Recent variations in the concentration of atmospheric carbon dioxide ([CO2]atm) have been shown to affect carbon fixation during photosynthesis and are correlated with anatomical and physiological changes observed in leaves of woody dicots. These include changes in stomatal frequency and size and in δ13C values. These attributes can be measured on organically preserved leaf fossils, and therefore can be used as proxies for Earth’s atmospheric CO2 history. This study tests the link between [CO2]atm and global temperature for the late Oligocene and early Miocene using fossil leaves collected from Ethiopia, by estimating [CO2]atm using a leaf gas exchange model. The results of this study show a statistically significant [CO2]atm rise from ∼390 ppm in the late Oligocene to ∼870 ppm in the early Miocene. These values demonstrate a positive correlation between [CO2]atm and global average temperature, contrary to some previous studies for this time interval. The results of this study have important implications for understanding future climate change driven by rising [CO2]atm.