Assessment and Constraint of Mesozooplankton in CMIP6 Earth System Models

Although zooplankton play a substantial role in the biological carbon pump and serve as a crucial link between primary producers and higher trophic level consumers, the skillful representation of zooplankton is not often a focus of ocean biogeochemical models. Systematic evaluations of zooplankton in models could improve their representation, but so far, ocean biogeochemical skill assessment of Earth system model (ESM) ensembles have not included zooplankton. Here we use a recently developed global, observationally based map of mesozooplankton biomass to assess the skill of mesozooplankton in six CMIP6 ESMs. We also employ a biome‐based assessment of the ability of these models to reproduce the observed relationship between mesozooplankton biomass and surface chlorophyll. The combined analysis found that most models were able to reasonably simulate the large regional variations in mesozooplankton biomass at the global scale. Additionally, three of the ESMs simulated a mesozooplankton‐chlorophyll relationship within the observational bounds, which we used as an emergent constraint on future mesozooplankton projections. We highlight where differences in model structure and parameters may give rise to varied mesozooplankton distributions under historic and future conditions, and the resultant wide ensemble spread in projected changes in mesozooplankton biomass. Despite differences, the strength of the mesozooplankton‐chlorophyll relationships across all models was related to the projected changes in mesozooplankton biomass globally and in regional biomes. These results suggest that improved observations of mesozooplankton and their relationship to chlorophyll will better constrain projections of climate change impacts on these important animals. Plain Language Summary Zooplankton are marine animals that have a key role in transferring carbon from the atmosphere deeper into the ocean. They also serve as a crucial link in food chains between microscopic marine plants (phytoplankton) and predators like fish and whales. Researchers have created mathematical representations (models) of the linked processes of the oceans, atmosphere, and land, most of which include zooplankton. Yet how well these models represent zooplankton has not been adequately tested. We compared observations of zooplankton biomass to model estimates. We explored if these models reproduce the observed relationship between zooplankton biomass and chlorophyll concentration, which is useful for a