Marine carbon cycling and sequestration is extremely sensitive to zooplankton grazing in biogeochemical models

Abstract Zooplankton grazing regulates marine carbon cycling by constraining phytoplankton populations and the subsequent transfer of carbon to depth and higher trophic levels. Yet, without robust in-situ data to constrain them, the grazing formulation in state-of-the-art climate models varies largely. We present a new metric to compare how fast zooplankton are assumed to graze in 10 state-of-the-art (CMIP6) biogeochemical models and find they differ by nearly 2 orders of magnitude, implying an ocean populated exclusively with everything from slow-grazing krill to rapidly-grazing ciliates. We use a global, coupled ocean-biogeochemistry model to test the sensitivity of marine carbon cycling to this uncertainty and find the Net Primary Production (NPP) and export efficiency collapse across the range of plausible values. Even when tuned to identical NPP by increasing phytoplankton growth rates, export and secondary production remain extremely sensitive to grazing, likely biasing predictions of future climate states and food security.