Any workable definition will require some combination of these ethical, social, economic, environmental, and operational components. Two questions arise: one of equity, the other of costs, and the two are closely related. The equity issues loom large in any discussion of responsibility and costs. The average environmental footprint created by a U.S. resident, for example, is twice that of a European, twelve times that of an Indian, and twenty-four times that of a Somali. The United States and Europe are prosperous, developed countries, India a growing, strongly developing country, and Somalia a poverty-stricken, war-torn, famine-ravaged country. Each is a polluter. Who pays, and what should be the relative contributions? The U.K.’s Stern Commission calculated that a restorative national investment now of 1 percent national GDP could avoid a potential 20 percent cost in subsequent liabilities for environmental restoration.
Nor can we isolate these sustainability questions from the broader question of how many humans Earth can sustain.
The most basic requirements for human survival have been estimated as follows, expressed as per person, per year:
- Nine hundred liters of relatively clean water;
- Three hundred kilograms of food, mostly from grain; and
- Adequate clothing and shelter from freezing temperatures.
Now these are basic, survival requirements. In contrast, the average U.S. resident in a typical state, Rhode Island, consumes each year:
- One hundred thousand liters of clean water;
- One thousand kilograms of food, including significant amounts of meat and imported fruits;
- Between five hundred and one thousand liters of gasoline for transportation;
- One to two thousand equivalent liters of gasoline for power; and
- Tons of metals, plastics, fabrics, chemicals, and construction materials.
Clearly, therefore, estimates of Earth’s carrying capacity involve social, ethical, and economic as well as physical and biological considerations—and, because there is no agreement on the relative priority of each, it is scarcely surprising that there is little agreement on Earth’s carrying capacity. Add to this uncertainty the speculative nature of cost estimates and prospects for improvements in technological efficiency (alternative fuels, more-efficient energy use, synthetic materials, new crops, and improved water safety, for example) and the problem of social sustainability becomes still more complex. Profoundly personal views and values thus have to be incorporated in any estimate of Earth’s carrying capacity. For example, is it better to have a smaller, prosperous, well-fed population, or a larger, poorer, less well-fed population? How does one decide? How do Earth’s people decide? What is the balance of personal reproductive choice and social responsibility?
These and other issues influence the widely varying estimates of Earth’s carrying capacity, but if one includes social sustainability as a requirement, estimates range from 0.5 billion to 14 billion, depending on the standard of living, the longevity of nonrenewable resources and the adequacy of renewable ones, and the likelihood of technological improvements. The medians of high and low estimates, however, range from 2.1 to 5.0 billion. That means our present population level of 7.0 billion is already unsustainable. That we are able to maintain this imbalance reflects our dependence on nonrenewable energy sources—especially petroleum and natural gas—and the huge social inequalities that our present global population represents.
Sustainability projections such as these inevitably involve assumptions about the global framework within which they will apply. Prolonged global conflict, for example, would clearly involve greater stress than a peaceful global society in which creativity and technology were nurtured and encouraged, free trade was embraced, a sense of global responsibility was fostered, and human population leveled off and stabilized. Given such perhaps utopian conditions, sustainability—though difficult—does not seem unattainable. The alternative—human population growth that exceeds a sustainable number, with the loss of the most vulnerable in large numbers, whatever is done—is simply unacceptable to all our deepest instincts and all our finest values.