Earth system sensitivity (ESS) is the long-term (>103 yr) equilibrium temperature response to doubled CO2. ESS has climate policy implications because global temperatures are not expected to decline appreciably for at least 103 yr, even if anthropogenic greenhouse-gas emissions drop to zero. We report quantitative ESS estimates of 3 °C or higher for much of the Cretaceous and early Paleogene based on paleo-reconstructions of CO2 and temperature. These estimates are generally higher than climate sensitivities simulated from global climate models for the same ancient periods (~3 °C). We conclude that climate models do not capture the full suite of positive climate feedbacks during greenhouse worlds. These absent feedbacks are probably related to clouds, trace greenhouse gases, seasonal snow cover, and/or vegetation, especially in polar regions. Continued warming in the coming decades as anthropogenic greenhouse gases accumulate in the atmosphere ensures that characterizing and quantifying these positive climate feedbacks will become a scientific challenge of increasing priority.