With the publication of "Expansion of C ecosystems as an indicator of global ecological change in the late Miocene" in 1993, Cerling, helped by Yang Wang and Jay Quade, made relevant studies relatively to carbon isotopes. Thanks to a deep analysis of palaeovegetation from palaeosols and palaeodiet measured in fossil tooth enamel, was demonstrated a global increase in the biomass of plants using C photosynthesis between 7 and 5 million years ago. The decrease of atmospheric  concentrations over the history below a threshold that favored the C-photosynthesizing plants was considered as a valid reason for the global expansion of C biomass.
The publication "Global vegetation change through the Miocene/Pliocene boundary" in 1997 confirmed these results, demonstrating even how at lower latitudes the change appeared to occur earlier because of the threshold for C photosynthesis is higher at warmer temperatures.