From the 1960s to the early 1990s it looked as if the world was heading for a population catastrophe. Paul and Anne Ehrlich’s The Population Bomb and The Population Explosion warned of a burgeoning global population of 10 billion or more by the middle of this century, driving the poorer nations into ecological and economic ruin. It was an apocalyptic vision - the world plagued by crippling pollution, famine, and political collapse.

However, the latest report by the aid organisation UN Population Fund (relating to the period 2000-02) suggests that the world may be able to avoid such extreme scenarios, for there has been a dramatic slow-down in the population growth rate. It now appears that numbers will peak at 8.9 billion by 2050. This is a marked decrease on the figure projected for 2050 just two years earlier (9.3 billion), but still a considerable increase on the present population of 6.3 billion.

While we may not be out of the crisis yet, this is welcome news indeed, and the Ehrlichs and their colleagues can be proud of the contribution they have made to awareness of the problem and the greater access to and use of family planning that followed, which accounts for half of the projected decline in growth. The other half, unfortunately, will be the result of Aids.

The new report indicates that we can no longer divide the world - from a population perspective at least - simply into the categories of developed and developing. A clear division is now emerging among developing nations, with many of the better off going through a rapid change that echoes the experience of the developed world decades earlier. China, for example, already has below-replacement fertility. A rump of predominantly (though not entirely) very poor countries will continue to grow at what is often thought of as “third world” rates. In 24 of these countries, women still bear an average of six children and half the population increase that is predicted over the next 50 years will take place in just eight of them.

A curious demographic split is also opening up in the “first world”. Here it is between the US and the rest, with Europe rapidly taking shape as the US’s demographic antithesis. As a result of its robust birth rate and large-scale immigration, the population of the US will continue to grow strongly. Indeed so high is the population growth that the US is one of just six countries that account for half the world’s current annual population increase of 77 million (the others are China, India, Pakistan, Bangladesh and Nigeria).

If US fertility levels are more characteristic of a developing nation than a developed one, so is its rate of Aids infection. It was the only developed nation whose infection rate made it worthy of consideration at the global level in the UN report, which indicates that about 400,000 of its citizens will die of Aids between 2000 and 2005.

The most extreme manifestation of Aids, however, will be in southern Africa. With about 30 per cent of its population already infected, Botswana is the most badly hit. Two years from now, life expectancy is expected to fall below 40 and population will start to decline. The situation is similar in many neighbouring countries, with life expectancy in Zimbabwe set to drop to 33 by 2005 if not before, and South Africa to 41.5 years between 2005 and 2010.

With the exception of the US, the population of developed countries is projected to contract before 2050. By then, Japan is expected to have 14 per cent fewer people, Italy 22 per cent, and Bulgaria, Estonia, Latvia, Georgia, the Russian Federation and Ukraine between 30 and 50 per cent.

Around 2030, most developed nations will begin a general population decline, while the developing world will still be experiencing a “robust” increase of 0.4 per cent.

Population ageing has long preoccupied the governments of these countries, but the new figures make it clear that the problem is not theirs alone. The greatest proportions of older people will soon be living in those developing nations entering the demographic transition. The total number of “aged” (classified as those over 60) is set to rise from the present 606 million to 1.9 billion, with eight out of every 10 of these living in the world’s less developed regions.

Centenarians may be an extreme example, but their projected distribution around the world is intriguing. Japan is projected to have a million, the US and China nearly half a million each, and India, France and Germany more than 100,000 each. Such extraordinary survival appears to be a function of both national population size and available health care.

The impact of immigration will continue to be significant. Today, about 175 million people live in a country that is not the one of their birth, a number that has doubled since 1975 and which looks set to keep expanding.

While the political debate surrounding migration has focused on refugees, only nine per cent of migrants are classified as such.

The importance of migration can be best understood by looking at its impact historically on a single nation. Australia has been one of the great importers of people during the past 50 years, and today some 44 per cent of its population is from first or second-generation migrant families. Such has been the impact of Australia’s post-war migration programme that its population would be about 7.6 million and declining (rather than almost 20 million and growing) had its demographic trends stayed as they were in the 1930s and early 1940s. If Australia’s immigration programme stays as it is, with about 100,000 migrants arriving each year, its population will reach 23 million by 2030, then begin to decline.

