The news on the population front sounds bad: birthrates 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 of the United Nations' World Population Prospects, two years ago, 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 4 billion people. (The current global population is 7 billion.) The headline number is deceptive,and conceals another, grimmer reality. Three-quarters of that growth will come in 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.