The world's population is growing at 78 million per year, the equivalent of a new Mexico City every six months. Virtually all that growth is happening in places we call "developing." In 1800 there were just one billion humans. Three billion was reached in 1960 and doubled in less than 40 years. In 2000 the fifth billionth person is just 13 years old; the fourth billionth just 25. Not only are there so many more people, but each is bigger, as measured by the energy and material used and the pollutants and wastes produced.
12 October 1999 was declared by the United Nations as decided to make an event out of the fact that "The Day of Six Billion" -- a projection to the date when the world's population would reach 6 billion..
We are, as far as we know, the first creatures on this planet evolved enough to realize that there is such a thing as a carrying capacity and that there are penalties for exceeding it. Scientists have begun to calculate how many of us at what standard of living the earth can support. They do not agree on an exact number, but there is clear evidence that we are already beyond it.