While the globe’s total population as analysed on a country-by-country basis is important, it does not give the whole picture, as internal migration is also picking up pace. In terms of ecological sustainability, it is important to consider whether people are living in cities or in villages, and this is changing rapidly.

Today, three quarters of the population of the most advanced nations live in cities, but by 2030 this will have increased to 83 per cent. In the less developed world the figure is 40 per cent, projected to rise to 56 per cent. If patterns of consumption for city dwellers remain unchanged, this could have a massive impact on global warming and congestion.

There are many assumptions made in population projections. The UN report focuses on what they call “medium” projections, but the figures are liable to change. If for example, the birth rate were to remain at today’s level (increasing at 1.2 per cent or a current 77 million per year), the global population would rise to 12.8 billion by 2050.

Even if the average number of children per woman were to increase by 0.5 above the estimate, planet earth could end up with 10.6 billion passengers in 47 years’ time. So, while it looks as if great strides are being made in limiting the increase in population, a slight setback could have grave consequences.

The issue underlying all discussions of population is sustainability - whether we will leave sufficient resources for future generations to live in dignity and comfort - and the signs are not good. As early as 1961, when the world population was little more than half the size it is today, humanity’s demands for resources consumed about 70 per cent of what the Earth could sustainably generate. By the 1980s, we were consuming close to 100 per cent, and by the end of the last decade, an excess over sustainability of 20 per cent. This means that we are now eating into the accumulated capital of all life on earth. If technology and patterns of affluence do not change to become more environmentally friendly, we are heading for disaster, even at current population levels.

The spread of a growing population is another worry. Biodiversity has never been under such threat, with everything from deep-sea fisheries to rainforest communities experiencing catastrophic collapse. In an effort to feed its billion-plus population, China is embarking on massive projects such as the Three Gorges dam, which threatens to extinguish such extraordinary species as the Yangtze River dolphin, the Chinese alligator and several of the world’s cranes. Such projects will be repeated over and over again in the developing world. By the time there are 8.9 billion people, it seems likely that much of the world’s unprotected rainforests will be felled, its last wild rivers dammed, and many of its precious species lost forever.

And it’s not just consumption that is a problem. Disposing of waste is one of the most formidable problems humanity will have to face. Global warming from emissions of industrial and other waste look set to heat the Earth by 6oC by 2100. To put that into perspective, during the last ice age, about 15,000 years ago, when glaciers blanketed much of North America, Britain and Europe north of the Alps, the earth was a mere five degrees cooler than it is today.

The climate change projected for this century will herald an era of instability and transformation that is barely imaginable from our present perspective. It is highly likely, for example, that the violence of many climatic events, such as cyclones and el Ni$o, will intensify and become more widespread. Cyclones may extend far to the north, ravaging Europe’s major cities, while drought may blight areas currently immune from its influence.

Such changes may make vast regions that are presently habitable quite marginal. Take one example - the warming of the oceans (without including the melting of the polar ice caps) will raise the sea-level sufficiently to flood 14 per cent of Bangladesh. The impact this will have on that densely populated country - about 135 million people - is likely to be catastrophic.

Patterns of disease incidence will change also, with tropical scourges such as dengue fever and malaria becoming more widespread. But it’s the things we’ve been unable to predict that should worry us most:never in the past half a million years has the Earth ever experienced a climatic shift such as is predicted for this century. And by 2050 humans will be intensively using more than 80 per cent of the Earth’s surface, making a large proportion of us acutely vulnerable in a changing world.

While the latest UN report gives us reason to hope, it leaves no room for complacency; looking at the overall issue of sustainability, even 8.9 billion is a frighteningly large figure. With global population now looking as if it is on track to stabilisation within 50 years, it’s time to turn to technology. If we are to avoid environmental disaster we must transform the world into a post-fossil fuel economy and rein in the patterns of consumption that are so extravagant in the developed world.

For more information on the global environmental threat, see WWF’s Living Planet Report for 2002 www.panda.org

Tim Flannery is director of the South Australian Museum and a professor at the University of Adelaide. His books include The Future Eaters, Throwim Way Leg, and The Eternal Frontier