It is true that between the years 1967 and 1997 the annual global population growth rage declined from 2.07% to 1.33%. That’s nearly a 40% reduction in only 30 years! Good for us! However, note that nearly the SAME NUMBER of people were added last year as were added in 1968, a year when the growth rate was nearly 40% faster than it is now…huh?

With a population of 4 billion (it was a little less than that in 1967), a 2.07% growth rate gives you a net addition of about 82 million people per year. At this rate, it would only take 24 years to add another 2 billion people to the human population. Race forward 24 years (or 30 years if we want to compare 1967 and 1997) and note that sure enough, you are have a human population of 6 billion (in 1999), but it is ONLY growing at a rate of 1.33% per annum (there’s that 40% reduction in growth rate)! Don’t get too excited, however. Do the math and you will note that we are still adding about 80 million people per year, suggesting it will only take another 24 years to add another 2 billion people to the population. This is an example of what we call lag time–like a fully-loaded ocean liner, these very large population sizes have momentum of their own that resist change in speed or direction, even when you have taken your foot off the accelerator (which we have not done yet).

How long does it take to slow this ship? Even at our current “decreasing” growth rates (that is, even if we continue to slow the growth rate to less that 1.33% per year), our population will increase by 2 billion people in under 25 years. With a population of 8 billion we would have to drop our growth rate to 1% per year just to maintain the current rate of number of individuals being added each year (about 80 million) in above scenario. In other words, with 8 billion people (in say, the year 2024) we would need to lower our net growth rate to 1% per year to add ONLY an additional 80 million people per year to our population. That’s only a 33% reduction of growth rate from 25 years earlier, so that should be easy, yes?

Let’s say we are can continue to do the amazing and reduce our populations growth rate by 40% each 25 years, when do we reach a population size where we are adding a minimal number of individuals each year, or, even reach zero population growth (ZPG, where there is no net increase in numbers of humans per year)? In other words, when does the ship come to a stop and just float? The current slowing rate (40% per 25 years) puts us at about 12-13 billion people in 150 years — at that point we will only be adding about 8 million added per year (an order of magnitude fewer than this year, or next year). Note that this is NOT ZPG, however, but it is a lot closer than we are now!

There is no reason to believe that we cannot decrease the population growth rate even faster than 40% per 25 years or 1.6% per year (that would be slowing the acceleration faster than we are slowing the acceleration now) and if so, we can hope that the population growth rate will slow to zero (no net growth) within the next 100 years. Even so, the global human population will be 10-12 billion people, twice what it is today.

NOTE: This question was answered by Dr. Alan Thornhill.

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ID: 440
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updated: 02 Jun 2001