Volcanic arcs in the humid tropics may contribute disproportionately to global CO2 consumption through silicate weathering. Accurately modelling the impact of volcanic-arc landscapes on the earth’s long-term carbon cycle requires understanding how climate and physical erosion control weathering fluxes and CO2 consumption from arc landscapes. We evaluate these controls by examining the covariation of stream solutes, runoff, and long-term physical erosion rates in volcanic watersheds in Puerto Rico. We find that CO2 consumption is controlled more strongly by physical erosion than by runoff or temperature. This indicates that volcanic-arc rocks in the humid tropics are not major participants in a negative silicate weathering feedback stabilizing global temperature. Consequently, tectonically driven uplift of volcanic-arc rocks may increase global CO2 consumption and contribute to transitions from greenhouse to icehouse conditions through earth history.