THE GROWING WORLD POPULATION
he world is in the midst of an unprecedented expansion of human numbers. It took hundreds of thousands of years for our species to reach a population level of 10 million, only 10,000 years ago. This number grew to 100 million people about 2,000 years ago and to 2.5 billion by 1950. Within less than the span of a single lifetime, it has more than doubled to 5.5 billion in 1993.
This accelerated population growth resulted from rapidly lowered death rates (particularly infant and child mortality rates), combined with sustained high birth rates. Success in reducing death rates is attributable to several factors: increases in food production and distribution, improvements in public health (water and sanitation) and in medical technology (vaccines and antibiotics), along with gains in education and standards of living within many developing nations.
Over the last 30 years, many regions of the world have also dramatically reduced birth rates. Some have already achieved family sizes small enough, if maintained, to result eventually in a halt to population growth. These successes have led to a slowing of the world's rate of population increase. The shift from high to low death and birth rates has been called the “demographic transition.”
The rate at which the demographic transition progresses worldwide will determine the ultimate level of the human population. The lag between downward shifts of death and birth rates may be many decades or even several generations, and during these periods population growth will continue inexorably. We face the prospect of a further doubling of the population within the next half century. Most of this growth will take place in developing countries.
Consider three hypothetical scenarios* for the levels of human population in the century ahead:
Fertility declines within sixty years from the current rate of 3.3 to a global replacement average of 2.1 children per woman. The current population momentum would lead to at least 11 billion people before leveling off at the end of the 21st century.
Fertility reduces to an average of 1.7 children per woman early in the next century. Human population growth would peak at 7.8 billion persons in the middle of the 21st century and decline slowly thereafter.
Fertility declines to no lower than 2.5 children per woman. Global population would grow to 19 billion by the year 2100, and to 28 billion by 2150.
The actual outcome will have enormous implications for the human condition and for the natural environment on which all life depends.
KEY DETERMINANTS OF POPULATION GROWTH
igh fertility rates have historically been strongly correlated with poverty, high childhood mortality rates, low status and educational levels of women, deficiencies in reproductive health services, and inadequate availability and acceptance of contraceptives. Falling fertility rates and the demographic transition are generally associated with improved standards of living, such as increased per capita incomes, increased life expectancy, lowered infant mortality, increased adult literacy, and higher rates of female education and employment.
Even with improved economic conditions, nations, regions, and societies will experience different demographic patterns due to varying
Population Reference Bureau, The U.N. Long-Range Population Projections: What They Tell Us, Washington, D. C., 1992.