The breakdown of volcanic rocks by water helps balance the climate of the Earth by sequestering atmospheric CO₂. The rate of CO₂ sequestration is referred to as "weatherability". Our modelling study finds that continental position strongly impacts CO₂ concentrations, that runoff strongly controls weatherability, that changes in weatherability may explain long-term trends in atmospheric CO₂ concentrations, and that even relatively localised changes in weatherability may have global impacts.