To improve future predictions of anthropogenic climate change, a better understanding of the relationship between global temperature and atmospheric concentrations of CO2 (pCO2), orclimate sensitivity, is urgently required. Analyzing proxy data from climate change episodesin the past is necessary to achieve this goal, with certain geologic periods, such as the midMiocene Climatic Optimum (MCO), a transient period of global warming with globaltemperatures up to ~7°C higher than today, increasingly viewed as good analogues to future climate under present emission scenarios. However, a problem remains that climate modelscannot reproduce MCO temperatures with less than ~800 ppm pCO2, while most previously published proxies record pCO2 <450 ppm. Here, we reconstructed MCO pCO2 with a multitaxon fossil leaf database from the well-dated MCO Lagerstätte deposits of Clarkia, Idaho,USA, using four current methods of pCO2 reconstructions. The methods are principally based on either stomatal densities, carbon isotopes, or a combination of both – thus offering independent results. The total of six reconstructions mostly record pCO2 of ~450–550 ppm. Although slightly higher than previously reconstructed pCO2, the discrepancy with the ~800 ppm required by climate models remains. We conclude that climate sensitivity was heightened during MCO, indicating that highly elevated temperatures can occur at relatively moderate pCO2. Ever higher climate sensitivity with rising temperatures should be very seriously considered in future predictions of climate change.