Humans are consuming and polluting resources – aquifers and ice caps, fertile soil, forests, fisheries and oceans – accumulated over geological time, tens of thousands of years or longer.
Wealthy countries consume out of proportion to their populations. As a fiscal analogy, we live as if our savings account balance were steady income.
According to the Worldwatch Institute, an environmental think tank, the Earth has 1.9 hectares of land per person for growing food and textiles for clothing, supplying wood and absorbing waste. The average American uses about 9.7 hectares.
These data alone suggest the Earth can support at most one-fifth of the present population, 1.5 billion people, at an American standard of living.
Water is vital. Biologically, an adult human needs less than 1 gallon of water daily. In 2010, the U.S. used 355 billion gallons of freshwater, over 1,000 gallons (4,000 liters) per person per day. Half was used to generate electricity, one-third for irrigation, and roughly one-tenth for household use: flushing toilets, washing clothes and dishes, and watering lawns.
If 7.5 billion people consumed water at American levels, world usage would top 10,000 cubic kilometers per year. Total world supply – freshwater lakes and rivers – is about 91,000 cubic kilometers.
World Health Organization figures show 2.1 billion people lack ready access to safe drinking water, and 4.5 billion lack managed sanitation. Even in industrialized countries, water sources can be contaminated with pathogens, fertilizer and insecticide runoff, heavy metals and fracking effluent.