The population bomb (…fizzled out in 1989)

Population growth peaked in 1989. The world’s population is already flattening out. Women’s education is the key factor driving lower fertility rates.

Type “population counter” into Google and you’ll find them, the doomsday clocks inexorably ticking up to some Malthusian catastrophe. Click, clickety-click they run, compulsive, metronomic, yet without comment or explanation. We’re left to draw our own conclusion; Ehrlich’s population bomb ^[1] is exploding. Numerous pundits make it more explicit. There’s a “population explosion”, population is growing “exponentially”, there’s “unending growth”, it’s the “elephant in the room”, it’s what “everybody knows” but can’t say.

What do the population statistics actually show ?  Is population growth “exponential” ?  Are we facing “unending growth” ?

No. In reality population growth is already decelerating; the population is levelling off. Let’s look at the numbers. The clearest way to see the trend is to look at the rate of change of population. Figures 1 and 2 show world population data from the U.S. Census Bureau ^[2].

Figure 1.  Annual population change [2]. Population growth peaked in 1989. The sharp dip in 1959–60 was due to the Great Leap Forward in China.		Figure 2.  Annual population change expressed as annual percentage growth [2].

Figure 1 shows the year-on-year rate of change of population from 1950 to the present day, and projected out to 2050. The rate of increase peaked in 1989, and is declining. For an exponentially growing population, the rate of growth would itself be an rising exponential curve. In fact, the rate of growth is falling. That means population growth is slowing. Figure 2 shows the same data, but plotted as an annual percentage change. For an exponentially growing population, this graph would be a horizontal line showing constant year-on-year percentage growth. In fact, it is declining sharply, again showing that the rate of growth is falling.

We can see from Figure 1 that the population curve went through an “inflection point” in 1989; it switched from an increasing growth rate to a decreasing growth rate. In other words, that’s when the world’s human population started levelling out.

Figure 3. An S-curve and its first derivative, a bell curve.

Just to elaborate, I’ll take a brief detour into school calculus. Figure 3 shows how the maths works. When a rate of change (i.e. the gradient or first derivative of a function) peaks and then declines in the form of a bell curve (lower graph), the function itself (upper graph) first increases ever more rapidly and then, beyond the “inflection point” – the peak of the derivative – it increases ever more slowly and flattens out. That’s the trajectory being followed by the population curve.

We can see this by looking at the population figures themselves. Figure 4 shows population estimates from the United Nations Population Division ^[3] for the same time period (1950–2050) as Figures 1 and 2. For the median estimate, the world’s population is growing at under 0.4% per annum by 2050. The world’s population is flattening out. In fact, Europe’s is already falling.

Figure 4. Population of the world and its regions.  [3] Population axis is in millions.   •  Solid line:  median variant.   •  Shaded region:  low to high variant.   •  Dashed line:  constant-fertility variant. The “World” population curve is the sum of the six regional population curves (note that the vertical axis is logarithmic).

Table 1.  World Population in 2050

Population (million)
Low variant	7,792
Median variant	9,191
High variant	10,756
Constant fertility variant	11,858
Data from World Population Prospects, United Nations Population Division.  [3] This table shows the same data as Fig. 4, for the year 2050. The population in early 2008 was 6,651 million.

On a longer timescale, Figure 5 shows a range of projections out to the year 2100. The data are from the IPCC Special Report on Emissions Scenarios ^[4]. The red, blue and green lines are the high, median and low projections used in the IPCC scenarios.

Figure 5. Population projections from the IPCC Special Report on Emissions Scenarios. [4] High and low projections from Lutz et al. [5]; median projection from the UN. The insert shows population development in the developed (industrialized) regions. Historical data from 1900 to 1990; The coloured lines are the three Special Report on Emissions Scenarios (SRES) population scenarios from 1990 to 2100.   •  Red line:  the high SRES projection (A2).   •  Blue line:  the median SRES projection (B2).   •  Green line:  the low SRES projection (B1). The three dotted lines are from the earlier IPCC IS92 scenarios. The range of possible outcomes has narrowed in the more recent projections.

The first thing to note is that the accuracy of the projections decreases with time. Population projections one generation ahead can be quite accurate, since the next generation of parents has already been born, so their numbers are known, and their fertility rate can be estimated from current trends. Moving three or four generations forward, the uncertainties accumulate, and the error bars on the estimates widen. This can be seen in Figure 5. The spread between the high and low projections widens until there is a factor of two between them by 2100. By then the uncertainty on the estimate exceeds ±40%, so there’s not much to be gained by calculating projections still further forward.

As for the core conclusion on population, the IPCC report ^[4] summarises it thus:

The B2 scenario family is based on the UN median 1998 population projection. The global population increases to about 9.4 billion people by 2050 and to about 10.4 billion by 2100. This population scenario is characteristic of recent median global population projections, which describe a continuation of historical trends toward a completion of the demographic transition that would lead to a constant global population, and is consistent with recent faster fertility declines in the world together with declining mortality rates.

