Visualizing Environmental Science

166 CHAPTER 7 Human Population Change and the Environment

(Figure 7.8). The estimates vary depending on fertility

changes, particularly in less developed countries, because

that is where almost all of the growth will take place.

Population projections must be interpreted with

care because they vary depending on what assumptions

are made. In projecting that the world population will

be 8.1 billion (their low projection) in the year 2050,

U.N. population experts assume that the average num-

ber of children born to each woman in all countries

will have declined to 1.7 by 2045–2050. The average

number of children born to each woman on Earth is

currently 2.5. If the decline to 1.7 doesn’t occur, our

population could be significantly higher. If the aver-

age number of children born to each woman declines

to 2.17 in 2045–2050 instead of 1.5, the 2050 popula-

tion will be 9.3 billion (the U.N. medium projection).

increased life expectancy for a great majority of the

global population (Figure 7.7).

Projecting Future

Population Numbers

The human population has reached a turning point. Al-

though our numbers continue to increase, the world

growth rate (r) has declined slightly over the past sev-

eral years, from a peak of 2.2

percent per year in the mid-1960s

to the current growth rate of 1.2

percent per year. Population ex-

perts at the United Nations and the

World Bank project that the growth

rate will continue to decrease slowly

until zero population growth is attained toward the end of

the 21st century. Exponential growth of the human popu-

lation will end, and the S curve may replace the J curve.

The United Nations periodically publishes popula-

tion projections for the 21st century. The latest (2010)

U.N. figures forecast that the human population will

reach 9.3 billion in the year 2050 (their “medium” pro-

jection), and could range between 8.1 billion (their “low”

projection) and 10.6 billion (their “high” projection) Year

1800 1900 2000

High 10.6

Medium 9.3

Low 8.1

2050

Human population (billions)

1

2

3

4

5

6

7

8

9

10

11

Based on data from

World Population Prospects, The 2004 Revision, United Nations Population Division

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A child in Bangladesh receives a dose of oral polio vaccine.

At one time, polio killed or crippled millions of children each

year. Polio is still endemic (constantly present) in Nigeria, India,

Afghanistan, and Pakistan, and it sometimes spreads from those

countries to other countries.

World population projections

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In 2010 the United Nations made three projections, each based

on different fertility rates.

zero population

growth The state in

which the population

remains the same size

because the birth rate

equals the death rate.

FARJANA KHANGODHULY/Stringer/AFP/Getty Images