There is virtually no possibility that global population will ever reach 15bn

The world’s population will soon pass 7 Billion, and the United Nations Population Fund will mark that milestone this week by releasing its annual State of World Population report. On October 22 the UK Guardian claimed that the report will contain a statistical bombshell. It headlined:

The headline in the Daily Mail, Britain’s largest circulation daily, was even more sensationalist:

The Guardian story tells us:

The Guardian‘s editors repeated the claim in an editorial on October 23. “Without radical action, the UN now predicts the world’s population doubling again before the end of this century.”

Population Matters – the brand-name recently adopted by the arch-populationists of Optimum Population Trust – quickly posted the Guardian October 22 article on its website.

Populationists around the world have jumped on the wagon: less than 48 hours after the Guardian article first appeared, a Google search for “15 Billion by 2100″ found “about 10,900″ results.

But the Guardian article isn’t true. The UN isn’t releasing a new population forecast this week, experts aren’t shocked, and there is virtually no possibility that global population will ever reach 15 Billion.

For starters, the United Nations Population Fund doesn’t compile population statistics or produce population forecasts. Any statistics it publishes come from a separate UN agency, the Population Division of the Department of Economic and Social affairs. The Population Division’s report, World Population Prospects, 2010 Revision, was published six months ago – another isn’t due until 2013.

In this year’s report, the Population Division says that if current population trends continue, the world’s population will be 9.3 Billion in 2050, and 10.1 Billion in 2100. Their projections stop there, but if the trends they describe continue, world population growth will stop early in the 2100s.

So where does 15 Billion in 2100 come from?

The 10.1 Billion figure, called the Mid-Range projection, is based on a careful, country-by-country analysis, combining the latest statistics with the Division’s considered assumptions about long-term trends. The UN has been making these calculations since 1950, and its projections have consistently been off by less than 4%.

But to show that the results aren’t certain, the Population Division also produces two other projections by simply assuming that each adult woman will have 0.5 more or fewer children than the detailed Mid-Range projection. The choice of 0.5 seems to be entirely arbitrary: I’ve been unable to find any explanation of why the UN uses it it instead of a larger or smaller number.

This year, that calculation produced projections for 2100 that range from a low of 6 Billion to a high of more than 15 Billion, as shown in this graph. (click image for a larger version.)

[caption id="attachment_3414" align="alignnone" width="550" caption="The highest line assumes that fertility doesn’t change at all from now on, taking us to 27 Billion in 2100. Not even the most extreme populationists quote that number."][/caption]

It’s important to understand that the 6 Billion to 15 Billion range is not comparable to the “margin of error” figure often reported in statistical studies. No probability whatsover is attached to it – it is just the result of a very crude calculation using an arbitrary adjustment.

In fact, the chance that population will reach 15 Billion this century is very close to zero. For that to happen, global fertility rates would have to be 20 to 25 percent higher than the UN’s best estimates, every single year for the next 90 years. Countries where birth rates have been falling for years would have to experience nine unprecedented decades of baby boom. Global birth rates, which have been declining for half a century, would have to reverse direction immediately, and stay high until the next century.

As noted above, previous UN Mid-Range projections have been accurate within 4%. Reaching 15 Billion in 2100 would be 50% off the mark. That’s extremely unlikely, to say the least.

The Guardian report is sloppy journalism, by reporters and editors who likely aren’t familiar with population projections.

But Optimum Population Trust claims to be a source of population expertise. For them to highlight the Guardian‘s grossly inaccurate article qualifies as either ignorance or deliberate misrepresentation. Either way, their judgement obviously can’t be trusted.

Unfortunately, as Mark Twain said, a lie can travel round the world while the truth is still lacing up its boots. Eventually the truth will win, but I expect we’ll see the “15 Billion by 2100″ lie quite a lot for a while.