The numbers of humans on Earth have increased enormously during the past several millennia, but especially during the past two centuries. By the end of the twentieth century, the global population of humans was 6.0 billion. That figure is twice the population of 1960, a mere 30 years earlier. Moreover, the human population is growing at about 1.5% annually, equivalent to an additional 89 million people per year. The United Nations Population Fund estimates that there will likely be about nine billion people alive in the year 2050.

In addition, the numbers of animals that live in a domestic mutualism with humans have also risen. These companion species must be supported by the biosphere along with their human patrons, and can be considered an important component of the environmental impact of the human enterprise. The large domestic animals include about 1.7 billion sheep and goats, 1.3 billion cows...