The magnitude of future warming is dependent upon carbon cycle feedbacks, which can either amplify or mitigate warming. The latest generation of CMIP6 models suggest that the combined effects of known climate feedback mechanisms is to amplify global warming. However, climate models are blind to the ‘unknown unknowns’ – these are the things we know little about and have potential to take future climate into unimagined directions. Transient warming events in the geological record (hyperthermals) capture the response of the Earth system to all the feedbacks in operation, including those that we are unaware of. In this talk, I will use coastal marine sediments deposited during the Paleocene-Eocene Thermal Maximum (56 million years ago) to reveal how the terrestrial carbon cycle operates in a warmer world. This talk will focus on two highly uncertain carbon cycle feedbacks: (1) the erosion and burial of terrestrial organic carbon (a potential CO2 sink), and (2) the oxidation of rock organic carbon (a potential CO2 source). I will show how organic geochemical techniques have the potential to illuminate these uncertain feedbacks during the PETM and other key climate events in Earth’s history.