World Population Projections for Greenhouse Gas Emissions Scenarios

Abstract

A survey is made of the latest world population projections issued by the United Nations, World Bank, U.S. Census Bureau, and International Institute for Applied Systems Analysis. Medium variants from all the organizations show excellent agreement with respect to many features of future world population growth. It appears that little would be gained by obtaining additional regional projections made by governments or organizations other than those listed above. In general, the new range of population projections that are candidates for forthcoming IPCC emissions scenarios are narrower and lower than the previous IPCC IS92 population range: a reflection of updated information on the decline of fertility rates in developing countries and the incorporation of a plausible correlation between mortality rates and fertility rates within the IIASA ‘rapid’ and ‘slow’ demographic transition variants. Comments are made on the schematic approach of forecasting CO2 emissions using multiplicative identities such as ‘IPAT’ (impact/emissions = population × affluence × technology). Although the unqualified IPAT model suggests that emissions should scale linearly with population, a number of caveats to this exist, the most important of which may be factor interactions. A brief review is made of conventional thinking about interactions between population growth and economic development. Correlation studies and theory suggest that population growth has a neutral or, at most, weak negative effect on economic growth. Conversely, it is well established that higher per capita incomes are well correlated with lower fertility and mortality rates in developing countries. Therefore, a plausible first-order relationship worth exploring in the next generation of IPCC scenarios is that scenarios with higher average economic growth rates in the developing world should be associated with lower fertility and mortality rates there. Calculations are presented that illustrate the effect this negative correlation could have had on the range of the older IS92 emission scenarios, assuming that all other factors are unchanged. Finally, some policy issues concerning population and global warming are reviewed in connection with the IPCC’s omission of population policy discussion in its 1995 Second Assessment Report.