I would like to follow up on the last sentence of the excellent presentation by Joel Cohen. It is indeed true that the lag between new methodological developments and their actual implementation by statistical agencies is regrettably long, but I think there is some hope that the speed of applying innovations has been accelerating over this century. This is the case in many other areas, and it would be surprising if it were not the case in the field of population forecasting. I would assess with a probability of well above 90 percent that by the end of this century, institutional procedures for projecting population will include probabilistic elements. ; In the following, I would like to mention three additional aspects [new methods, extreme events, and projections of population by level of education] complementing the important remarks made by Cohen.Demography ; Economic conditions

Wolfgang P. Lutz

English

Research Papers in Economics

WORLD POPULATION IN 2050: ASSESSING THE
PROJECTIONS. DISCUSSION
Wolfgang P. Lutz*
I would like to follow up on the last sentence of the excellent
presentation by Joel Cohen. It is indeed true that the lag between new
methodological developments and their actual implementation by statis-
tical agencies is regrettably long, but I think there is some hope that the
speed of applying innovations has been accelerating over this century.
This is the case in many other areas, and it would be surprising if it were
not the case in the ﬁeld of population forecasting. I would assess with a
probability of well above 90 percent that by the end of this century,
institutional procedures for projecting population will include probabi-
listic elements.
In the following, I would like to mention three additional aspects
complementing the important remarks made by Cohen.
NEW METHODS
Actually, things are evolving rapidly with respect to the application
of new forecasting methods. A recent issue of the international scientiﬁc
journal Nature features an article, “The End of World Population
Growth,” that presents a new method of probabilistic population projec-
tions (Lutz, Sanderson, and Scherbov 2001). This method represents a
possible synthesis of several competing approaches to dealing with the
issue of uncertainty in future population trends. I will quickly describe
the alternative methods and present some results from the synthesis
approach that underlies the Nature article.
*Leader, Population Project, International Institute for Applied Systems Analysis,
Laxenburg, Austria.Since the cohort component method of projection is taken as a
standard, the differences between alternative probabilistic approaches to
population forecasting refer only to the modeling of future fertility,
mortality, and migration rates. Here one can distinguish between (a) the
speciﬁc process chosen for representing the time series of these vital rates,
and (b) the basis for the speciﬁc assumptions made about the future range
of uncertainty.
The literature essentially has presented two ways of specifying the
time series of vital rates: (1) processes with annual ﬂuctuations (see Lee
1998; Lee and Tuljapurkar 1994; Lee and Carter 1992; Alho 1990) and
(2) piece-wise linear scenarios (see Lutz, Sanderson, and Scherbov 1997,
1996; Lutz and Scherbov 1998). While the second way has the advantage
of conforming to the current practice of scenario deﬁnition in statistical
ofﬁces around the world (including the United Nations 1999 and the
World Bank 2000), the ﬁrst way can produce more realistic annual
outcomes, given that the possible levels are bounded. We have chosen a
moving-average model with annual ﬂuctuations.
The literature suggests three approaches to deriving assumptions
about the future range of uncertainty surrounding the components:
(1) compute a measure of the future error from the ex post analysis of past
projections (Stoto 1983; Alho 1997; Keilman 1999), (2) apply time series
models (Lee 1999; Tuljapurkar, Li, and Boe 2000; Lee and Tuljapurkar
1994), and (3) have well-informed experts make assumptions based on
explicitly stated substantive arguments (Lutz and Scherbov 1999). The
three approaches are not mutually exclusive, and (1) and (2) also include
expert judgment.
In the Nature study we take a step toward a synthesis of the three
approaches. Our process speciﬁcation uses a time series model. Given
that, at the level of world regions, lack of data makes empirical estimation
impossible, we explicitly consider existing national-level parameter esti-
mates (Lee and Tuljapurkar 1994; Lee and Carter 1992). The ex post
analysis of past errors enters our study in two ways: (1) The substantive
assumptions made on fertility and mortality changes are informed by the
analysis of past errors in those components (Keilman 1999; Bongaarts and
Bulatao 2000), and (2) Our results at the regional level are compared to
the results of an ex post error analysis of global UN projections docu-
