Exploratory Extension of IIASA's World Population Projections: Scenarios to 2300

Lutz, W. ORCID: https://orcid.org/0000-0001-7975-8145 & Scherbov, S. ORCID: https://orcid.org/0000-0002-0881-1073 (2008). Exploratory Extension of IIASA's World Population Projections: Scenarios to 2300. IIASA Interim Report. IIASA, Laxenburg, Austria: IR-08-022

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Abstract

This paper describes an exploratory extension of IIASA's world population projections for two centuries beyond their regular time horizon. IIASAs World Population Program recently published its newest 2007 projections in "Nature" (February 7, 2008) in the form of fully probabilistic projections for 13 world regions for single years of age with assumptions defined until 2080 and extended results presented up to 2100. The extensions presented here do not attempt to define uncertainty ranges for the more distant future but rather apply alternative fertility and mortality assumptions starting from the median of the probabilistic projections as well as from the upper and lower end of the 95 percent range in 2080, which is the last year for which substantive assumptions were defined in the IIASA projections. After that we apply several different fertility levels ranging from 1.0 to 2.5 children per woman, cross-classified with two alternative mortality assumptions assuming maximum life expectancies of 90 and 120 years. The results for the year 2300 range from a world population of merely 40 million to one of 56 billion. The assumptions of likely future fertility, mortality and migration trends based only on substantive reasoning can have a maximum time horizon of 30-50 years which, when explicitly accounting for uncertainty, could possibly be stretched to 70-80 years. Therefore, the extensions presented here should not be viewed as likely or even meaningful projections, but only as a sensitivity analysis of what would be the long term outcome of certain alternative assumptions.

The range of alternative fertility and mortality assumptions considered here is much broader than that of the recent UN (2004) projections to 2300 which consider 1.85 as the lowest possible long term fertility level. But similar to the UN study, a main result of our exercise is to demonstrate the great sensitivity of long term population trends to very small differences in fertility assumptions. In addition, our study shows that a world population size of around 3 billion, which several ecologists propagate as a sustainable population level, need not necessarily be reached through disastrous mortality increase - as many of them believe - but could be reached before 2300 through the benign fertility reduction to a level of around 1.7 children per woman.

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