There may be over 10 billion of us by 2100

Climate change may be the existential threat, but underlying this is, of course, population size. And this is a problem that never seems to go away. There are of course two ways, broadly speaking, to limit population growth aside from draconian policies governing reproduction (such as China's One Child policy). One is sometimes called the demographic transition. This is when a combination of factors including so-called modernization which may involve increase quality of health care in combination with increased social equality lead to lower birth rates. The other is when things go badly wrong and interruptions in the food supply, warfare genocide, and epidemic disease simply cull out large parts of the population.

The following chart shows the effect of two major famines on the overall population increase in Ethiopia since 1950. If you squint and look at it kinda sideways you can see the slowing of population increase during this period. But really, it made little difference.

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And here is the population of Rwanda since 1950, with the 1994 Genocide marked in. Here the event is much more clearly indicated but again, over the long term, there is not that much of an effect, just a delay in reaching some high future number.

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Until recently, experts on population had estimated that world population would reach about 9 billion by around 2050, and then level off. But a new study by the UN indicates that population is likely to increase to more than 10 billion or so by the end of the century. The study came out in this week's issue of Science (Gerland, P., Raftery, A. E., Šev, H., Li, N., Gu, D., Spoorenberg, T., … Heilig, G. K. (2014). Reports World population stabilization unlikely this century, (September), 1–5. doi:10.1038/42935, with additional information here). Here is what the projection looks like for the world, but you can go to the link and see the projections for each country, various regions, and projections for other demographic values.

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From the Abstract:

The United Nations recently released population projections based on data until 2012 and a Bayesian probabilistic methodology. Analysis of these data reveals that, contrary to previous literature, world population is unlikely to stop growing this century. There is an 80% probability that world population, now 7.2 billion, will increase to between 9.6 and 12.3 billion in 2100. This uncertainty is much smaller than the range from the traditional UN high and low variants. Much of the increase is expected to happen in Africa, in part due to higher fertility and a recent slowdown in the pace of fertility decline. Also, the ratio of working age people to older people is likely to decline substantially in all countries, even those that currently have young populations.

The new study is done differently than most earlier studies. According to Patrick Gerland, a UN demographer, “Earlier projections were strictly based on scenarios, so there was no uncertainty. This work provides a more statistically driven assessment that allows us to quantify the predictions, and offer a confidence interval that could be useful in planning.” Also earlier studies made unrealistic assumptions about fertility, allowing the entire world to have a higher or lower fertility to develop a range of outcomes. Author Adrian Raftery notes, "“In a given year and country the fertility rate might be half a child higher, but the probability that it would be half a child higher in all countries in all years in the future is very low."

We need to be working towards a more rapid demographic transition, which in large part involved education of girls and access to good health care so babies survive better, and good reproductive services so women can no be so easily coerced into being baby factories.

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Back in the 70's, Sokol, I think it was, did the math, and came to the conclusion that the most effective way to slow population growth was delay of first birth, lengthening generation time. This fits in with your comments about empowering women, and having babies survive better is a good thing also. If they live, you don't have to have as many.

By Jim Thomerson (not verified) on 20 Sep 2014 #permalink

Maybe i am just not getting the joke, but, uhm, 10 billion by 2011??

Sokol's analysis was later overturned; the most effective way to reduce fertility is to increase inter-birth intervals.

From the Rwanda trend, it seems the population grows quickly to the limit of food production, (it blipped up after the genocide), and we've been getting better everywhere at producing food by about 1% a year. The populations of first world countries aren't going down either, they're growing by 1% p.a. too, through immigration.

What happens if global warming reduces our food production limit by 1% in a year? That's about 80 million fewer people, all in one year. Our population shouldn't some down because it's being forced down from above by food limits. We should draw it down from below, until it's much lower than the food producing limit.

The political problem is someone's always going to be unhappy they can't have a baby, even though they could clearly feed one, and someone else is always going to be unhappy they can't have a new worker, even though they could clearly pay one. From there comes the impulse to denialism: "we're sure there's nothing to be worried about, you're just being meeean and you hate babies!"