Sometime towards the end of 2011, the 7 billionth living person will be born. The number of people on earth took very roughly 250,000 years to reach a billion in 1800. More than a century pass before it reached 2 billion.

Click here to subscribe to The Economist on YouTube: http://econ.trib.al/rWl91R7

The billion after that took less than a third of the time, and the one after that took only half as long. And the following stages to five billion and six billion took 13 and 12 years.

And the population seems to be rising as fast as ever. The most recent billion again took only a dozen years. Yet the era of acceleration is over and this is just as important to understand as the continued rise in the population. The population has gone on rising because of what happened in the 50s and 60s; this was the period of the baby boom in rich countries and very high birth rates in poor ones.

But momentum is shifting. The additional numbers added each year to the total are now dropping. This is because the total fertility rate, that's the number of children a woman can expect to have during her lifetime, has been falling for a long time. That means families are getting smaller and the underlying impetus towards population growth has dropped. In some countries the speed of change has been breathtaking. In Iran fertility fell from seven to below two.

Sometime in the next few years the world will reach another milestone half of mankind will be living in countries or regions where fertility is at or below 2.1. That is the replacement rate for which your country is having only enough children to keep the population stable

It means population growth will slow down and give the world a bit more time to deal with its demographic and environmental problems. The current increase in population rise is the last that will happen in such a short space of time. The next billion will take slightly longer, about 14 years, and the billion after that which raises the population to 9 billion will take 20 to 25 years.

By 2050 the momentum of population growth may have slowed almost a zero. It will be a more crowded but more stable world

Daily Watch: mind-stretching short films throughout the working week.

For more from Economist Films visit: http://films.economist.com/
Check out The Economist’s full video catalogue: http://econ.st/20IehQk
Like The Economist on Facebook: https://www.facebook.com/TheEconomist/
Follow The Economist on Twitter: https://twitter.com/theeconomist
Follow us on Instagram: https://www.instagram.com/theeconomist/
Follow us on LINE: http://econ.st/1WXkOo6
Follow us on Medium: https://medium.com/@the_economist