As absolute energy and water use increases due to population growth and economic growth, it is worth considering what the ultimate carrying capacity of the earth is. That determination depends on factors such as how many people inhabit the earth and the quantity of resources they use for their existence. It turns out that those values are not consistent from place-to-place and that they change over time.
Four macro trends that have been underway for decades and are likely to continue include urbanization, industrialization, electrification, and mobilization, all of which increased overall demand for energy from 1800 to the present day and are expected to continue that trajectory into the future. Economic growth typically implies higher per capita energy consumption as people gain affluence, and that outcome is evident for many decades since the early 1800s. However, in many medium economic developed countries (MEDC), per capita consumption leveled off or decreased slightly since the energy crises of the 1970s. Since those years, the industrial mix in the United States shifted from very energy-intensive industries such as manufacturing and chemical production to less energy-intensive, more service-oriented industries such as banking and research.
Overall, the world consumes more than 550 Exajoule (EJ) of energy. U.S. consumers use the same types of energy as global citizens but on a different scale. Roughly speaking, the average global citizen consumes about 75 EJ each year. The average U.K. resident consumes twice as much as the average global citizen. The average U.S. resident consumes twice as much as the average U.K. resident. On average, U.S. residents consume four times as much energy per person than the typical resident of China or India.
If everyone on Earth consumed energy at British rates, global energy demands would double. If the world’s population consumed at U.S. rates, energy demand would quadruple. Population growth compounds the pressure. By 2050, between nine and eleven billion people will inhabit Earth. With an increasing population and increasing resource demand as more people consume more energy per person, it is possible to imagine a moment at which Earth’s atmosphere, oceans, and resources reach their capacity to provide resources or to take society’s waste products, much of which is from the energy system.
The concept of an ecological footprint provides a framework to help model carrying capacity, or the maximum number of humans that can be sustainably supported by the global environment. The ecological footprint model approximates the area of water and land required to support a defined human population at a given standard of living. In the example above, if the entire Earth is the ecosystem and the population doubles, the ecological footprint of the new population is two Earths. If the population remains the same but demand quadruples, the ecological footprint of the new demand is four Earths. This conundrum of carrying capacity and the footprint of the global population is the grand challenge of the twenty-first century. Increasing energy consumption brings many good aspects, such as economic mobility and improved health. However, with energy each benefit carries a tradeoff. As energy consumption grows, so do energy imports, environmental degradation, and emissions, which contribute to global climate change. The challenge that remains is to answer the question “How does society bring the value of energy consumption—clean water, indoor lighting, quality of life—to every global citizen in a way that does not leave behind a wake of environmental destruction from energy consumption or require more than one planet on which to live?”