During the 20th century, the world’s population grew by the factor 3.5. What permitted this growth was the agricultural advances of the 20th century; without those advances the population would not have grown as it did, from 1.7 billion in 1900 to 6 billion in 2000. Now in the 21st century, the ability to secure adequate food and water worldwide is threatened by continued rapid population growth, which has been steady at ~80 million per year for each of the last 50 years. For some countries the growth has instead been nearly exponential, as in the Philippines, whose population grew from 7 million in 1900 to 100 million in 2014, by doubling four times.
Noting the regional diversity in fertility rates, and the mathematical result that the global average fertility rate can rise even if each country’s fertility rate is falling, and consideration of pandemics and war, lead to the likelihood that world population will instead be limited only by food supply. This conclusion supports an explanation for the failure of the Search for Extraterrestrial Intelligence (SETI), namely that technological civilizations have short lifetimes.
Reference: S.G. Warren, 2015: Can human populations be stabilized? Earth’s Future 3, 82-94, doi:10.1002/2014EF000275.
What this paper does:
(1) presents equations relating family size to population growth rate
(2) explains that the UN projections are based on a logical fallacy
(3) offers mathematical exercises to illustrate population dynamics, suitable as homework problems in an undergraduate class