There will soon be seven billion people on the planet. By 2045, the global population is projected to reach nine billion. Can the planet take the strain? The first attempt to estimate the human population may have been carried out by Antoni Van Leeuwenhoek in the 17th century. Based on his calculations, Leeuwenhoek concluded triumphantly, there could not be more than 13.385 billion people on Earth - a small number indeed compared with the 150 billion sperm cells of a single codfish.
Historians now estimate that in Leeuwenhoek’s day, there were only half a billion or so humans on Earth. After rising very slowly for millennia, the number was just starting to take off. A century and a half later, when another scientist reported the discovery of human egg cells, the world’s population had doubled to more than a billion. A century after that, around 1930, it had doubled again to two billion. The acceleration since then has been astounding. Before the 20th century, no human had lived through a doubling of the human population, but there are people alive today who have seen it triple. According to the U.N. Population Division, by the end of 2011, there will be seven billion of us.
The population explosion, though it is slowing, is far from over. Not only are people living longer, but so many women across the world are now in their childbearing years - 1.8 billion - that the global population will keep growing for another few decades at least, even though each woman is having fewer children than she would have had a generation ago. U.N. demographers project that the population may reach nine billion by the year 2045. The eventual tally will depend on the choices individual couples make when they engage in that most intimate of human acts, the one Leeuwenhoek interrupted so carelessly for the sake of science.
With the population still growing by about 80 million each year, it is hard not to be alarmed. Right now on Earth, water...