In future oceans, aquatic animals and marine ecosystems will have to cope with combined effects of ongoing warming and CO2 accumulation. This study investigated possible effects of ambient CO2 on the thermal tolerance window of the North Sea edible crab Cancer pagurus. To enhance ecological relevance, the temperature ramp was chosen to mimic daily temperature changes and allow for acclimation effects. Applied temperatures ranged from 12°C to 20°C with 2°C warming steps (within 2 hours). Temperature steps were held for 10 hours including 5 hours acclimation time and 5 h measurement time. Animals were exposed to present day normocapnia (ca. 480 μatm) and CO2 levels projected beyond 2100 (ca. 1400 μatm). Using a multi-parameter approach (measured/calculated parameters: metabolic rate, heart rate, cardiac stroke volume proxy, cardiac output proxy) enabling non-invasive recording of highly resolved and continuous data, it was investigated how aerobic metabolic rate and the cardiovascular system as key components of animal performance are modulated on a daily basis by changes in temperature and ambient CO2. As non-invasive measurements only allowed for detection of relative changes in cardiac stroke volume, an approach for non-invasive quantification using cine magnetic resonance imaging (MRI) is presented. This approach further revealed complex inner structures of the ventricle likely enhancing efficiency of the pumping mechanism. Despite high interindividual variability, exemplary analysis of the heart rate over time could reveal, that increasing amplitudes of rhythmic patterns (representing aerobic metabolism) in response to rising temperatures mostly depended on increases at the maximum level of performance, whereas minimum levels remained stable. Additionally, temperature-dependent changes from low to high frequencies could be observed. Beside these temperature-dependent effects, it could be shown, that CO2-concentrations around 1400 μatm can have light narcotic effects on the heart activity over time of C. pagurus. The diverse responses of the other parameters and no apparent CO2-dependent shifts in the performance response to temperature imply, that projected CO2 levels in the North Sea beyond the year 2100 do not narrow the thermal tolerance window of C. pagurus.