The latest UN population projection is out, and people are preparing to flip that leading digit as we hit 7,000,000,000 people on October 31, 2011.
From the press release [PDF]:

The current world population of close to 7 billion is projected to reach 10.1 billion in the next ninety years, reaching 9.3 billion by the middle of this century, according to the medium variant of the 2010 Revision of World Population Prospects, the official United Nations population projections prepared by the Population Division of the Department of Economic and Social Affairs, which is being launched today. Much of this increase is projected to come from the high-fertility countries, which comprise 39 countries in Africa, nine in Asia, six in Oceania and four in Latin America.

(See also coverage from The NY Times and Grist. You can get the data from the projections in Excel files here.)

A few observations:

If this projection is accurate, then Person Seven Billion has already been conceived, and his or her mother is roughly three months pregnant as I type this.

If PSB is part of a multiple birth, then birth order will decide who is number seven billion and who is just another statistic. This seems trivial, until things like book and movie deals come into play.[1]

Then we have the projection that we could get all the way to 10.1 billion people by the end of this century. What ever happened to the notion that we would peak around 2050 at “only” 9.0 to 9.5 billion people? The Grist article, linked below, explains:

The surprising assumptions underlying some countries’ fertility rates reflect one of the key features of the population projections. In the past, the projections were constructed using a technique that, in the U.N. medium fertility projection, assumed all countries would move toward a universal fertility rate of 1.85 children per woman. High and low fertility projections only varied from the medium by 0.5 children per woman in either direction.

This universal rate was highly improbable among countries at the demographic extremes, and the U.N. has now recognized that its past projections did not allow for unpredictable demographic transitions within individual countries. Uganda is a dramatic example of this. The previous revision of the U.N.’s World Population Prospects showed that the country’s fertility rate had fallen by about half a child per woman — less than 8 percent-between 1950 and 2010. Yet that report projected that the country’s fertility rate would decline by 60 percent, to less than three children per woman, by 2050.

Starting with today’s new set of projections, the U.N. has shifted to a “probabilistic” model for its medium-fertility scenario. This allows the pace of each country’s fertility decline to be calculated individually, based on new estimates of historic fertility rates, allowing for much more variance across countries. The new method also assumes that fertility rates will eventually balance out around 2.1 children per woman, a level where couples would “replace” themselves in the population, rather than 1.85. And the projections now extend out to 2100, and incorporate life expectancies ranging as high as 90+ years.

I’ve seen other comments floating around for some time that the old estimates were too low because of incorrect assumptions. It seems the UN has reached the same conclusion and updated their methodology.

Everyone who’s read almost anything about world population has seen the set of dates showing when we reached each multiple of a billion people. From the FAQ for the UN report:

3 Billion: 20 October 1959
4 Billion: 27 June 1974
5 Billion: 21 January 1987
6 Billion: 5 December 1998
7 Billion: 31 October 2011
8 Billion: 15 June 2025
9 Billion: 18 February 2043
10 Billion: 18 June 2083

If population gets that high by mid century and then keeps growing, we’re in a world of hurt. But wait a minute — what are the chances that world population will actually get to 8 or 9 or 10 billion? That’s where things go from a fascinating academic exercise that (hopefully) will help inform policymakers to a very grim discussion that will (hopefully) get the undivided attention of policymakers. As best I can tell, these UN projections are based solely on current demographics and assumptions about future fertility rates in each country. Even that is a gargantuan task, and if you disagree, then tell me how many children women in every country on the planet will be having, on average, for each year through 2100. Daunting, isn’t it?

The problem with this study, as with so many others I talk about on this site, is that it isn’t broad enough in scope. You can’t look at just peak oil, for example, without also considering climate change (and vice versa). You can’t look at energy issues without considering water as a complicating factor (and vice versa) — that’s the whole energy/water nexus thing I yammer about endlessly. And you can’t talk about population near or past the carrying limit of the planet without talking about fresh water supply and food, and therefore climate change and energy and the nearly impossible to predict intervening issues of economics, public policy, and psychology. If you draw out the causality chain of issues as a series of labeled circles with arrows indicating which circles influence which other circles, you quickly realize that everything in this discussion is a function of everything else, with future population being the circle most likely to change in response to one of the other circles.

So, you’re wondering, what do I think world population will do in the next 89 years? Even without invoking some nightmare scenario like a major nuclear war, I think it’s very hard to see how we get to eight billion people, let alone nine or ten.[2] The combination of peak oil, various fresh water supply issues, and huge, not wholly predictable additional costs imposed by climate adaptation (largely due to rising sea levels), large portions of the world population eagerly moving to a more American lifestyle, and insane policies (like the US turning 35% of its corn crop into motor vehicle fuel), will create a food supply bottleneck. This probably won’t happen right away — we’ll find ways to muddle through for probably at least another ten to 20 years, at least until major factors like the depletion of some “fossil water” aquifers in the US, India, and China start to give out. But then things will get exceedingly interesting unless we’ve had the foresight to take significant action well before it happens — as in right now.

[1] Yes, I’m kidding. The utter absurdity of seven billion people being alive on the planet at one time and just a few months from now and not in some vision of a far-off future arising from the mists of computer modeling, is mind bending and more than a little terrifying. So I joke as a coping mechanism.

[2] Please don’t leap to the conclusion that I’m agreeing with the more apocalyptic projections we hear of a worldwide population crash to one or two billion people in the next 50 to 75 years. And please don’t conclude from what I just typed that I’ve ruled it out. I think it’s possible, but it still seems very unlikely. As always, one of the recommended books on world population is Joel E. Cohen’s How Many People Can the Earth Support?

Photo by lusi .