| Summary: | During recent decades, the Arctic Ocean has experienced continuous warming and freshening which influence the biogeochemical states such as nutrient supply, light availability, chlorophyll, and productivity. Although the Arctic marine productivity is projected to increase by expanding the open ocean and increasing chlorophyll concentration, uncertainties of chlorophyll and nutrients may distract the fidelity of productivity in current earth system models (ESMs). Here, using the 26 ESMs participated in Coupled Model Intercomparison Project 5 and 6 (CMIP5 and CMIP6), we analyze the existing uncertainty in the Arctic chlorophyll projections. The uncertainty in the Arctic chlorophyll projections in CMIP6 ESMs becomes greater than that in CMIP5 ESMs because of increasing uncertainty in background nitrate. Especially, the large uncertainty in chlorophyll changes among ESMs is associated with the different seasonality of nutrient limitation. A significant relationship between background nitrate and projected chlorophyll with observational climatology of nitrate is demonstrated. Based on this tight relationship, the emergent constraint is applied to reduce the uncertainty of chlorophyll projections. The estimation in chlorophyll concentration based on emergent constraint is increased chance to be further decreased in the future than (51.0 %) the present day, which is about three-fold than the multi-model mean projection (−13.5 %). Comparing cumulative density functions before and after the emergent constraint, the probability of the decreasing chance of chlorophyll is increased about 36% from 61% in prior CMIP5,6 to 90-97% after constraint. Our results imply that reducing nitrate uncertainty can elevate the fidelity of future projections of Arctic ecosystem in the ESMs.
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