The news on the population front sounds bad: birth rates are not dropping as fast as expected, and we are likely to end up with an even bigger world population by the end of the century.

The last revision, two years ago, of the United Nations' World Population Prospects predicted just over 10 billion people by 2100. The latest revision, just out, predicts almost 11 billion.

That's a truly alarming number, because it's hard to see how the world can sustain another four billion people. (The current global population is seven billion.) But the headline number is deceptive, and conceals another, grimmer reality. Three-quarters of that growth will come in just one continent: Africa.

The African continent currently has 1.1 billion people. By the year 2100, it will have 4.1 billion - more than a third of the world's total population. Or rather, that is what it will have if there has not already been a huge population dieback in the region. At some point, however, systems will break down under the strain of trying to feed such rapidly growing populations, and people will start to die in large numbers.

It has happened before - to Ireland in the 1840s, for example - and it can happen again. In fact, it probably will. When you look more carefully at the numbers, you can even identify which regions will be hardest hit, because even in Africa there are large areas where population growth is low and dropping.

None of the Arabic-speaking countries of northern Africa will increase its population by more than one-third by 2100, and some will even be declining. South Africa, at the other end of the continent, will only add another 10 million people by the century's end. It's in the middle belt of Africa that things will get very ugly.

Between now and 2100, six countries are expected to account for half of the world's projected population increase: India, Nigeria, the United States, the Democratic Republic of Congo, Tanzania and Uganda. Four of the six are in central Africa.

In this area, where fertility is still high, the numbers are quite astonishing. Most countries will at least triple in population; some, like Uganda, Tanzania and Zambia, are predicted to grow fivefold. That is on top of populations that have already tripled, quadrupled or quintupled in the past half-century. Uganda had five million people at independence in 1962; it is projected to have 205 million in 2100.

The numbers are simply preposterous. Niger, a desert country whose limited agricultural land might feed 10 million people with good management, a lot of investment, and good luck with the weather, already has twice as many as that. By the end of the century it will have 20 times as many: 204 million people.

All these numbers are based on assumptions about declining birth rates: if we all just carried on with the birth rates of today, there would be 25 billion people on this planet by the end of the century. The key question is: how fast is fertility declining - and all the numbers in this column so far are from the UN's "medium estimates," i.e. the moderately optimistic ones. The "high estimate" for Niger gives it 270 million people by 2100: an extra 70 million.

It makes no practical difference. Even the "low estimate" of 150 million people in Niger by 2100 is never actually going to happen. That is 15 times too many people for the available land, and Niger certainly cannot afford to import large amounts of food. Even without reckoning in the huge negative impact of climate change, large numbers of people in Niger (and quite a few other African countries) will begin starving long before that.

So the real picture that emerges from the UN's data is rather different. It is a world where two-thirds of the world's countries will have declining populations by 2100. China and Russia will each be down by a third, and only the United States among the major developed countries will still have a growing population: up from 320 million now to 460 million. (By the way, that means there will only be twice as many Chinese as Americans by then.)

In terms of climate change, the huge but ultimately self-limiting population growth in Africa will have little impact, for these are not industrialized countries with high rates of consumption and show no signs of becoming so. The high economic growth rates of African countries in recent years are driven mostly by high commodity prices, and will probably not be sustained.

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It is the developed and rapidly developing countries whose activities put huge pressure on the global environment, not only by their greenhouse gas emissions but also by their destructive styles of farming and fishing. Their populations are relatively stable but their actual numbers are already very large, and each individual consumes five or ten times as much as the average African.

So the frightening numbers in the UN's latest population predictions are mostly of concern to Africa - but the rest of the world is still in deep, deep trouble on many other fronts.