29 JULY 2011 VOL 333 SCIENCE www.sciencemag.org 540

IN 1900, THERE WERE 1.6 BILLION PEOPLE ON EARTH. By 2000, that number had skyrocketed to 6.1 billion. This astounding rate

of growth has slowed, but the trend is still heading dramatically upward.

It varies substantially by region, however, with the less developed coun-

tries growing rapidly and the more developed countries growing slowly,

if at all. World population is expected to pass 7 billion in late October

and is projected to top 9 billion by 2050; the latest U.N. projections put

it at about 10 billion in 2100. In truth, no one knows exactly how high

population will grow or when it might fl at-line. All population projections

are uncertain, as they are entirely dependent on assumptions about the

future—for instance, how many children a woman will have 20 or 30 years

hence. In that sense, these numbers can be considered best scientifi c

guesses, not destiny. What’s more, the further out one looks, the cloudier

these projections become. Still, they offer a window into what the world

might look like in 2050. –LESLIE ROBERTS

9 Billion?NEWS

0

1

2

3

4

5

6

7

8

9

10

India

China

Africa

Other less developed countries

More developed countries

World Population Growth, 1950–2050 (medium variant)

1950 1970 20101990 2030 2050

SOURCES: UNPD, 2011

9.3 Billion

0

1

2

3

4

5

6

7

8

9

10

Po

pu

lati

on

(b

illio

ns)

Historic and Projected Population Growth

13 years

To add the next 1 billion people

SOURCES: CARL HAUB, POPULATION REFERENCE BUREAU (PRB), 2010; U.N. POPULATION DIVISION (UNPD), 2011

1+ Million years 6000 B.C.E. 4000 B.C.E. 2000 B.C.E. C.E. 1 C.E. 2000

7 Billion 2011

6 Billion 1999

5 Billion 1987

3 Billion 1960

2 Billion 1930

4 Billion 1974

1 Billion 1800

8 Billion 2024

1950 1975 2000 2025 2050 2075 2100

0

5

10

15

20

25

U.N. Population Projections, 1950–2100

SOURCE: UNPD, 2011

High-fertility variant, 2.5 children

Medium-fertility variant, 2.0

Low-fertility variant, 1.6

Constant fertility variant

*Total fertility rate: the average number

of children women would bear in their

lifetime if the birth rate of a particular

year were to remain unchanged.

TFR* 2.5

Constant

2.0

1.6Po

pu

lati

on

(b

illio

ns)

Population is growing —fast ...

Different assumptions, different scenarios. The United Nations has peered out to 2100, but those projections are even more uncertain than those for 2050. The medium variant most commonly used assumes the average woman in 2100 will have two children. If she had half a child more, or less, the picture would change dramatically. And if fertil-ity remained constant at current levels …

Trends in Life Expectancy at Birth, 1950 and 2010

Africa Asia Europe N. America WorldLatin Amer. andthe Caribbean

1950–1955 2005–2010

0

20

40

60

80

47.7

67.9

42.9

69.0

38.2

55.251.3

73.4

SOURCE: UNPD, 2011

68.778.2

65.675.4

Ye

ars

Behind the growth. Much of the population growth of the past 50 years is due to the spectacular gains in life expec-tancy in developing countries, refl ecting advances in public health and medicine.

Going up. The world has never seen anything like the population explosion of the past century. Four billion people have been added to the planet since 1950, and the time it takes to add 1 billion people has dropped to 13 years. World population could reach 9.3 billion by 2050, according to U.N. projections.

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SPECIALSECTION

Fertility and Education, 2007Percentage of girls enrolled in secondary school

Total fertility rate

100

80

60

40

20

0

0 1 2 3 4 5 6 7 8

Total fertility rate

100

80

60

40

20

00 1 2 3 4 5 6 7 8

Fertility and Poverty, 2007Percentage of population living on <$2 per day

SOURCE: PRB, 2007

SOURCE: PRB, 2010

Fewer than 1.5 births per woman

1.5–2.1 births per woman

2.2–2.9 births per woman

3.0–4.9 births per woman

5.0 or more births per woman

Fertility Rates Vary Across the Globe

Total Fertility Rate, 2010

Global Decline in Fertility, 1950–2010

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

0

1

2

3

4

5

6

7

SOURCE: UNPD, 2011

More developed countries

World

Less developed countries(excluding China)

To

tal fe

rtili

ty r

ate

0.0

0.5

1.0

1.5

2.0

2.5

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Population Growth Rate, 1950–2010

SOURCE: UNPD, 2011

Gro

wth

ra

te (

% p

er

yea

r)

More women, fewer kids. The global fertility rate has dropped from 5 to 2.5 in roughly 50 years, and the average woman in developing countries (outside of China) now has three children, down from six.

Stark contrasts. But that downward trend masks sharp regional differences. Since 1970, Asia and Latin America have seen the steepest drops; in sub-Saharan Africa, fertility remains high, in part refl ecting desired family size.

Strong predictors. High fertility rates are associated with poverty and low levels of educational attainment for girls.

Tipping point. The period of most rapid population growth is behind us. Since its peak from 1965–70, the growth rate has declined, falling roughly by half in 40 years as women have had fewer children.

