<H1>The Solution to Many Problems: One Billion Persons on Earth.</H1>
<H2> How many humans are too many?</H2>
The question of optimum human population size on Earth comes up fairly regularly. There are two parts to this question: what is best for humans and what is best for the natural world. I'm convinced that the answer to both questions is one billion persons on Earth.
For comparison population expert Paul Ehrlich says 1.5 to 2 billion. Other experts come up with larger numbers: 2 to 4 billion. These larger numbers are generally derived from estimates of "carrying capacity" of humans. In other words how many humans can we cram on Earth without using up all our resources in the foreseeable future. But in my mind we should have no more humans on Earth then can coexist with an undiminished natural world. A world in which extinctions of animals and plants and other organisms are close to zero.
To determine that criteria we should look at the history of extinctions. When did they become a serious problem? My answer is 1800. When you look at the attached chart of extinctions there is a noticeable uptick in the rate around 1800. (See graph.)
You could argue that the rate of extinctions before 1800 was already above the background rate. This is true, but I can respond that by using advanced technology and renewable energy (and avoiding some of the stupidities of the past), a reduced population of humans would be capable of coexisting with the natural world even better than we did before 1800. Consider the huge Steller’s sea cow, needlessly hunted into extinction by 1768, for example.
So I suggest that in the year 1800 humans were exhibiting an approximate balance with the natural world. The population on Earth in 1800 was 1 billion.
Overpopulation of humans doesn’t just cause trouble for animals. Overpopulation causes air and water pollution which threatens human lives. Every human has a carbon footprint, but the richest persons have a much larger effect . In total humans are inexorably raising the concentration of CO2 in the atmosphere which leads to global warming. Population pressures lead to deforestation which turns rainfall into devastating floods. Destruction of mature forests makes forest fires burn hotter and longer. Encroaching on natural areas causes pathogens and viruses to jump from animals to humans causing pandemic disease. Too many mouths to feed encourages factory farming and soil depletion.
We have too many humans on the earth, but it is possible to have too few? Another constraint on optimum population size is maintaining a highly advanced technological and scientific society. I was born in late 1942. The Manhattan Project was in full swing. Science in general was advancing rapidly. The population of the U.S. in 1942 was 135 million. Reducing world population proportionately from 8 billion to 1 billion would imply a U.S. population 1/8 of its current size of 330 million or 40 million. Compare to Israel which has a robust scientific establishment with a national population of less than ten million.
Given the enormous communication capacity that we have now, I have no doubt that a U.S. population of approximately 40 million educated persons (along with all the other scientifically-advanced nations) could continue advancement of science and technology at a rapid rate.
One might reasonably demand to know: In what way are humans good for nature? One simple answer: for protection from planet-killer asteroids. The next time a Chicxulub-sized asteroid (or larger) comes our way, humans should be prepared to deflect it.
Ergo: the ideal human population size on Earth is one billion persons.
Furthermore a world fertility rate of 1.4 (slightly less than one and a half children per woman on average) would result in 1 billion persons on Earth around the year 2300. Taiwan, South Korea, and ten other countries already have fertility rates of 1.4 or lower.
At some point in the future human populations in space habitats could expand indefinitely using the (biologically dead) resources of the solar system and the enormous energy of our sun to replicate the Earth ecosystem. Ultimately humans and their habitats could travel to dead star systems, potentially filling our Galaxy with life.