About a thousand years ago, the human population began exponential growth. By increasing the carrying capacity of the environment and by immigrating to previously unoccupied habitats, the following factors made exponential growth possible:

1. Increases in food supply. By domesticating animals and plants, humans were able to change from a hunter/gatherer lifestyle to one of agriculture. In the last hundred years, food output from agriculture was increased as a result of technological advances made during the industrial and scientific revolutions.

2. Reduction in disease. Advances in medicine, such as the discoveries of antibiotics, vaccines, and proper hygiene, reduced the death rate and increased the birth rate.

3. Reduction in human wastes. By developing water purification and sewage systems, health hazards from human wastes were reduced.

4. Expansion of habitat. Better housing, warmer clothing, easy access to energy (for heating, cooling, and cooking, for example) allowed humans to occupy environments that were previously unsuitable.