazing is the largest source of uncertainty for marine carbon cycling in CMIP6 models

The current generation of Earth system models used by the United Nations to project future climate scenarios (CMIP6) relies heavily on marine biogeochemical models to track the fate of carbon absorbed into the oceans. Here we compare 11 CMIP6 marine biogeochemical models and find the largest source of inter-model uncertainty in their representation of the marine carbon cycle is phytoplankton-specific loss rates to zooplankton grazing. This uncertainty is over three times larger than that of net primary production and driven by large differences in prescribed zooplankton grazing dynamics. We run a controlled sensitivity experiment in a global marine biogeochemical model and find that small changes in prescribed grazing dynamics (roughly 5% of what is used across CMIP6 models) can increase secondary and export production by 5 and 2 PgC yr−1, respectively, even when tuned to identical net primary production, likely biasing predictions of future climate states and food security.