Population Projections, IPCC ^[4]

These projections have 1990 as their starting point. More recent population projections have revised these estimates downwards and reduced the uncertainties. Here Lutz et al. ^[6] present an update in Nature:

There has been enormous concern about the consequences of human population growth for the environment and for social and economic development. But this growth is likely to come to an end in the foreseeable future. Improving on earlier methods of probabilistic forecasting, here we show that there is around an 85 per cent chance that the world’s population will stop growing before the end of the century. There is a 60 per cent probability that the world’s population will not exceed 10 billion people before 2100, and around a 15 per cent probability that the world’s population at the end of the century will be lower than it is today.

The End of World Population Growth,
W. Lutz et al., Nature, 2001 ^[6]

Most probably, the Earth’s population will have stopped growing at a level slightly below 10 billion people by the year 2100. There’s less than 50% of growth left in it. That brings us to the next question: why are human fertility rates falling?

Why Are Human Fertility Rates Falling?

Income per person (fixed PPP$)

Figure 6. Graph of children per woman (vertical axis) vs. income per person by country for 1950–2005, displayed using Gapminder World. Click on the image to view the animated Gapminder World graph.

Wealthy societies have fewer children per family than poorer societies ^[7] [8]. This inverse correlation between wealth and fertility, shown in Figure 6 (thumbnail), is occasionally called the “demographic-economic paradox”. It’s only a paradox if seen from the Malthusian viewpoint where greater means should lead to more offspring. In reality, of course, there is no paradox. Women’s education is the main explanatory factor accounting for falling fertility rates in Asia and Africa ^[9] [10].

Female education influences fertility in a number of ways. It is associated with later age at marriage, desire for smaller families and increased acceptance of and access to contraception. It enhances women’s choices in the matter of family formation. Even in societies where marriage is a poor indicator of the onset of sexual activity, better educated women are older when they have their first child. A later transition to motherhood is likely to have an influence on family size.

U.N. Press Release ^[9]

Taking India as a specific example:

There has been a significant decline in fertility in many parts of India since the early 1980s. [...] We find that women’s education is the most important factor explaining fertility differences across the country and over time. Low levels of child mortality and son preferences also contribute to lower fertility. By contrast, general indicators of modernization and development such as urbanisation, poverty reduction, and male literacy bear no significant association with fertility.

J. Drèze and M. Murthi ^[11]

The trend is toward a flat population, but this isn’t an argument for doing nothing. Trends don’t just “happen”, they are driven by policy choices. Existing policies across Asia and parts of Africa are moving toward the “demographic transition” of a constant global population by the end of the century. Jeffrey Sachs makes a case for accelerating this trend with greater policy intervention on four fronts ^[12]:

1. promote child survival;
2. promote girls’ education;
3. promote availability of contraception;
4. raise farm productivity.

His focus is on population growth in Africa: “Fifty percent of the projected global population increase by 2050 will fall within Africa and the Middle East, the world’s most politically and socially unstable regions.” He takes political and social instability as a given, and sees population control as the policy intervention of choice. It’s a curiously fatalistic view of political instability. One might approach this problem from the other end, and seek to address the social and political instability in Africa. Agriculture needs political stability. A farmer must have confidence he will have the use of his land from one year to the next.

To wind up this post, here is Prof. Hans Rosling, professor of global health at the Karolinska Institute and a cofounder of Médecins Sans Frontières Sweden, discussing how fertility rates have changed over the last thirty years. He is speaking at TED 2006. The fertility discussion starts about 3 minutes in, but the talk provides a broader context too:

Debunking Third World Myths H. Rosling, TED 2006

References

1. The Population Bomb, Paul R. Ehrlich, Ballantine Books (1970)
2. World Population Trends, U.S. Census Bureau (2007)
3. World Population Prospects, The 2006 Revision Population Database, United Nations Population Division
4. 8.1. Population Projections, N. Nakicenovic et al., IPCC Special Report on Emissions Scenarios, Technical Summary, Chapter 8. Main Scenario Driving Forces Based on the Literature (2000)  (and references therein)
5. World population scenarios for the 21st century, W. Lutz, W. Sanderson, S. Scherbov and A. Goujon, In “The Future Population of the World. What Can We Assume Today? (Revised Edition)”, Ed. W. Lutz, pp. 361–396. London: Earthscan. (1996)
6. The end of world population growth, W. Lutz, W. Sanderson and S. Scherbov, Nature 412, 543–545 (2001)
7. Total fertility (children per woman), World Population Prospects: The 2006 Edition, United Nations Population Division (2006)
8. A Gapminder data set for GDP per capita by purchasing power parities, M. Lingren, Gapminder Foundation (2008)
9. Study Explores Negative Correlation Between Educational and Fertility Levels among Countries, Regions, U.N. Press Release DEV/2092 POP/605, February 7, 1996
10. Fertility Change in Asia and Africa, M. Murthi, World Development 30, Issue 10, 1769–1778 (2002)
11. Fertility, Education and Development: Further Evidence from India, J. Drèze and M. Murthi, STICERD – Development Economics Papers 20, Suntory and Toyota International Centres for Economics and Related Disciplines, LSE, revised (2000)
12. Lower Fertility: a Wise Investment, J.D. Sachs, Scientific American 295 (3), 42 (2006)

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