mented in the National Research Council (NRC) report.
For the speciﬁc purpose of estimating the probabilities that popula-
tion growth in different world regions will end during this century, we
wanted to err on the side of higher variance (that is, lower probability of
population growth ending in this century); accordingly, we followed the
general rule to produce intervals that were at least as large as those in the
NRC report at the level of major world regions. Combining this with
argument-based expert judgment (Bongaarts and Bulatao 2000; Lutz
1996), we saw substantive reasons for assuming a larger uncertainty in
124 Wolfgang P. Lutzmany regions as a result of new factors such as HIV/AIDS, the situation
in the former USSR, and the indeterminancy of long-range, post-transi-
tional fertility levels affecting an increasing number of countries.
Figure 1 shows forecasted distributions of world population sizes
(fractiles), as taken from Lutz, Sanderson, and Scherbov (2001). The
results show about an 85-percent probability that world population will
reach a maximum over the course of this century and then start to decline.
There is also about a 60-percent probability that world population will
not reach 10 billion before 2100, and around a 15-percent probability that
in 100 years, world population will be even lower than today.
EXTREME EVENTS
Cohen identiﬁed a very important qualiﬁcation that needs to be
made with respect to conventional population projections, namely, the
assumed absence of extreme events. High-impact, low-probability events
such as a lethal global pandemic, a major war, or a serious meteoric
impact certainly have the potential for altering the future course of
population signiﬁcantly.
What should scientists do with this qualiﬁcation? Should we just live
with it and ignore it for our practical work, only remembering in the back
of our minds that things could indeed turn out differently? Another
WORLD POPULATION IN 2050: ASSESSING THE PROJECTIONS: DISCUSSION 125possible approach is to try to deﬁne low-probability scenarios that
assume certain hypothetical demographic consequences of certain hypo-
thetical events of unknown magnitude, structure, and timing. Because of
the many unknowns, a science ﬁction novel could be more consistent
than such scenarios. In the ﬁrst edition of The Future Population of the
World (Lutz 1994), we showed some examples of the possible conse-
quences of major disasters, such as a Malthusian overshooting of carrying
capacity in Africa that would lead to the immediate death of half of the
population between the years 2060 and 2065. We did this mostly to show
the outside world that demographers are not completely ignoring the
possibility of catastrophic events. Although it is of some interest to study
what such a hypothetical event might do to the age structure and
population dynamics after the event, we decided not to include these
calculations in the 1996 revision of the book (Lutz 1996) because we felt
that they were simply too arbitrary—assumptions made out of the blue
without any empirical or theoretical basis.
Moreover, there may be another, more promising way to deal with
this issue, in the context of probabilistic population projections. If the
uncertainty distributions for future vital rates are deﬁned in such a way
that, at the very tail ends of the distributions, some very high-impact,
very low-probability events are included, this may in fact capture the
issue to some degree. Of course, we know little about the probabilities of
such events or their magnitude, but our knowledge may be more than
zero. It may be possible to calculate the risk of a major meteorite based on
past observations, for instance; it is more difﬁcult with respect to possible
new diseases or possible rapid increases in life expectancy. But engineers
did not shy away from calculating in exact quantitative terms the risks for
certain nuclear accidents (and they have been taken as the basis for
serious discussions in national parliaments) even though very little is
known about the probabilities of the different factors that can lead to such
a disaster. I doubt that the intentional shutting down of the security
system that led to the Chernobyl accident was factored into these
calculations. Still, it may be worth thinking a bit further along these lines.
As a matter of fact, we do have such long tails in the distributions
underlying the Nature article, and in one out of the 2,000 simulations, we
actually get a mortality increase of catastrophic magnitude (higher than
the impact of AIDS in Southern Africa).
PROJECTIONS OF POPULATION BY LEVEL OF EDUCATION
Another area where conventional population projections are in need
of renewal and expansion of focus is the explicit incorporation of other
major covariates in addition to age and sex. Level of education and
rural/urban place of residence are two prime candidates for such explicit
