10 Billion R Us?

How many humans can fit on Earth?

It's not a logistical question (heel-toe, or nose-to-nose?). It's a question of resources. Every other species on the planet lives with a natural limitation. Say you're a snail that lives in a scummy pond and eats algae. There's plenty of algae available, and soon there are snails everywhere. But as your population booms, and you start bumping shoulders with other snails more often, you find there's not enough algae to go around. A lot of snails die or fail to reproduce. The algae population rebounds while the snail population fades, and the cycle continues.

Humans have beaten this system--for now. We clear forests, irrigate deserts, and blow the tops off of mountains to get the resources we need. This means we've stretched our upper limit. And according to a new report from the United Nations, there's no end in sight.

Previously, the UN had predicted that the world's population would reach 9 billion during this century, then level off. The projected upper limit wasn't because we were due to run out of food or space, though. Demographer John Bongaarts says in an interview with ScienceInsider that the UN's projection, which they revisit every two years, comes from a combination of mortality rate and fertility rate: how quickly are people dying, and how many babies are they having?

As nations invest money in family planning and women gain access to birth control, the fertility rate (how many children the average woman has) tends to decline. Bongaarts says that in Africa, where most of the world's "high-fertility" countries are, there has been a lack of investment in family planning programs recently. The Bush administration, for example, cut funding for contraceptives to African nations. Additionally, Bongaarts says AIDS hasn't affected population growth in Africa quite as researchers expected it to.

The result is that the UN now expects the world's population, rather than peaking around 9 billion, to continue on to 10 billion by the end of this century. And there's no peak--the population will still be increasing in 2100.

To come up with their projections, the UN says, "Account is taken of past fertility trends in a given country plus the past experience of all other countries in the world. The model was used to generate 100,000 trajectories for future fertility for each country." Out of those 100,000 projections for each country, they used the median values to make an overall projection.

The UN's model assumes that over time, the low-fertility and medium-fertility countries will level out to their "replacement rate"--that is, each generation will have enough children to exactly replace itself. For a country where most people make it to adulthood, that means 2.1 kids per mom. Low-fertility countries include most of Europe, as well as Iran, Brazil, and (by design) China. Countries with intermediate fertility include India, Mexico, Egypt, and the United States. High-fertility countries include 39 African nations.

A small wobble away from the predicted fertility rates could have a huge impact on the global population. The UN report points out that if they've overestimated global fertility by half a child per woman, the population will peak at 8 billion and swing downward again to 6 billion by the end of the century. But if they've underestimated by a half a child, we could hit almost 16 billion by the year 2100.

Mortality is important, too. It's assumed that life expectancy will continue to increase globally. The highest-fertility countries also happen to be some of those with the lowest life expectancy, thanks to killers such as HIV and malaria. Currently, life expectancy among all high-fertility countries is just 56 years. (Low-fertility countries, where people presumably have the best access to health care, have the highest life span: 74 years, compared to 68 years for those of us in the middle group.)

All these projections, too, hinge on there not being a global cataclysm in this century that removes a large portion of our population, as the Black Death or the 1918 flu pandemic did. Just because it's the twenty-first century doesn't mean this risk is gone. Viruses are mutating all the time, bacteria are developing resistances to most of our antibiotics, climate change is altering the life cycles and habitats of animals that carry diseases--and, of course, people are getting closer together.

What's the lowest global population you remember? The world reached 3 billion people in 1959, and 4 billion in 1974. In 1987 we reached 5 billion. I remember hearing from Bill Nye (the Science Guy) that the population was almost at 6 billion; it reached that number in 1998. We're currently expected to hit 7 billion this fall, 8 billion in 2025, and 9 billion in the 2040s. Get ready to bump some shoulders.