There is scope for land-use changes to increase or decrease CO2 concentrations in the atmosphere over the next century. Here we make simple but robust calculations of the maximum impact of such changes. Historical land-use changes (mostly deforestation) and fossil fuel emissions have caused an increase in atmospheric concentration of CO2 of 90 ppm between the pre-industrial era and year 2000. The projected range of CO2 concentrations in 2100, under a range of emissions scenarios developed for the IPCC, is 170–600 ppm above 2000 levels. This range is mostly due to different assumptions regarding fossil fuel emissions. If all of the carbon so far released by land-use changes could be restored to the terrestrial biosphere, atmospheric CO2 concentration at the end of the century would be about 40–70 ppm less than it would be if no such intervention had occurred. Conversely, complete global deforestation over the same time frame would increase atmospheric concentrations by about 130–290 ppm. These are extreme assumptions; the maximum feasible reforestation and afforestation activities over the next 50 years would result in a reduction in CO2 concentration of about 15–30 ppm by the end of the century. Thus the time course of fossil fuel emissions will be the major factor in determining atmospheric CO2 concentrations for the foreseeable future.