The early Eocene Climatic Optimum (EECO) represents the peak of Earth’s last sustained greenhouse climate interval. To investigate hydroclimate variability in western North America during the EECO, we developed an orbitally resolved leaf wax δ ² H record from one of the most well-dated terrestrial paleoclimate archives, the Green River Formation. Our δ ² H _wax results show ∼60‰ variation and evidence for eccentricity and precession forcing. iCESM simulations indicate that changes in the Earth’s orbit drive large seasonal variations in precipitation and δ ² H of precipitation at our study site, primarily during the summer season. Our findings suggest that the astronomical response in δ ² H _wax is attributable to an asymmetrical climate response to the seasonal cycle, a “clipping” of precession forcing, and asymmetric carbon cycle dynamics, which further enhance the influence of eccentricity modulation on the hydrological cycle during the EECO. More broadly, our study provides an explanation for how and why eccentricity emerges as a dominant frequency in climate records from ice-free greenhouse worlds.