Emergent Constraint for Future Decline in Arctic Phytoplankton Concentration

Abstract

In recent decades, the Arctic Ocean has experienced continuous warming and freshening, affecting biogeochemical factors such as nutrient supply, light availability, chlorophyll, and productivity. While Arctic marine productivity is projected to increase due to the expansion of the open ocean and increased chlorophyll concentration, uncertainties related to chlorophyll and nutrients may distract the fidelity of productivity in current Earth system models (ESMs). Here, we analyze the existing uncertainty in the Arctic chlorophyll projections using the 26 ESMs participating in Coupled Model Intercomparison Projects 5 and 6 (CMIP5 and CMIP6). We found that the uncertainty in the Arctic chlorophyll projections in the CMIP6 ESMs is greater than in the CMIP5 ESMs due to increasing uncertainty in the background nitrate concentration. A significant relationship between background nitrate and projected chlorophyll (r = 0.86) is demonstrated using the observational climatology of nitrate. Based on this strong relationship, the emergent constraint is applied to reduce the uncertainty of future chlorophyll projections. Declines in chlorophyll concentration based on emergent constraint are estimated to be further decreased in the future (44.9% +/- 29.1% to 50.9% +/- 27.6%) than at present, which is about three-fold larger than the multi-model mean projection (-13.5% +/- 48.7%). Comparing cumulative density functions before and after the emergent constraint, the probability of the decreasing chance of chlorophyll is increased by approximately 36% from 60% in prior CMIP5,6 to 93%-96% after constraint. Our results imply that reducing the uncertainty in background nitrate concentration can improve the fidelity of future projections of the Arctic ecosystem in the ESMs. The Arctic Ocean environment has undergone changes in response to human-induced greenhouse gases, such as dramatic warming and sea-ice retreat. Recently, the chlorophyll concentration, the proxy of the phytoplankton biomass, has increased leading to an increase in marine productivity in the Arctic Ocean. However, there is a large uncertainty in the current earth system models (ESMs) regarding future changes in phytoplankton biomass. In this study, we analyze the 26 ESMs and estimate the future changes in phytoplankton biomass based on the relationship between current climate and future changes, which is known as the "emergent constraint." We find a strong relationship between the level of current nitrate levels and future chlorophyll changes in the Arctic Ocean. Based on this relationship, we estimate the uncertainty of the Arctic chlorophyll decline, which is about three times larger than the multi-model mean projection. Our results suggest that reducing the uncertainty of present-climate nitrate in the ESMs is important for projecting Arctic productivity.