Emissions Scenarios - IPCC

108 Scenario Driving Forces
SA90 scenarios all used the same median population projection
- the World Bank 1987 projection (Zachariah and Vu, 1988).
The IS92a-f series made use of three different projection
variants, the World Bank (World Bank, 1991) 1991 projection
and the United Nations (UN, 1992) 1992 medium-high and
medium-low projections.
Wexler (1996) surveyed the world population projections used
in GHG emissions scenarios since 1990. Of the models
surveyed, all but one employed the World Bank central
projections. The sole exception - the DICE model of
Nordhaus (1993; Nordhaus and Yohe, 1983) - used
algorithmic projections based on assumed declining
population growth rates. As noted in Chapter 2, many longterm
emission scenarios available in the literature do not even
report their underlying population projections. In general.
World Bank projections have been more heavily employed
than the UN projections, apparently because of the shorter
time horizon and longer cycle time of the UN Long Range
projections. UN Revisions until 1994 extended only to 2025
(now to 2050), which is too short for emissions scenarios. The
UN Long Range series is revised less frequently, with a sixyear
interval between the previous two UN Long Range
projections. The World Bank, in contrast, updates its published
projection every two years, and it has always been a longrange
projection, out to year 2150. In addition, the World Bank
maintains a country-level disaggregation throughout its
projection, unlike the UN Long Range series, which switches
to a nine-region summary.
In 1994, however, the World Bank discontinued the publication
of population projections, even if these continue to be
generated for internal uses. In contrast to the past dominance of
World Bank projections, the SRES scenario series instead
employs published projections from the International Institute
for Applied Systems Analysis (IIASA) along with the UN's
Medium Long Range projection.
3.2.3.2. Currentiy Available Population Projections
World population projections are currently generated by the
following institutions:
• United States Census Bureau (USCB);
• World Bank;
• UN; and
• IIASA.
The main defining characteristics of the individual projections
and a detailed description of each projection is given in Gaffin
(1998).
Figure 3-2 displays these latest world population numbers from
all four demographic organizations. Two of the four
projections, from the World Bank and USCB, contain only one
central estimate and are unpublished. Tliese are generated
currently for internal organizational purposes and are not
further considered here.
The only population projections that cun^ently )псофога1е
variation of the long-range fertility rates are those produced by
UN and IIASA (Gaffin, 1998). For the UN, these variants
result in four additional projections to the UN medium
projection and are referred to as "low," "medium low,"
"medium high" and "high." The designations reflect the
average world fertility rate relative to the medium projection.
For IIASA, the primary variants also refer to world average
fertility (and mortality) rate relative to the medium projection
and are referred to in Figure 3-2 as "high" and "low."
The central projections in Figure 3-3 show excellent agreement
over the next 100 years, with the exception of the "overshoot"
in the IIASA projection, discussed below. Although significant,
such an agreement does not imply certainty or accuracy.
Rather, it reflects the use of similar methodologies and
UN 98 High
IIASA High
- D - UN 96 High
- K - UN 96 Low
UN 98 Medium High
— 0 — UN 96 Medium
—Л—
-o—
US Census
IIASA Medium
UN 98 Medium
_ 4 . . World Banlc
2000 2050 2100 2150
-Ф—
IIASA Low
UN 98 Medium Low
UN 98 Low
Figure 3-2: Most recent world population projections from the four mam demographic organizations, including high and low
variants. Two of the projections (USCB and World Bank) will not be published. Variations in future world populations are
largely determined by different assumptions concerning the demographic transition in developing countries as well as longrange
fertility rates worldwide.