Warming of the oceans and consequent loss of dissolved oxygen (O<inf>2</inf>) will alter marine ecosystems, but a mechanistic framework to predict the impact of multiple stressors on viable habitat is lacking. Here, we integrate physiological, climatic, and biogeographic data to calibrate and then map a key metabolic index-the ratio of O<inf>2</inf> supply to resting metabolic O<inf>2</inf> demand-across geographic ranges of several marine ectotherms. These species differ in thermal and hypoxic tolerances, but their contemporary distributions are all bounded at the equatorward edge by a minimum metabolic index of ∼2 to 5, indicative of a critical energetic requirement for organismal activity. The combined effects of warming and O<inf>2</inf> loss this century are projected to reduce the upper ocean's metabolic index by ∼20% globally and by ∼50% in northern high-latitude regions, forcing poleward and vertical contraction of metabolically viable habitats and species ranges.