Emissions Scenarios - IPCC

Scenario Driving Forces 111
deficit in world population growth rate will be only -0.4
persons per thousand in 2005. The reason for the modest
impact is that birth rates in many developing countries are
much higher than the death rates, so the AIDS increase in
mortality only partially offsets this larger difference.
3.2.3.33. The correlation between fertility and mortality
In the recently published IIASA population projections (Lutz,
1996), the correlations of fertility and mortality rates are also
different to those of the UN projections. Within IIASA the
main scenarios are labeled "central," "rapid transition," "slow
transition," "high," and "low." In the two "transition" scenarios,
mortality rates and fertility rates are correlated so that low or
high mortality accompanies low or high fertility, respectively,
in line with conventional wisdom among demographers that
fertility declines are associated with mortality declines. This
correlation narrows the range of projected population size as
compared with an anticorrelation assumption. The "high" and
"low" scenarios, by contrast, anticorrelate mortality and
fertility and are considered quite unlikely.
3.2.3.3.4. Probabilistic population projections
Another recent development in demographic projections is that
of probabilistic scenarios. Lutz et al. (1997) consider, based on
their probabilistic population projections, a doubling of world
population unlikely. Their scenarios use fertility, mortality, and
migration rate assumptions based on a Gaussian fit to a survey
of demographic experts who were asked to give a range of rates
for each region that they considered to cover the 90* percentile
probability range. Given the Gaussian cui-ve fits to the expert
data, a Monte Carlo simulation was run to generate 4000
scenarios, with five-year timesteps, which have a probability
distribution attached to them. The branch points in the fertility,
mortality, and net migration rate curves, based on the expert
data, were set in 1995, 2000,2030, and 2080. The probabilistic
projections extend to 2100. The 5* and 95* percentile intervals
are between 6.7 to 15.6 billion people by 2100, a range that
usefully covers current knowledge on the uncertainty of future
world population levels to be considered in SRES. It should be
noted that such probability assignments derived from expert
opinion are inherently subjective and do not necessarily
suggest a corresponding likelihood of future оссштепсе.
3.2.4. Other Aspects of Population: Aging and
Urbanization
3.2.4.1. Aging
Population aging has widely discussed implications for social
planning, health care, labor force structural changes, and
entitlement programs. As shown in Figure 3-6, percentage
growth in the elderly age cohorts is predicted strongly by all
projections. The figure shows the percentage of elderly age
cohorts using the medium projection data. Importantiy, it will
be a continuous process over the entire 2P' century, even
though total population size is forecast in these cases to
stabilize during the latter half of the century.
The detailed economic effects of such a profound and rapid
change in social structure are not well understood (Eberstadt,
1997). The problem is considered below in the discussion of
the impact of population dynamics on economic development.
In short, conventional wisdom takes a more or less neutral view
of the effect of population growth, including the impacts of
aging, on the rate of economic growth (Hammer, 1985; Kelley,
1988; National Research Council, 1986). Lowered population
growth rates (and concomitant aging) might have a beneficial
effect on the economy through reduced youth dependency
ratios, which result in higher savings rates (Higgins and
Williamson, 1997). Also, population aging could reduce labor
supply and thus reduce potential economic growth. Against this
argument, labor scarcity induces higher wages that in tum are
a powerful incentive to increase labor productivity. Increasing
30
IIASA % over 60
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20 -
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WB%>60
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USCB%>65
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I I 1 1 1 1
2000 2025 2050 2075 2100
Figure 3-6: Percentage of world population over 60 or 65 years of age with time for the central scenarios. The World Bank data
are from 1994 (Bos et al., 1994) because the results from 1996 are not published. The USCB data are from McDevitt (1996).