Calcifying phytoplankton play an import role in long-term CO₂ removal from the atmosphere. We therefore studied the ability of a representative species to continue sequestrating CO₂ under future climate conditions. We show that CO₂ sequestration is negatively affected by both an increase in temperature and the resulting decrease in nutrient availability. This will impact the biogeochemical cycle of carbon and may have a positive feedback on rising CO₂ levels.