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Aim Climate change will reshape marine ecosystems over the 21st century through diverse and complex mechanisms that are difficult to assess quantitatively. Here, we characterize expectations for how marine community biomass will respond to the energetic consequences of changes in primary production and temperature-dependent metabolic rates, under a range of fishing/conservation scenarios. Location Global ocean. Time period 1950-2100. Major taxa studied Commercially harvested marine ectotherms ('fish'). Methods We use a size-structured macroecological model of the marine ecosystem, coupled with a catch model that allows for calibration with global historical data and simulation of fishing. We examine the four energetic mechanisms that, within the model framework, determine the community response to climate change: net primary production, phytoplankton cell size, and the temperature dependencies of growth and natural mortality. Results Climate change decreases the modelled global fish community biomass by as much as 30% by 2100. This results from a diminished energy supply to upper trophic levels as photosynthesis becomes more nutrient limited and phytoplankton cells shrink, and from a temperature-driven increase of natural mortality that, together, overwhelm the effect of accelerated somatic growth rates. Ocean circulation changes drive regional variations of primary production, producing patterns of winners and losers that largely compensate each other when averaged globally, whereas decreasing phytoplankton size drives weaker but more uniformly negative changes. The climate impacts are similar across the range of conservation scenarios but are slightly amplified in the strong conservation scenarios owing to the greater role of natural mortality. Main conclusions The spatial pattern of climate impacts is mostly determined by changes in primary production. The overall decline of community biomass is attributed to a temperature-driven increase of natural mortality, alongside an overall decrease in phytoplankton size, despite faster somatic growth. Our results highlight the importance of the competition between accelerated growth and mortality in a warming ocean.