Joel Cohen is the professor and mathematician who most famously tried to calculate a definitive number, in his book How Many People Can the Earth Support?
His success was in admitting that he couldn’t determine a sustainable limit for human population because that limit depends on how people live their lives and what they choose to consume. Are they city-dwelling vegetarians who live in tiny apartments and take public transportation, omnivorous homesteaders who grow their own food and make their own clothes and furniture, wealthy estate owners who take private jets to play golf every weekend (ahem, Donald Trump), or somewhere in between? The carrying capacity estimates of 94 scientists range from 500 million to 1 sextillion (that’s 21 zeroes) and the factors listed above illustrate why.
Some scientists who look at population issues believe that we’re already past the “tipping point,” where natural systems are so overstressed that they cannot be recovered, thus effectively changing renewable resources into non-renewable ones. Already, we’re consuming the Earth’s renewable resources at one and a half times the sustainable rate. And that’s with billions of people living in poverty, consuming next to nothing. Imagine what would happen if desperately poor people were fortunate enough to live a middle class lifestyle. And then imagine what would happen if poor people joined the middle class, AND the human population grew from today’s 7.5 billion to 9, 10, or 11 billion.
Another reason nobody knows the upper limit for the human population is that it depends on what that limit protects. Is it human life or is it the non-human animals that are dying off at such an alarming rate? Animal populations are, on average, less than half the size they were in 1970, and we’re currently witnessing the sixth mass extinction, wiping out entire species with our destructive activities. How long before human numbers and activities push so many species to extinction that it begins to negatively affect the natural ecosystems we all depend upon?
Family planning programs and contraceptive services have had enormous success in promoting health, development, and gender equality since they were introduced in the 1960s. They’ve also had a tremendous effect on the trajectory of human population growth. The global fertility rate has declined from 4.92 children per woman in 1965-1970 to 2.47 today, and the rate of population growth has declined from its peak of 2.06% a year to 1.08%. With a dedicated investment in domestic and international family planning, we could bend that growth trajectory downward, leveling it off before it hits 9 billion and before we discover our planet’s true carrying capacity.