The UN estimates that the world's population will increase by 1.3% in 1999. That's 78 million people. It forecasts an average increase of 1.4% a year for the period 1995-2000.

This explosion in population is relatively recent. Two millennia ago, around the time of Christ, the Earth had around 300 million people, about as many as there are in northern America today. The figure remained steady for a thousand years. It grew slowly to half a billion by 1500 and one billion by 1803.

But it then took only 124 years to reach the second billion, in 1927. In 1960, world population was 3 billion, and in 1974, it hit 4 billion. It passed the 5 billion mark in 1987 and in October 1999, it will reach 6 billion, meaning it's only taken 12 years to gain an extra one billion people.

The balance of the global population is shifting too. A hundred years ago, there were three times as many Europeans as Africans. Now there are three times as many Africans as Europeans.

The biggest population explosion came after the Second World War, when death rates in developing nations declined sharply. The peak growth came in the period 1965-70, when the world grew in numbers by 2% per year. Since then, education and development programmes have steadily reduced fertility rates.

Future growth is difficult to predict. If fertility rates (that is, the number of babies born as a percentage of the total population) remain at the 1990-95 levels, then we would hit nearly 15 billion by 2050, and a staggering 296 billion by 2150. The problems that would cause are so enormous that it's almost impossible to believe we would realistically reach that figure.

The UN's low-end estimate of population trends is that world numbers would decrease to only 3.6 billion by 2150. Its medium estimate of fertility puts the 2050 figure at around 9 billion people, growing to 10.8 billion by the year 2150 and ultimately stabilising at 11 billion by the year 2200.

One important factor is the epidemic of HIV/AIDS in developing nations, especially sub-Saharan Africa. With 30 million people infected worldwide, the disease is cutting average life expectancy by as much as ten years in the worst-hit countries.

Extract from the BBC's overpopulation site