6.4: A global transition
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In Figure \(\PageIndex{1}\) we add data from the mid-1960s to the present day. People living in the 1960s were completely unaware of the great demographic transition that was developing. For hundreds of years prior to this time, human populations were stuck on an orthologistic path, with a singularity ever looming and guaranteed by the positive slope. In most of the world, however, the slope abruptly turned about and negative. Not all countries of the world turned about, but on average the world did. Humanity started down a logistic-like path.
Where the downward-sloping line crosses the horizontal axis is where population growth would cease. From this simple r + sN model, it appears that world’s population will stabilize between 10 and 12 billion. That is in line with other recently published projections.
Prior to the 1960s there were dips in the increasing growth, with World Wars I and II leveling the rate of increase worldwide, though population continued to grow rapidly. The rate also fell in 1960, corresponding to extreme social disruptions in China.
What caused this great demographic transition, averaged over the globe? The “Four Horsemen” commonly expected to check human populations were not a primary cause. In many regions birth control, became more available. Education slowed reproduction because people got married later. Modern medicine raised survival rates, making large families unnecessary. The space program looked back at Earth and projected a fragile dot suspended in the black of space, viewed by billions. China’s one-child policy had a noticeable effect. However, so did HIV, one of the few Horsemen that has made a noticeable comeback.
Plants and other animals have logistic growth forced upon them because of overcrowding. In humans, however, logistic growth has been largely voluntary. And there could be further developments in a lifetime. In many nations, birth rates are presently below replacement rates. In fact, in all nations with a gross national income above 16K dollars per person, the birth rate is at or below the replacement rate of 2.1 lifetime births per female (Figure \(\PageIndex{2}\)).
This change in demographic rates could conceivably allow present and future generations to voluntarily adjust the population to whatever is desired. The new question just may be: what is the minimum world population we dare have?
Returning to your supervisor’s questions, you can now tell her that, in 2100, the world’s population will be between 10 and 12 billion. And you can say “The other population projections are not far off. They are slightly different from what we calculate using this method. But they use very complicated methods so you have to cut them a little slack!”
