Emissions Scenarios - IPCC

по Scenario Driving Forces
IIASA Slow
UN High
UN Medium
--«-- World Bauk
IIASA Central
UN Low
2050 2075 2100 2150
IIASA Rapid
Figure 3-5: Projected world average TFRs from the UN, IIASA, and World Bank. The older 1994 World Bank projections are
shown, because the latest (1996) projection methodology is not published. USCB fertility rates are not readily available and so
are omitted. UN fertility assumptions are shown only for the 1996 Revision as full detail of the 1998 data were unavailable at
the time of drafting this report.
fertility assumption causes the projection to decline and to
"hit" the World Bank and UN curves in 2100. This "overshoot"
occurs because the IIASA central scenario assumptions rely on
an expert poll conducted in 1993 (Lutz, 1994), whereas other
projections incorporate more recent information into their
future assumptions (more rapid recent fertility declines in
many developing countries; see Courbage, 1998). However,
among demographers disagreement persists on the timing and
rates of demographic transition in the developing countries, in
particular- between the demographers at IIASA and those at the
UN and World Bank (see the discussion in Lutz, 1994) on the
differences among altemative population projections). This
disagreement reflects important uncertainties in the projection
of future demographic developments. The beiow-replacement,
long-term fertility level in the IIASA central estimate is a
hallmark of the institute's work. Demographers at IIASA
strongly adhere to the view that there is little reason to expect
developed nations to return to replacement-level fertility, while
much evidence suggests that low long-term rates will persist
(Lutz et ai, 1996). One issue here, however, is that the TFR
measure is currently depressed in Europe, in part because
women are delaying their childbearing until later ages
(Bongaarts, 1998). Thus, the TFR may rise if these women do
have children, and such a conection has to be incoфol•ated into
the long-term TFR assumptions (e.g., see Courbage, 1998).
Important uncertainties are by how fast and how much the TFR
will rise.
3.2.33. Recent Developments in Demographic Projections
3.2.3.3.1. Downward revisions in population projections
The UN 1996 Revision generated substantial press attention in
1997 because it forecast nearly 500 million fewer people in
2050 than it had in 1994. The base year of data for the UN 1996
Revision is 1995, whereas in the 1994 Revision fertility.
mortality, and migration rates for 1995 are forecast. The
reduction in the population projection largely results from
more accurate data available for 1995 (UN, 1997b, 1997c). The
major change was the lower-than-anticipated world average
fertility of 2.96 children per woman during the period
1990-1995, as compared with 3.10 children per woman
assumed in the 1994 Revision. The main reason for this
decrease is a faster-than-anticipated decline in fertility in a
number of couniries in south central Asia, Bangladesh and
India, and Sub-Saharan Africa, Kenya, and Rwanda (UN,
1997c). Other important regional declines in fertility also took
place in Brazil, the former Soviet republics, and the newly
independent states in eastern and southern Europe (Haub,
1997). Higher-than-anticipated mortality rates in a number of
countries afflicted by wars and the spread of AIDS also
contributed to the downward population revision.
3.2.3.3.2. Demographic impact of the HIV and AIDS epidemic
Both the UN and the Wofld Health Organization (WHO)
conduct ongoing surveys of the global HIV (human
immunodeficiency virus) and AIDS epidemic. The most recent
surveys indicate that at the beginning of 1998 30.6 million
people were infected with HIV, the vhus that causes AIDS, and
11.7 million people have already lost their lives to the disease
(UNAIDS and WHO, 1998).
Since the impact of HIV and AIDS on mortality rates is
greatest in the sub-Saharan region, the impact of the epidemic
on population growth will be greatest there. One study
(Bongaarts, 1996) suggests that by 2005 the annual population
growth rates (expressed as "persons increase per thousand
population") in sub-Saharan Africa will be about 1.4 persons
per thousand lower than would have occurred in the absence of
the disease. Other regions, however, will experience a much
smaller impact in population growth so that the AIDS-related