Population Projections for the IPCC Special Report
on Emissions Scenarios (1996)
The task of the World Population Program in contributing to Special Report on Emission Scenarios was basically to decide which population scenarios developed by IIASA would be more consistent with the range of long-term emission scenarios developed by the SRES team. IIASA population scenarios were chosen as low/high brackets whereas the United Nations Population Division provided the central demographic scenario.
IIASA population projections are particularly adapted to the SRES for the following reasons.
- They are long range up to the year 2100
- The spatial disaggregation in 13 regions matches those of most models used to develop the SRES;
- Fertility, mortality, and migration are co-varied in a consistent way that leaves room for substantive considerations of justifiable alternative future trends in the components.
- Finally, a large number of scenario variants are available.
The SRES was developed along four story lines representing the playing out of certain economic, social and environmental paradigms. Two population scenarios were adopted from the population projections realized at IIASA in 1996 and presented in The Future Population of the World: What Can We Assume Today? Revised and Updated Edition edited by Wolfgang Lutz and published by Earthscan.
The two IIASA scenarios bracket the upper and lower bound of future population developments in the SRES.
One is the high fertility, high mortality, central migration scenario, in which global population increases to some 15 billion people by the end of the 21st century (SRES scenario A2 ). The other scenario is a variant of the low fertility, low mortality, central migration scenario described in Lutz (1996). Some modifications were implemented to reflect the converging trends in socio-economic development described in the respective SRES storylines (SRES scenarios A1 and B1). In particular mortality rates were assumed to decline to similar levels across all regions by the end of the 21st century.
Under this (modified) scenario, the word population peaks in 2055 at 8.7 billions and decreases to 7.1 billion in 2100 as a result of below-replacement fertility levels, characteristic of the IIASA demographic projections. (This compares to a level of world population of 8.5 and 6.5 billion, respectively, in the original scenario reported in Lutz 1996).
POPULATION PROJECTIONS FOR SRES-SCENARIOS A1-B1
The SRES A1 and B1 storylines and scenario families describe a convergent world with substantial reductions in regional, social, economic, and development disparities.
The population projections for these scenarios adopted a variant of the IIASA low fertility/low mortality/central migration scenario. It combines:
- Low fertility levels for all developing regions, with the total fertility rates reaching between 1.5 (Centrally Planned Asia and China) and 2.0 (Central Asia, Middle East & North Africa, and Sub-Saharan Africa) in 2030-35;
- Central fertility levels in developed regions with stabilization at present below replacement fertility levels. Total fertility rates reach between 1.7 (Europe and Pacific OECD) and 1.85 (North America);
- Low, converging mortality levels in all regions with gains in life expectancies between 2 and 4 years per decade until 2030-35;
- Central migration patterns in all regions.
Under this scenario, the word population peaks in 2055 at 8.7 billion and decreases to 7.1 billion in 2100.
POPULATION PROJECTION FOR SRES-SCENARIO A2
The SRES A2 storyline and scenario family describes a world of high population growth with more concern for strengthening regional cultural identities, and less concern for rapid economic development.
The population projection adopted for this scenario and for all regions was the IIASA high fertility/high mortality/central migration variant. It entails a fertility increase for presently developed countries. Total fertility rates would reach between 2.1 (Europe and Pacific OECD) and 2.3 (North America). Fertility rates in developing countries would slightly decrease to reach 4.0 in North Africa & Middle East, sub-Saharan Africa and 3.0 in South Asia and Latin America. It would on the contrary increase to reach 3.0 in China, Pacific Asia, and 4.0 in Central Asia. The high mortality scenario assumes declines in life expectancy of two years per decade in sub-Saharan Africa. One reason could be that infectious disease, especially the AIDS epidemic may spread further. In other regions, life expectancy is modeled to stagnate at present levels (e.g. Pacific Asia) or increase moderately of one year per decade.
According to this scenario, the world population would double in 2050 and reach 15.1 billion people by the year 2100.
RESULT TABLES
The results are arranged in five-year time intervals (1995–2100) for each region. The results include the total population for male, female and both sexes as well as the population by age and sex.
Available here for downloading are two compressed file archives in zip format, each containing the population results as an Excel97 spreadsheet file and as a delimited text file.
