Emissions Scenarios - IPCC

Scenario Driving Forces J 09
UN 1998 Low Range
Medium
IIASA 1996
2000 2050 2100 2150
Figure 3-3: Central projections extracted from Figure 3-2 show excellent agreement among the central projections over the
next 100 years (with the possible exception of the IIASA projection).
databases, the dominance of population momentum associated
with a young population age structure, and the assumption of
replacement-level fertility in the long term.
Figure 3-4 shows the breakdown of population growth
projections in industrial countries (IND) and developing
countries (DEV) as defined by the UN. The key conclusion to
draw from Figure 3-4 is the dominance of the DEV population
on future world population growth.
The most important variable to detennine future population
levels is fertility. To a lesser extent, future population also
depends on mortality and migration rates. Figure 3-5 shows the
world-average total fertility rate (TFR), the average number of
births per woman, assumed for the various projections.
Overall, a fairly broad range of assumptions about future world
average fertility is encompassed by the projections. Future
population size is very sensitive (see Figure 3-2) to
comparatively small changes in the long-range fertility rate.
For example, within the UN projections, a decrease of the
asymptotic fertility rate of less than half-a-birth per woman
(from 2.1 to 1.7) decreases population in 2100 by 46% (UN,
1998). Only the replacement-level fertility of about 2.1 results
in a stable population in the long run.
Such sensitivity to small asymptotic fertility-rate changes
indicates the high and low projections from UN and IIASA are
all feasible scenarios of the future population (Gaffin and
O'Neill, 1998).
Up to 2050, the IIASA central TFR is high compared with the
UN and World Bank central projection. Later the IIASA central
TFR dechnes to below replacement-level feitility rate, as
depicted in Figure 3-5. As a result, the IIASA central forecast
lies considerably above ail the others throughout the 21'*'
century. After that, IIASA's below-replacement, long-term
-b- UN 96 DEV
IIASADEV
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-o- USCBDEV
-0- WB DEV
_u_ UN 96 IND
— IIASAIND
-o- USCBIND
- - 0 - WBIND
2000 2050 2100 2150
Figure 3-4: Industrial (IND) and developing (DEV) countries' population projections from the medium (central) variants. Note
that regional definitions differ in IIASA compared to UN projections.