ABSTRACT: Model projections for the Southern Ocean indicate that light, iron (Fe) availability, temperature and carbon dioxide (CO₂) will change concurrently in the future. We investigated the physiological responses of Southern Ocean phytoplankton to multiple variables by culturing the haptophyte Phaeocystis antarctica and the diatom Chaetoceros flexuosus under various combinations of light, Fe, temperature and CO₂. Using statistical models, the influence of each environmental variable was analysed for each physiological response, ultimately predicting how ‘future’ conditions (high temperature and high CO₂) would influence the 2 phytoplankton species. Under future conditions, cellular chlorophyll a and carbon to nitrogen molar ratios were modelled to increase for both species in all light and Fe treatments, but at times were inconsistent with measured values. Measured and modelled values of the photochemical efficiency of photosystem II (F_v/F_m) declined in cultures of P. antarctica due to concurrent increases in temperature and CO₂, under all light and Fe treatments. The trends in F_v/F_m for C. flexuosus were less clear. Our model and observations suggest that when temperature and CO₂ are concurrently increased, the growth of both species remains largely unchanged. This modelling analysis reveals that high CO₂ exerts a strong negative influence on the growth of both phytoplankton, and any ‘future’ increase in growth can be attributed to the positive effect of warming rather than a CO₂ fertilisation effect.