To improve future predictions of anthropogenic climate change, a better understanding of the relationship between global temperature and atmospheric concentrations of CO2 (pCO(2)), or climate sensitivity, is urgently required. Analyzing proxy data from climate change episodes in the past is necessary to achieve this goal, with certain geologic periods, such as the Miocene climatic optimum (MCO), a transient period of global warming with global temperatures up to similar to 7 degrees C higher than today, increasingly viewed as good analogues to future climate under present emission scenarios. However, a problem remains that climate models cannot reproduce MCO temperatures with less than similar to 800 ppm pCO(2), while most previously published proxies record pCO(2) < 450 ppm. Here, we reconstructed MCO pCO(2) with a multitaxon fossil leaf database from the well-dated MCO Lagerstatte deposits of Clarkia, Idaho, USA, using four current methods of pCO(2) 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 pCO(2) of similar to 450-550 ppm. Although slightly higher than previously reconstructed pCO(2), the discrepancy with the similar to 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 pCO(2). Ever higher climate sensitivity with rising temperatures should be very seriously considered in future predictions of climate change.