In natural ecosystems, the efficiency of energy transfer from resources to consumers determines the biomass structure of food webs. Generally, about 10% of the energy produced in one trophic level makes up the following1–3. Recent theory suggests that energy transfer may be further limited as rising temperatures increase metabolic growth costs4, although experimental confirmation is lacking in whole ecosystems. We quantified the efficiency of nitrogen transfer (a proxy for overall energy transfer) in freshwater plankton in artificial ponds exposed to 7 years of experimental heating. We provide the first direct experimental evidence that 4 ° C heating, relative to environmental conditions, can reduce trophic transfer efficiency to 56%. In addition, both phytoplankton and zooplankton biomass were lower in the heated dams, indicating major shifts in energy uptake, transformation, and transfer.5.6. These new findings reconcile the observed heat-driven changes in growth costs at the individual level and the efficiency of carbon consumption in different taxa4.7–10 with increases in the ratio of total respiration to gross primary production at ecosystem level11–13. Our results imply that an increasing proportion of the carbon determined by photosynthesis is lost to the atmosphere as the planet warms, which impairs energy flow through food chains, with negative implications for larger consumers and the functioning of entire ecosystems.