Mark your calendars for Oct. 31 and get the birthday candles ready. On that date — give or take a few days — the world’s 7 billionth person will be born. Or at least that’s what the U.N. Population Fund has been publicly predicting, and news outlets have been excitedly trumpeting the news. But how airtight is this sort of forecast likely to be?

(Manish Swarup/AP)

“Take China, the world’s largest country. Raw census data suggest that the average woman has 1.2 children, but this hides a multitude of problems,” reports Fred Pearce in the latest issue of New Scientist. “State demographers believe people are hiding tens of millions of babies to evade the one-child policy, and so estimate that the rate is 1.8. But Zhongwei Zhao of the Australian National University in Canberra says other figures in the 2010 census suggest the raw data may be nearer the truth. The UN currently plumps for 1.5 children per woman.” A million kids here, a million kids there, and soon you’re talking about real errors.

In any case, these arcane disputes are fascinating, but do they matter much? Actually, yes. Back in May, the U.N. announced that it was revising its long-term forecast. Instead of expecting the population to peak at 9 billion or so, the U.N. now expects the global population to pass 10.1 billion by 2100. Birth rates in places like Africa haven’t been falling as expected. Another surprise: wealthier countries like the United States and Britain saw birth rates boom during the housing bubble (although they’ve fallen again recently). But it makes a big difference if we should expect 7 billion or 10 billion or 15 billion people.

For instance, a team of scientists in a recent issue of Nature looked at whether it was even possible to feed a fast-growing population without destroying the planet. The conclusion was yes, but barely. We’d need to stop farming in places like the rain forests, boost crop yields in places like Africa and Eastern Europe, stop wasting so much food (right now about one-third of all food grown gets spoiled or tossed) and—this is the one everyone always hates—eat less meat. It’s a daunting task. And it becomes all the harder if the world’s population grows even faster than we thought.