Cerling's research interests are primarily focused on Earth surface geochemistry processes and on the geological record of ecological change.
[3] Particularly, working on conservation biology, Cerling has analyzed the modern animal diet and physiology by using stable isotopes as natural tracers as well as studying dietary changes of different mammalian lineages extending over millions of years.
With the publication of "Expansion of C4 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 C4 photosynthesis between 7 and 5 million years ago.
The decrease of atmospheric CO2 concentrations over the history below a threshold that favored the C3-photosynthesizing plants was considered as a valid reason for the global expansion of C4 biomass.
This was the first step for the discovery they made which was first published on February 25, 2008, by the "Proceedings of the National Academy of Sciences" with the title "Hydrogen and oxygen isotope ratios in human hair are related to geography".
Cerling discovered that a strand of hair could provide valuable clues about a person's travels by studying the variation of hydrogen-2 (δ2H) and oxygen-18 (δ18O) isotopes and comparing them to the ones in the drinking water.