There. Another five babies were born in the last second, and two people died. That means the world's population grew by three. The same thing happens each and every second - so this year, the human race will increase by 78 million people.

The numbers are staggering. The United Nations says that this year, on 12 October 1999, the planet's population will reach 6 billion for the first time in history. Imagine a Wembley Stadium totally full of people. Now, for each person there, imagine another whole Wembley Stadium full of people. That's 6 billion.

Estimates of exactly how many people the earth can support range wildly from four billion (meaning we're already in crisis) to 16 billion or more. But where the ultimate limit lies is a theoretical question. The immediate issue is that population growth is already causing enormous problems. It is gobbling up natural resources, putting enormous pressure on public services such as health and education, and making it almost impossible for hundreds of millions of people to break free from poverty.

Economic growth averaging between 1% and 2% a year over a long period is impressive to economists - but if population is increasing by 1.4% a year, as the United Nations estimates, then we need impressive economic growth just to feed, clothe and house people at our current uneven levels of prosperity.

The challenge is biggest in the poorest countries, which have some of the highest population growth rates. The UN estimates that population growth will average 0.3% between 1995 and 2000 in 'more developed regions' but 2.7% in the 'least developed countries'.

The good news is that population growth has slowed significantly in the last thirty years. The UN's projected rate for 1995-2000 of 1.4% a year compares with a peak of 2% a year in 1965-1970.

Economic development speeds up population growth by raising living standards and improving health services, so fewer children die in infancy and adults live longer. But it gradually also slows the growth down again by raising standards of education and creating more prosperous lifestyles in which people postpone having children and limit the size of their families. Vigorous campaigns promoting family planning have helped to speed up this 'demographic transition'.

The bad news is that the rapid growth of the past has created a vast wave of people who are now entering child-bearing age - one billion people aged between 15 and 24. Their decisions on how soon they want to start families and how many children they have will be critical for our future.

Ultimately, as the United Nations Population Fund emphasises, population growth is not about numbers but about people. The only reason the numbers matter is they affect the health, living standards and happiness of individuals. A world in which women control their own lives, every pregnancy is intended and every child is wanted will produce very satisfying statistics. 

But will it be too late?

Extract from the BBC's overpopulation site