The rising birth rates in Asia and Africa are unsustainable in the long term, writes Joe Chamie

THE MOST rapid period of world population growth yet recorded - nearly quadrupling - occurred during the 20th century. The planet now supports 6.7 billion inhabitants. With an annual increase of 78 million, the world population will soon reach seven billion, which is likely by 2011. After that, where is it headed?

While future world population trends remain uncertain, several things are clear today. First, the current rates of population growth are unsustainable in the long term.

For example, if fertility rates were to remain fixed at today's levels, then the world population would nearly double by the middle of the century - and leap to 44 billion by 2100.

Furthermore, while some countries such as Germany, Italy, Japan and Russia would decline to a fraction of their current size, the population of others, such as Ethiopia, Nigeria, Pakistan, Saudi Arabia and Yemen, would increase many times over.

Within the next five years, there is little doubt that the world population will reach seven billion. Will it reach eight billion? Most demographic observers would say that such a scenario is highly likely, perhaps within the next two decades. After that, things become less clear.

On average, fertility levels around the world are indeed declining. However, considerable variation exists across and within regions. In more developed countries, rates are often below replacement levels (about two children to each woman).

The average level for Europe, for example, is 1.4 children to each woman. A notable exception to this trend is the US, where fertility has been around the replacement level for a number of years. Fertility rates in less developed countries, in contrast, are often three or more children to each woman.

In sub-Saharan Africa for example, the average fertility level is five children to each woman. Again, a notable exception to this general pattern of relatively high fertility is China, where fertility is 1.8 children to each woman.

Many believe that world fertility will remain above the replacement level for some time to come. They point out that nearly all of Africa and most of south and west Asia have high fertility levels. Others, however, see below-replacement fertility becoming the global norm in the coming decades.

Current world fertility, they note, is about half the level it was in the 1950s. Also, they draw attention to the fact that approximately 45 per cent of the world's population, or almost three billion people, are living in countries with below-replacement fertility - versus less than 1 per cent in the 1950s.

It is important to bear in mind that in the long term, outcomes other than replacement fertility inevitably lead to ever-increasing or ever-declining populations. For instance, if fertility rates eventually settled at about a quarter child above replacement, or 2.35 children to each woman, the world's population would more than double to 14 billion by the century's end - and continue growing thereafter.

On the other hand, if fertility rates of all countries eventually settle at about one-quarter child below replacement fertility, or 1.85 children to each woman, the world population would be about 5.5 billion by the end of the century - and continue to decline thereafter.

In other words, barring dramatic changes in mortality, replacement fertility for the long term is essentially the tipping point for population change: above it there will be continued growth and below it continued decline.

Many demographers making long-range population projections see fertility levels settling close to replacement levels. To do otherwise, as noted above, leads in the long run to either extremely large, expanding populations or very small, shrinking populations.

Assuming that fertility rates come to land around the replacement level during the coming decades - and subsequently remain close to that level - then world population would eventually settle at around nine to 10 billion inhabitants.

Will high fertility rates, especially in sub-Saharan Africa and many parts of south Asia, gravitate to around replacement levels in the long term? This uncertainty about the future of currently high fertility levels is clearly a paramount issue of the 21st century.

Joe Chamie is the former head of the UN Population Division and is currently director of research at the Center for Migration Studies