Current trends of global climate change affect marine ectothermal animals not only through the increase in ambient temperature. Synergistic effects of carbon dioxide and temperature changes as well as more frequent hypoxia events must also be considered. As a first attempt, the combined effects of warming and elevated CO2 concentrations were investigated in the edible crab (Cancer pagurus). Arterial oxygen tension (PaO2) in the haemolymph was recorded on-line during a progressive warming scenario from 10 to 22 °C and cooling back to 10 °C. Hypercapnia (1% CO2) caused a significant reduction of oxygen partial pressure in the haemolymph as well as a large, 5 °C downward shift of upper thermal limits of aerobic scope. The present findings are the first to show that hypercapnia causes enhanced sensitivity to heat and thus, a narrowing of the thermal tolerance window of a marine ectotherm. Such interactions of ambient temperature and anthropogenic increases in ambient CO2 concentrations will need to be considered during future investigations of the effects of climate change on ecosystems. © 2007 Elsevier Ltd. All rights reserved.