Atmospheric carbon dioxide levels are controlled over long time scales by the transfer of carbon between the atmosphere, oceans, and sedimentary rocks— a process referred to as the CO2geochemical cycle. Carbon dioxide is injected into the atmosphere‐ocean system by volcanism; it is removed by the weathering of silicate rocks on the continents followed by the deposition of carbonate minerals on the sea floor. Humans are currently perturbing the natural carbon cycle by burning fossil fuels and deforesting the tropics, both of which add CO2to the atmosphere. The effects of human activities on future atmospheric CO2levels can be estimated by including anthropogenic emissions in a model of the long‐term carbon cycle. The model predicts that CO2concentrtions could increase by a factor of six or more during the next few centuries if we consume all of the available fossil fuels. Preserving existing forests and/or reforesting parts of the planet could mitigate the CO2increase to some extent, but cannot be depended on to make a significant difference. Because the removal processes for atomspheric CO2are slow, the maximum CO2level reached is relatively insensitive to the fossil fuel burning rate unless the burning rate is many times smaller than its present value. The model also predicts that hundreds of thousand of years could pass before atmospheric CO2returns to its original preindustral level. Implications of these results for future energy and land use policies are discussed.