I explore how fossil plants can be used to reconstruct ancient environments (especially CO₂, temperature, and climate sensitivity), and the (paleo-) physiological underpinnings behind these plant-environment relationships. Recent and current projects include the reconstruction of early Paleogene and middle Miocene atmospheric CO₂ levels from the stomatal distributions in plant leaves, and the development of mechanistically-grounded proxies for climate and leaf ecology from the size and shape of fossil leaves. I also compile Phanerozoic CO₂ records and investigate the strength of CO₂-temperature coupling over multi-million-year timescales.