Changing the supply of silica to the ocean may alter pCO2 levels. The increase in dust delivered to the ocean during glacial times increased the availability of silica for biological uptake. The increased silica levels shifted species composition: Diatom populations increased and coccolith populations decreased. Decreasing the population of coccoliths decreased the flux of calcite to the sediments, which, in turn, lowered pCO2 levels enough to explain the glacial-interglacial pCO2 transition. Furthermore, the contemporary increase in dust delivered to the ocean's mixed layer may be removing significant amounts of carbon dioxide from the atmosphere at present. To set the stage, this silica hypothesis is compared with the iron fertilization and nitrogen fixation hypotheses. ¿ 2000 American Geophysical Union