A missing link in the carbon cycle: phytoplankton light absorption under RCP scenarios

Abstract. Marine biota and biogeophysical mechanisms, such as phytoplankton light absorption, have attracted increasing attention in recent climate studies. Under global warming, the impact of phytoplankton on the climate system is expected to change. Previous studies analyzed the impact of phytoplankton light absorption under prescribed future atmospheric CO₂ concentrations. However, the role of this biogeophysical mechanism under freely-evolving atmospheric CO₂ concentration and future CO₂ emissions remains unknown. To shed light on this research gap, we perform simulations with the EcoGEnIE Earth system model and prescribe CO₂ emissions out to 2500 following the four Extended Concentration Pathways (ECP) scenarios, which for practical purpose we call RCP scenarios. Under all RCP scenarios, our results indicate that phytopankton light absorption weakens the biological carbon pump while it increases the surface chlorophyll, the sea surface temperature, the atmospheric CO₂ concentrations and the atmospheric temperature. Under the RCP2.6, RCP4.5 and RCP6.0 scenarios, the magnitude of changes due to phytoplankton light absorption is similar. However, under the RCP8.5 scenario, the changes in the climate system are less pronounced due to temperature limitation of phytoplankton concentration, highlighting a reduced effect of phytoplankton light absorption under strong warming. Additionally, this work highlights the major role of phytoplankton light absorption on the climate system, suggesting highly uncertain feedbacks on the carbon cycle with uncertainties that maybe in the range of those known from the land biota.