Why was the 20th century unique? Well, OK, England won the World Cup, and that may never happen again. But I am thinking about something even more profound, something fundamental to the story of Homo sapiens. The 20th century saw by far the largest growth in world population in history. Furthermore, since population growth is now slowing down sharply, the last century will always retain its crown. Hitler, Stalin, Mao, eat your hearts out - the defining feature of the 20th century will forever remain the explosive growth of our species.

The United Nations Population Division reckons that it was only in 1800 that global population reached 1 billion for the first time. Since life expectancy at birth for most of mankind's history was only about 10 years, it took a birth rate of over 80 per 1,000 even to prevent the species from becoming extinct. But after 1800, in a succession of countries, a mixture of medicine, sanitation and nutrition began to reduce the death rate. And a common feature of almost every country is that the death rate falls for a long period before economic progress begins to cut the birth rate. Only once the birth rate starts to fall toward the long-term stabilisation rate of two children per woman per lifetime does the population growth rate start to fall. Then it takes many decades for the total population to stabilise.

It took 127 years after 1800 for the world population to rise from 1 billion to 2 billion. The subsequent time periods to add an extra billion were 34 years, 13 years, 13 years and 12 years, which means we passed the 6 billion threshold in 1999. The total increase in global population during the 20th century was 4.31 billion, seven times the increase in the previous century.

But we are now at a crucial inflection point. The time it will take to add the next billion will rise to 14 years, and then the gaps will be 15, 26 and 129 years respectively. The UN reckons that we might reach long-term stabilisation at just over 10 billion, two centuries from now. It will therefore take us the whole of the next two centuries to add the number of people that we managed in the past 100 years.

You may well ask how on earth the UN thinks it knows this. A fair point, but we can already see that in most rich countries, the population has now stabilised, and the birth rate in some countries, like Italy, has now dropped to only 1.3 children per woman, which is probably the lowest fertility rate in human history. A decline in the birth rate is also happening in most developing nations, but at a much slower rate. Therefore, the population of the developed world will be static at 1.2 billion between now and 2050, while that of the developing regions will rise from 5.2 billion to 7.8 billion.

Some may say that none of this really matters. But apart from the effect of population size on global warming and the sustainability of the planet, I ask you this: how can England be expected to win the World Cup when we will be so heavily outnumbered by the footballers of the developing world in centuries to come?