Plants are likely to have had a profound influence on Earth’s long-term climate through their role in drawing down CO2 and emitting O2 into the atmosphere and their interactions with soils and minerals. Local climatic factors, including water availability, light, and temperature, play a key role in plant physiology and growth and have fluctuated substantially over geological time. However, the impact of these key climate variables on global plant biomass across the Phanerozoic have not yet been established.