inclusion in population projections, because both fertility and mortality
126 Wolfgang P. Lutztend to vary greatly with level of education and place of residence.
Making this important hidden source of heterogeneity explicit and
including it in the form of multistate population projections signiﬁcantly
affects the projected population dynamics. If, for example, the proportion
of women of reproductive age with secondary education increases in a
certain country (say, Algeria) with high educational fertility differentials,
then over time fertility will decrease, even if fertility within each
educational group remains constant.
In addition to this purely demographic effect, variables such as place
of residence and education are very important in their own right. In the
June 2001 issue of Population and Development Review, we produced the
ﬁrst global population projections by level of education. I would like to
share with you one of the ﬁndings that relates to human capital in
international economic competition.
The changing educational composition of the population is very
signiﬁcant at different levels: It matters greatly for individual develop-
ment and individual life changes. It makes a big difference for a nation’s
institutional and economic performance. It also matters for the relative
weights, productivity, and competitiveness of major world markets. In
this context, it is useful to look at absolute numbers of workers by skill
levels rather than at the proportions. For three economic mega-regions of
the future (Europe and North America together, China, and South Asia),
Figure 2 compares trends in the size of the working-age population (ages
20 to 65) by educational attainment. The data are taken from the
“American scenario,” which assumes convergence by 2030 to today’s U.S.
school enrollment levels. At present, China clearly has the most numer-
ous working-age population of the three regions, but its educated
population (secondary and tertiary together) is still smaller than that of
Europe and North America together. South Asia is far behind, with less
than half the size of the other two regions.
Over the next twenty years, South Asia is expected to surpass China
in terms of the total size of its working-age population. But in terms of
educational composition, the difference between the two regions will be
stunning. While in China in 2030, 73 percent of the working-age popu-
lation will be better educated (secondary plus tertiary), in South Asia the
better-educated share will be only 40 percent. The main reason for this
divergence lies in differences between the two regions in investment in
primary and secondary education over the past two decades.
Among the three major world regions, Europe and North America
will continue to have the highest educational levels of working-age
population, but in terms of absolute numbers of educated people, they
will clearly fall behind China. Over the next three decades China’s
educated working-age population is likely to increase from 390 million to
750 million, while that of Europe (without the former Soviet Union) and
North America together will hardly increase, from 430 million in 2000 to
WORLD POPULATION IN 2050: ASSESSING THE PROJECTIONS: DISCUSSION 127510 million in 2030. These signiﬁcant future changes in the numbers of
skilled workers are likely to have far-reaching consequences for the
regions’ roles in the global economic system.
In conclusion, many exciting new issues still remain in the ﬁeld of
population projections. Resolving them can contribute to making demo-
graphic forecasts more informative and useful for organizations, like the
Federal Reserve, that need such data.
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WORLD POPULATION IN 2050: ASSESSING THE PROJECTIONS: DISCUSSION 129

A universal pattern of mortality decline

Beyond Six Billion: Forecasting the World’s Population.

Doubling of world population

Expert-based probabilistic population projections.” In

How accurate are the United Nations world population projections?”

Long-Range World Population Projections: Based on the

Modeling and forecasting the time series of U.S.

Probabilistic approaches to population forecasting.” In

Probabilistic population projections based on expert opinion.” In

Stochastic methods in population forecasting.”

Stochastic population forecasts for the United States: Beyond high, medium and low.”

The accuracy of population projections.”

The End of World Population

The Future Population of the World. What Can We Assume Today?

The world’s changing human capital stock: Multi-state projections by educational attainment.”

World population in 2050: assessing the projections: discussion

http://www.bostonfed.org/economic/conf/conf46/conf46d3.pdf

World population in 2050: assessing the projections: discussion

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