Maximum impacts of future reforestation or deforestation on atmospheric CO₂

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.