Environmental Engineering Reference
In-Depth Information
For example, in 2005 the world's population grew by
1.2%. If that rate continues, the earth's population will
double in about 58 years (70/1.2).
The population of Nigeria is increasing by 2.4% per
year. How long will it take for its population to double?
12
11
High
Medium
Low
High
10.6
10
9
Medium
8.9
8
Declining Fertility Rates: Fewer Babies
per Woman
The average number of children that a woman
bears has dropped sharply since 1950, but is not low
enough to stabilize the world's population in the near
future.
Fertility
is the number of births that occur to an indi-
vidual woman or in a population. Two types of fertility
rates affect a country's population size and growth rate.
The first type,
replacement-level fertility,
is the
number of children a couple must bear to replace
themselves. It is slightly higher than two children per
couple (2.1 in developed countries and as high as 2.5 in
some developing countries), mostly because some fe-
male children die before reaching their reproductive
years.
Does reaching replacement-level fertility bring an
immediate halt in population growth? No, because so
many future parents are alive. If each of today's cou-
ples had an average of 2.1 children and their children
also had 2.1 children, the world's population would
continue to grow for 50 years or more (assuming death
rates do not rise).
The second type of fertility rate, the
total fertility
rate (TFR),
is the average number of children a woman
typically has during her reproductive years. In 2005,
the average global TFR was 2.7 children per woman:
1.6 in developed countries (down from 2.5 in 1950) and
3.0 in developing countries (down from 6.5 in 1950).
Although the decline in TFR in developing countries is
impressive, this level of fertility remains far above the
replacement level of 2.1.
How many of us are likely to be here in 2050? An-
swer: 7.2-10.6 billion people, depending on the world's
projected average TFR (Figure 7-3). The medium pro-
jection is 8.9 billion people. About 97% of this growth is
projected to take place in developing countries, where
acute poverty (living on less than $1 per day) is a way
of life for about 1.4 billion people.
7
Low
7.2
6
5
4
3
2
1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050
Year
Figure 7-3
Global outlook:
United Nations world population
projections to 2050, assuming the world's total fertility rate is 2.6
(high), 2.1 (medium), or 1.5 (low) children per woman. The most
likely projection is the medium one—8.9 billion people by 2050.
(Data from United Nations,
World Population Prospects: The
2000 Revision,
2001)
dren per woman. Since then, it has generally declined,
remaining at or below replacement level since 1972.
The drop in the TFR has led to a decline in the rate
of population growth in the United States. But the coun-
try's population is still growing faster than that of any
other developed country and is not close to leveling off.
Nearly 2.9 million people were added to the U.S.
population in 2005. About 59% of this growth occurred
because births outnumbered deaths; the rest came from
legal and illegal immigration.
According to U.S. Census Bureau, the U.S. popu-
lation is likely to increase from 296 million in 2005 to
457 million by 2050 and then to 571 million by 2100. In
contrast, population growth has slowed in other major
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
2.1
Baby boom
(1946-64)
Replacement
level
Case Study: Fertility Rates
in the United States
Population growth in the United States has slowed
but is not close to leveling off.
The population of the United States grew from 76 mil-
lion in 1900 to 296 million in 2005, despite oscillations in
the country's TFR (Figure 7-4). In 1957, the peak of the
baby boom after World War II, the TFR reached 3.7 chil-
1920 1930 1940 1950 1960 1970 1980 1990
Year
2000 2010
Figure 7-4
Total fertility rates for the United States, 1917-2005.
(Data from Population Reference Bureau and U.S. Census
Bureau)
