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This year will see the world鈥檚 7-billionth person. If only population growth forecasts were as definite, says demographer John Bongaarts

By how much do you think the world鈥檚 population will grow this century?
The that we will reach 9.1 billion in 2050, then the peak will occur around 2070 at 9.4 billion. But it is so far ahead and the margin of uncertainty is huge. We could easily end up with 1 or 2 billion more, or less.

What factors could lead to 11 billion people?
The UN assumes fertility in Africa, for example, will decline steadily from current levels of around four to five children, to around two. It might well go slower than that. Kenya, for example, had a fertility rate of nearly 8 in the 1960s, which came down to around 5 in the 1990s and has remained at that level. This was not expected by the UN, which expects things to go down steadily.

Are there any other reasons why global population might overshoot the UN estimate?
Governments in countries with very low birth rates are now considering implementing pro-natal policies. Most governments in Europe, I think, would like to see their birth rates go up a little bit. If that happens, there will be more Europeans, Japanese and Russians. Also, people might live longer than the UN predicts. The UN assumes that future increases in life expectancy will be slower than in the past. That may be wrong.

Some people think the population will in fact undershoot. What are your thoughts on this?
There are equally possible reasons for undershooting. The most obvious is that fertility in Africa and everywhere could follow the low fertilities of Italy and Spain. This is conceivable but I would be a little doubtful. Countries don鈥檛 like having low fertility. It leads to a smaller and ageing labour force and a large number of retired people, which is difficult to manage economically.

Isn鈥檛 it also true that many countries have experienced sharp declines in fertility rates?
Yes, take Asia. If you go back to the 1950s and 1960s, South Korea, Thailand and Indonesia looked just as undeveloped as some African countries now, with high fertility rates. Today Korea and Thailand have fertility rates below 2. So this is quite possible, and depends on how countries develop. It was a very rapid change, though, and could well turn out to be slower elsewhere.

Does HIV/AIDS have an impact on population?
HIV is an example of a disease that people thought would stop growth or reduce Africa鈥檚 population. That didn鈥檛 happen. Population growth has such a powerful momentum even such a lethal and massive epidemic had only a fairly modest impact.

The bottom line is, what鈥檚 your money on if you had to bet on the 2050 world population?
There are very few things that you can predict with any degree of accuracy 40 years from now. But the UN has a good record. In a projection made in 1950 it came close to predicting the actual 2000 population, so I would bet on there being a little over 9 billion people by 2050. Even so, the margin is still plus or minus a billion at least.