Doubling Time:
Many people hear that the U.S. population is growing by only 1.1% and think that this is of no concern. To approach the U.S. population growth problem so sanguinely is dangerous and foolhearty. We must consider the mechanics of exponential growth. Even the smallest fraction of steady growth leads eventually to doubling and redoubling; perpetual population growth due to the phenomenon of exponential growth "starts slow and finishes fast." Even a country whose population is increasing by "only" 1% per year will double in size in 72 years! A small number of doublings produces a large number very quickly.
To find the doubling time of a population at any given annual rate of growth, divide 72 by the annual percentage growth rate (in this case 1.1%). Current trends are as follows:
D = 72 and P=1.1
then D/P = about 65 years
(D = doubling time) and (P = annual percentage growth rate).
Because of our high rate of growth, the U.S. is one of the fastest growing countries in the industrialized word  we have grown from 150 million in 1950 to 275 million in 2000. It took all of human history for world population to reach 2.5 billion in 1950. Then, doubling just once during the next forty years, world population increased by 2.5 billion (a number equivalent to all preceding growth). U.S. population added 100 million people between 1950 and 1990, and if current rates of growth continue, will double in size to 550 million in the next 65 years. Examination of these numbers provides some understanding of the explosive nature of exponential growth; unfettered population growth is something the U.S. can illafford.
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