How many people can the Earth support? This question has interested scientists for many years. The concept of carrying capacity has been used in this context. Carrying capacity is a term used by ecologists to describe the maximum number of animals of a given species that a habitat can support indefinitely, without permanently degrading the environment.

In the 17th century, Dutch microbiologist Anton van Leeuwenhook (1673-1723) estimated that the Earth could support a maximum of 13.4 billion people. Another 17th-century scientist, Gregory King, predicted the Earth could support 6 to 12 billion humans. In the mid-19th century, German chemist Justus von Liebig (1803-1873) formulated his Law of the Minimum, based on the realization that the addition of a single fertilizer will increase crop yield only if a particular soil can deliver all the other necessary nutrients. Any of the essential minerals, such as nitrates, phosphates, potassium, etc., could become the controlling factor in plant growth. Liebig's Law of the Minimum has been applied to the study of animal populations. Using Liebig's law, we can say the population of humans, or any other species, will be constrained by whatever survival resource is in shortest supply. Using this approach, modern estimates for human carrying capacity have ranged from 1 or 2 billion people living in prosperity, to 33 billion people fed on minimum rations and using all suitable land for high-intensity food production. Many scientists now believe that the human carrying capacity of Earth may be approximately 12 billion.

However, population numbers alone do not inform us of the total burden we place on our environment. Resource use and waste output per person vary tremendously from country to country, and will probably change in the future. For each necessary resource, it is important to determine the burden we are now imposing, as well as the maximum sustainable burden beyond which that resource will degrade. For example, we are near the maximum sustainable burden for ocean fishing, and beyond the maximum sustainable burden for carbon dioxide emissions. In order to plan for a sustainable future, we must carefully assess each area of resource use and waste output, and the impacts on Earth's ecosystems.