Ocean warming and acidification occur at global scales and, in the case of temperature, have already caused shifts in marine ecosystem composition and function. In the case of CO2-induced ocean hypercapnia and acidification, however, effects may still be so small that evidence for changes in the field is largely lacking. Future scenarios indicate that marine life forms are threatened by the specific or synergistic effects of factors involved in these processes. The present paper builds on the view that development of a cause and effect understanding is required beyond empirical observations, for a more accurate projection of ecosystem effects and for quantitative scenarios. Identification of the mechanisms through which temperature- and CO 2-related ocean physicochemistry affect organism fitness, survival and success, is crucial with this research strategy. I suggest operation of unifying physiological principles, not only of temperature but also CO 2 effects, across animal groups and phyla. Thermal windows of optimized performance emerge as a basic character defining species fitness and survival, including their capacity to interact with other species. Through effects on performance at the level of reproduction, behaviour and growth, ocean acidification acts especially on lower marine invertebrates, which are characterized by a low capacity to compensate for disturbances in extracellular ion and acid-base status and sensitivity of metabolism to such disturbances. Available data suggest that one key consequence of these features is a narrowing of thermal tolerance windows, as well as a reduced scope for performance at ecosystem level. These changes in bioenvelopes may have major implications for the ranges of geographical distribution of these organisms and in species interactions.