Stuart R. Gaffin

World Population Projections for Greenhouse Gas Emissions Scenarios

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.

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 populations 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.

Comment on: 'The lifetime of excess atmospheric carbon dioxide' by Berrien Moore III and B. H. Braswell

Article original: Moore, B. III, and B. H. Braswell, Global Biogeochem. Cycles, 8, 23-38, 1994.

Pulsations in seafloor spreading rates and transit time dynamics

Correlation studies and theoretical models suggest the major plate driving forces, slab pull and ridge push, are closely linked to seafloor age at subduction zones (= transit time and hereafter denoted τ). Based on this fact it has been proposed that seafloor spreading rate fluctuations should be stably damped because a speedup should lead to rejuvenation of τ and hence a weakening of driving forces. If true, such stability could imply the first-order cycle in Pacific spreading rates during the Mesozoic-Cenozoic (“the pulse of the Earth”) was externally forced by mantle dynamics. However we show that a crucial assumption in this stability argument is invalid: increasing spreading rates do not necessarily cause transit time to decrease. The transit time dynamics of seafloor are complex; simple plate models suggest that, rather than stabilizing spreading rate, τ-dependent plate driving forces may be destabilizing leading to self-sustained and aperiodic oscillations. These results raise questions about the cause of spreading rate cycles. Most importantly, are the plates themselves, through age dynamics, actively contributing to “the pulse of the Earth” ?

Comments on “The significance of coral reefs as global carbon sinks—response to greenhouse” by D.W. Kinsey and D. Hopley

Article original : Kinsey, D.W. and Hopley, D., 1991. Palaeogeogr., palaeoclimatol., Palaeoecol., 89: 363-377.