Population and global warming with and without CO2 targets

The sensitivity of future global warming to variable population growth rates is reexamined as part of an ongoing debate over the extent to which climate change should be added to the list of concerns surrounding population growth. The UN 1992 low, medium and high population projections out to the year 2150 are run through an integrated climate-economics model which allows the effect of population variability to be traced through to CO2 emissions, concentrations, warming and economic growth. We treat separately the cases of population's role in global warming, first without and then with specified atmospheric targets. Without targets, modeled CO2 concentrations in year 2150 show great Variability ranging from 600 ppm (UN low projection) to 1375 ppm (UN high projection). Such numbers suggest the potential effect of variable population growth on climate is large and that population policy options carry with them a significant, longterm, global warming mitigation component. The range of global warming achieved is not as sensitive to population because of weakened radiative absorption at high CO2 levels. With respect to targets, with low population, stabilization at 650-750 ppm is achieved with relatively modest cuts in carbon intensity. Stabilization at 350-450 ppms requires steep cuts in emissions that are only weakly affected by the full range of variable population growth rates. Stabilization at 550 ppm is thus a transitional point between these end-member roles for population. Future work needs to address cost issues which could change this assessment of the role of population with CO2 targets.