evidence: Over much of the last 100 million years of Earth's history, greenhouse gas levels and global temperatures were higher than those of the present day (Zachos et al., 2008; Foster et al., 2017). Because greenhouse gas levels are currently well above anything experienced during the modern natural climate state, ancient climate archives hold important clues to our possible future climate (IPCC, 2013). This is particularly true for those past times when climate was considerably warmer and greenhouse gas levels considerably higher than those of the present day. For instance, such intervals can provide information about the sensitivity of the climate to greenhouse gas forcing (e.g., Rohling et al., 2012; Zeebe, 2013; Caballero and Huber, 2013; Anagnostou et al., 2016) or reveal the behavior of carbon cycle feedbacks under super-warm climate states (e.g., Carmichael et al., 2017). These times of past warmth also provide a powerful means to test the outputs of climate models because they represent actual realizations of how the Earth system functions under conditions of greenhouse forcing comparable to the coming century and beyond. [...] Pearson, P. N. and Palmer, M. R.: Atmospheric carbon dioxide concentrations over the past 60 million years, Nature, 406, 695–699, 2000. [...] Zeebe, R. E.: Time-dependent climate sensitivity and the legacy of anthropogenic greenhouse gas emissions, P. Natl. Acad. Sci. USA, 34, 13739–13744, https://doi.org/10.1073/pnas.1222843110, 2013.