Landmark #1: Population

Population 101

It's impossible to predict which of the three scenarios on the previous page will come to pass, but it is clear that overpopulation is one of our most serious problems, and a problem which will get worse before it gets better. We do understand a few important aspects of population growth, which should get more media attention than they do.

Population Growth Rate

The human population was roughly 5.7 billion in 1995. About 149 million children were born that year, and about 56 million people died, which means the population grew by 93 million. This represents a growth rate of 1.6 percent (93 million / 5.7 billion). The growth rate of the human population has been roughly the same for many decades.

Exponential Population Growth

The human population grows in proportion to its size -- the population in any given year is equal to one plus the growth rate, times the previous year's population. Anything growing in proportion to its size is growing exponentially.

Doubling Time

Any population growing exponentially has a fixed doubling time. If the human population grew by 1.6% every year, then in about 40 years the population would double in size. And indeed, the world population did double in size in the 37 years between 1950 and 1987. Unless we slow the growth rate the population will double again in 40 years and when I'm in my seventies the world will have 11 billion people.

Zero Population Growth (ZPG)

To maintain the population at a given level we must have a population growth rate of 0%, commonly called Zero Population Growth (ZPG).

Replacement Reproduction

In order to stop population growth we need to achieve replacement reproduction, which occurs when the average couple has just two children who survive to have children of their own. Unfortunately, achieving replacement reproduction doesn't immediately yield Zero Population Growth, because of population momentum.

Population Momentum

This next bit isn't so obvious. If you can achieve replacement reproduction, why doesn't the population stop growing? In the long run the population will stop growing, but not immediately. Why not? Because it's mostly young people who have babies, and mostly old people who die. So if the population has a high proportion of young people -- as it has now -- then even after replacement reproduction is achieved, many babies will be born for every old person who dies.

Population Distribution Projected age distribution of the world's population in the year 2000

In 1989, 40% of the population of the average less-developed nation was under fifteen years of age, and estimates indicate 40% of all 14-year-old girls alive today will be pregnant at least once by the age of 20. Over a billion young people in the world have yet to enter their prime reproductive years. Even if we were to achieve replacement reproduction soon -- it would take another 50 or 60 years to achieve Zero Population Growth.

The Bottom Line
If we want to stabilize the world population at 8 billion in 50 years we need to start working toward that goal immediately.

You can copy freely from this site. This work has been dedicated to the Public Domain by the author, Brian Douglas Skinner.