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Monday, December 10, 2012

Population Primer

Our friends at recently posted reposted an article about the growth of the world's population over the past twenty centuries and to the end of the current one. (The article also has a lot of links to other useful articles about population.) I quickly copied the numbers into Excel to make the following simple graph.
Viewed from Year 1 (A.D. or C.E. -- Current Era) until 2083, population takes the form of the letter J, or perhaps a hockey stick. That is, it grew very gradually for 1800 years (and for about 8,000 years before that), and really grew significantly only in the past century or two. The population has more than doubled, in fact, since the members of the EarthView team were born. This is not true of the Wranglers who are still in college, though the planet now has almost 2 billion people on it than when they were born!

A J-curve represents exponential growth, which shows no sign of stopping. A more careful look at these numbers, however, reveals that the growth is actually not shaped like a J. By graphing the data only since 1804, a subtle but important shift in the pattern can be more easily seen.
These are the same numbers, except that the earlier, long period of very slow growth has been removed. Rather than a J, the growth resembles an S, which is why this pattern is called sigmoidal, after sigma, which is the Greek letter equivalent of S.

This kind of growth occurs with many living things, including humans. Rapid growth often occurs when resources are abundant, and then slows as resources become scarcer. What looks like unstoppable growth slows down when limitations are reached.

In the case of humans, agriculture allows for significant growth, but industrialization allows for growth that is much more rapid, at least initially. Many regions have experienced what geographers call a demographic transition, as the rapid growth eventually slows. As explained on the BBC "Bitesize" article on population change and structure, these changes do not result from rising birth rates, but rather from falling death rates.

Most of the world's industrial countries -- such as the United Kingdom, the United States, Italy, and Japan --
have been through some version of the demographic transition and now have relatively slow growth, or even negative growth. Many developing countries -- such as Mexico and Brazil -- are near the end of such a transition. Most importantly, even in countries where population growth rates remain high -- such as Kenya and Jordan -- the transition is beginning, and growth is beginning to slow. (See the Census Bureau International Database detailed estimates of population change in these and all other countries.)

Mexico's current population pyramid shows that the country's population has
 been growing in recent generations, but that growth in the future
will be slower. This kind of graph is more properly called an
age-structure diagram because not all countries resemble pyramids.
The Census IDB generates diagrams for individual countries by year.
National Geographic provides a
 lesson on making andinterpreting the diagrams.
The result is that although human population is expected to grow by an additional 3 billion people before this century is ended, it is not expected to grow much beyond that.

For geographers and other scientists who are concerned about the demands a growing population places on natural resources, it is encouraging to see that the growth of population is slowing down. Biologist E.O. Wilson has described this curve as a "bottleneck" because it shows a period of rapid growth that is lasting for only a couple of centuries. For a long time before and after this period, human population growth was and will be very slow. Most of the plants, animals, and resources that survive this rapid change are likely to survive long-term, but survival during this period requires a lot of care and protection.

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