... But more slowly than in the recent past

By the numbersHow Many Children Do Women Say Are

“Ideal”? (selected countries and dates*)

*Niger 2006, Uganda 2006, Jordan 2007, Egypt 2008, U.K. 2006, Austria 2006

9.1Niger

3.2Jordan

5.3Uganda

2.4U.K.

1.6 Austria

3.0Egypt

SOURCES: DEMOGRAPHIC AND HEALTH SURVEYS; EUROBAROMETERS

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29 JULY 2011 VOL 333 SCIENCE www.sciencemag.org 542

3%Population 65+,sub-Saharan Africa

LATIN

AMERICA

2010590,082

Population

2050750,956

Projected

NORTH

AMERICA

2010344,529

Population

2050446,862

Projected

AFRICA

20101,022,234

Population

20502,191,599

Projected

ASIA

20104,164,252

Population

20505,142,220

Projected

EUROPE

2010738,199

Population

2050719,257

Projected

100+

26–99

1–25

<1

Percent PopulationGrowth,

2005–2050

Males

Females

Population (millions) Population (millions)

DevelopedCountries

2010

DevelopingCountries

2010

300 200 100 0 100 200 300 300 200 100 0 100 200 300

Age

85+

80–84

75–79

70–74

65–69

60–64

55–59

50–54

45–49

40–44

35–39

30–34

25–29

20–24

15–19

10–14

5–9

0–4

Different rates. Virtually all population growth from now until 2050 will occur in developing countries—with the largest growth concentrated in the poorest countries of the world.

Projected Population Growth by Country2005–2050SOURCE: PRB, 2011

Young and growing; aging and stable. In developing countries, the large pro-portion of young people ensures rapid population growth. The aging population in more developed countries, with few future parents, spells little or no growth.

Sheer numbers.* Because of sub-Saharan Africa’s high fertility rate, the fastest growth will occur there. But even with its slower growth rate, Asia—largely India and China—will still account for the bulk of the world’s population in 2050.

Where is it growing?

43% Population <15, sub-Saharan Africa

By the numbers

16% Population 65+,Europe

16% Population <15, Europe

SOURCE: PRB, 2010

SOURCE: UNPD, 2011

SOURCE: UNPD, 2011

*Numbers in thousands

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SPECIALSECTION

Percentage Residing in Urban Areas, 1950–2050100

WorldMore developedLess developed

1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050

90

80

70

60

50

40

30

20

10

0

The Demographic Divide

NIGERIA JAPAN

Population 2010 (millions)

Lifetime births per woman

Annual number of births

Percentage of population below age 15

Percentage of population over age 65

Life expectancy at birth

Infant death per 1000 births

Annual number of infant deaths

Population 2050 (millions)

158

5.7

6,700,000

43

3

47

75

500,000

326

127

1.4

1,090,000

13

23

83

2.6

2,830

95

Nigeria2010

15 10 5 0 5 10 15

Population (millions)

0–4

85+

80–84

Age

Japan2010

Population (millions)

0-4

85+

80–84

Age

Males

Females

15 10 5 0 5 10 15

City bound. More and more people will be living in cities and towns, with the fastest rate of urbanization occurring in less developed countries.

A tale of two countries. A look at Nigeria and Japan today suggests what’s ahead. Given Nigeria’s high birthrate and large number of women of childbearing age, the population is expected to more than double by 2050, while the population of Japan is expected to decline.

Higher costs, fewer workers. In 2050, developed countries will not have enough workers to support the higher costs of their aging populations. Developing coun-tries with young populations will not have enough jobs. International migration is set to increase.

The World Ahead

By the numbers

6:1 Population of developing to developed countries, 20502:1 Population of developing to developed

countries, 1950

12 Biggest Cities

1975 2025

26.61

15.88

10.69

9.84

9.61

8.93

8.74

8.56

7.89

7.62

7.56

7.55

Population

(millions)

Tokyo

N.Y.-Newark

Mexico City

Osaka-Kobe

São Paulo

L.A.-Long Beach

Buenos Aires

Paris

Kolkata

Moscow

Rio de Janeiro

London

City

Tokyo

Delhi

Mumbai

São Paulo

Dhaka

Mexico City

N.Y.-Newark

Kolkata

Shanghai

Karachi

Lagos

Kinshasa

37.09

28.57

25.81

21.65

20.94

20.71

20.64

20.11

20.02

18.73

15.81

15.04

City Population

(millions)

0

5

10

15

20

World More developed Less developed Least developed

9

4 4

2

11

4

17

9

Number of Working-Age Adults per Older Adult, 2010 and 2050

2010

2050

37Million

Number of people that developing coun-tries could be adding each year in 2050

0 Number of people that developed coun-tries could be adding each year in 2050

By the numbers

SOURCE: PRB, 2010

SOURCE: UNPD, 2011

SOURCES: (CHART) PRB, 2010; (POPULATION PYRAMIDS) UNPD, 2011

SOURCE: PRB, 2010

SOURCE: UNPD, 2009 SOURCE: UNPD, 2009

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