The IIASA probabilistic world population projections as a basis

The projection exercise described in this paper presents an exploratory extension of IIASA’s world population projections for two centuries beyond their regular time horizon. IIASA’s World Population Program recently published its newest 2007 projections 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. These 2007 projections, published under the title “The coming acceleration of global population ageing” (Lutz et al. 2008b) are an update of earlier projections published under the title “The end of world population growth” in Nature in 2001 and in a full length book under the same title in 2004 (Lutz et al. 2001, 2004). The methods used for this set of probabilistic projections are comprehensively described in that book.

It also includes lengthy substantive justifications of the specific fertility, mortality and migration assumptions. The 2007 update of the 2001 projections includes one methodological innovation and two noteworthy revisions of the fertility assumptions.

Methodologically, the new projections also consider uncertainty ranges for the starting conditions of the projections for countries with unreliable information about current conditions. This is particularly relevant for China, where published fertility levels for around 2000 range from 1.2 to 1.8. After the analysis of about 30 different estimates of Chinese fertility levels, we assumed a median TFR of 1.5 with an 80 percent uncertainty range from 1.3 to 1.7 for the year 2000 (Lutz et al. 2007). This downward revision of Chinese fertility has visible impacts on the projected world population because of the great weight of China. However, this is partly compensated by an upward revision for African fertility rates which did not decline as rapidly as assumed in 2001. This is partly due to stalled fertility declines in some African countries due to worsening overall conditions, and in particular, a stagnation or even decline in female basic education.

But the overall picture drawn by the 2001 and 2007 projections is very similar.

The probability that the world population will peak in size before the end of the century has now increased from 86 to 88 percent. Figure 1 puts the results of these probabilistic projections to 2100 into a millennial perspective, plotting world population size from 1000 AD to 2100. It shows that for centuries, world population has been below half a billion until around 1800, when population growth started to take off. The increase accelerated tremendously during the 20^th century, when world population increased from 1.6 billion in 1900 to 6.1 billion in 2000. At the global level, the population growth rate was highest during the late 1960s, peaking at just over 2 percent per year.

Since then the growth rate has been declining. Absolute increments started to decline around 1990. The graph also shows the decilies of uncertainty distribution of future world population based on IIASA’ probabilistic projections. It shows that the 80 percent range for 2100 goes from less than 6 billion to more than 12 billion. The line with dots in the center gives the median of the 2000 independent simulation runs. It shows a peak of 8.9 billion around 2070 and then falls to 8.4 by 2100. The 80 percent uncertainty range for 2100 is from 6.2 to 11.1 billion, i.e. a range of five billion. This quantification of uncertainty helps to put our thinking about long term population trends into perspective.

Figure 1. Historical world population growth and the results of IIASA’s probabilistic projections: Plotting world population size from 1000 AD to 2100.

Within this century the uncertainty of the future proportion of elderly in our society is even larger. Figure 2 plots the resulting uncertainty ranges for the proportion of the population above the age of 80 in Western Europe. The proportion is almost certain to increase over the coming decades from currently around 3 percent of the population to between 6 and 14 percent in 2050. But during the second half of the century the uncertainty will explode to an incredible 80 percent range; from 8 to 37 percent of the entire population will be above the age of 80. This large uncertainty in the future proportion of elderly is a function of the current scientific uncertainty about the likely future course of old age mortality. While one school of thinking assumes that in the West we will not see much further increase in life expectancy, the other school assumes sustained further increases, if not an acceleration in the increase of life expectancy due to bio-medical progress. Without being able to scientifically reject one or the other view, the only thing a forecaster can do is to try to honestly reflect this uncertainty in the projections themselves and not pretend to know more than we know.

Figure 2 also shows two horizontal lines in 2100 which represent the values given by the high and low variants of the UN long term population projections. These variants differ from our projections in two ways: First, they are lower because the UN assumes a slower increase in future life expectancy. Second, the interval between the two variants is very narrow because the UN does not consider alternative mortality scenarios, as

0 2 , 0 0 0 4 , 0 0 0 6 , 0 0 0 8 , 0 0 0 1 0 , 0 0 0 1 2 , 0 0 0 1 4 , 0 0 0

1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0

Y E A R

WORLD POPULATION (IN MILLIONS)

mentioned above. The difference between the two lines is only a consequence of alternative fertility assumptions, i.e., different total population sizes by which an identical absolute number of elderly will be divided to calculate the proportion. For this reason the UN projections cannot be expected to describe the uncertainty associated with future ageing in a meaningful way.

Figure 2. The uncertainty distribution of the proportion of the population above age 80 in Western Europe. Source: Lutz et al. (2008b: 718).

For the exploratory extensions of these projections to 2300, we considered two alternative approaches:

(1) Start the alternative assumptions in the year 2005. For this century, have them cover the full uncertainty range of the probabilistic projections while assuming continuations at the same levels for another 200 years. This full range would, however, result in an absurdly broad range of results which would likely generate feedback mechanisms that would check population growth or keep our species from voluntary extinction. This is the reason why the UN (2004), who start their alternative scenarios in 2000, chose to define such extremely narrow fertility ranges. As a compromise, we will present selected scenarios with more moderate fertility levels that originate at the high and low ends of the 95 percent distribution in 2080 when those projections stop making substantive assumptions.

(2) Leave the uncertainty ranges of the 2007 IIASA projections as they are and only define alternative scenarios starting from the median of the distribution in 2080.

By doing so it will be evident to the user that these scenarios are not intended to cover the full range of uncertainty but are indeed only exploratory exercises in population dynamics. We chose this approach for most of our scenarios precisely for this reason, so that users do not mistake it for a comprehensive assessment of possible future ranges as can easily happen when presented with any range of alternative trends.

In the following projections we study a cross-classification of five different fertility scenarios with two mortality scenarios. Some of the assumptions have been applied as starting in 2005, and all assumptions have been applied as starting from the median in 2080.