The Second Axiom
Population growth and/or growth in the rates of consumption of resources cannot be sustained.
Human population growth has been sustained up to the present. How can we be sure that it cannot be sustained into the indefinite future? Simple arithmetic can be used to show that even small rates of growth, if continued, add up to absurdly large—and plainly unsupportable—population sizes and rates of consumption. For example, a simple 1 percent rate of growth in the present human population (less than the actual current rate) would result in a doubling of population each 70 years. Thus in 2075, Earth would be home to 13 billion humans; in 2145, 26 billion; and so on. By the year 3050, there would be one human per square meter of Earth's land surface (including mountains and deserts). Virtually no one expects this to occur—at some point, population growth will cease. Similar calculations apply to consumption rates.
The Third Axiom
To be sustainable, the use of renewable resources must proceed at a rate that is less than or equal to the rate of natural replenishment.
Renewable resources are exhaustible. Forests can be overcut, resulting in barren landscapes and shortages of wood (as occurred in many parts of Europe in past centuries), and fish can be overharvested, resulting in the extinction or near extinction of many species (as is occurring today globally).
This axiom has been stated (in somewhat different ways) by many economists and ecologists and is the basis for “sustained-yield forestry” (see above) and “maximum-sustainable-yield” fishery management.
The term “rate of natural replenishment” requires some discussion. The first clue that harvesting is proceeding at a rate greater than that of natural replenishment is the decline of the resource base. However, a resource may be declining for reasons other than overharvesting; for example, a forest that is not being logged may be decimated by disease. Nevertheless, if the resource is declining, pursuit of the goal of sustainability requires that the rate of harvest be reduced, regardless of the cause of resource decline. Sometimes harvests must drop dramatically, at a rate far greater than the rate of resource decline, so that the resource has time to recover. This has been the case with regard to commercial wild whale and fish species that have been overharvested to the point of near extinction; a moratorium on harvesting these species was necessary for them to recover. If the remaining breeding population is too small, however, even a moratorium is insufficient and the species cannot recover.
The Fourth Axiom
To be sustainable, the use of nonrenewable resources must proceed at a rate that is declining, and the rate of decline must be greater than or equal to the rate of depletion.
The rate of depletion is defined as the amount being extracted and used during a specified time interval (usually a year) as a percentage of the amount left to extract.
No continuous rate of use of any nonrenewable resource is sustainable. However, if the rate of use is declining at a rate greater than or equal to the rate of depletion, this can be said to be a sustainable situation because society's dependence on the resource will be reduced to insignificance before the resource is exhausted.
Estimates of the “amount left to extract,” mentioned in the axiom, are disputable for all nonrenewable resources. Unrealistically robust estimates would tend to skew the depletion rate in a downward direction, undermining efforts to attain sustainability via a resource-depletion protocol. It may be realistic to assume that people in the future will find ways to extract nonrenewable resources more thoroughly, with amounts that would otherwise be left in the ground becoming economically recoverable as a result of higher commodity prices and improvements in extraction technology. Exploration techniques are likely to improve as well, leading to further discoveries of the resource. Thus, realistic estimates of ultimately recoverable quantities should be greater than what is now known to be extractable using current technology and at current prices. However, it is unrealistic to assume that people in the future will ever be able to economically extract all of a given resource or that limits of declining marginal returns in the extraction process will no longer apply. Moreover, if discovery rates are currently declining, it is probably unrealistic to assume that discovery rates will increase substantially in the future. Thus, for any nonrenewable resource, prudence dictates adhering to conservative estimates of the “amount left to extract.”
The Fifth Axiom
Sustainability requires that substances introduced into the environment from human activities be minimized and rendered harmless to biosphere functions.
In cases where pollution from the extraction and consumption of nonrenewable resources has proceeded at expanding rates for some time and threatens the viability of ecosystems, reduction in the rates of extraction and consumption of those resources may need to occur at a rate greater than the rate of depletion.
If the second, third, and fourth axioms are followed, pollution should be minimized as a result. Nevertheless, these conditions are not sufficient in all cases to avert potentially collapse-inducing impacts.
It is possible for a society to generate serious pollution from the unwise use of renewable resources (the use of natural tanning agents on hides damaged streams during preindustrial times), and such impacts are to be avoided. Likewise, especially where large numbers of humans are concentrated, their biological wastes may pose severe environmental problems; such wastes must be properly composted.
The most serious forms of pollution in the modern world arise from the extraction, processing, and consumption of nonrenewable resources. If (as outlined in the fourth axiom) the consumption of nonrenewable resources declines, pollution should also decline. However, in the current instance, where extraction and consumption of nonrenewable resources have been growing for some time and have resulted in levels of pollution that threaten basic biosphere functions, heroic measures are called for. This is, of course, the situation with regard to atmospheric concentrations of greenhouse gases, especially in relation to the burning of coal; it is also the case with regard to hormone-mimicking petrochemical pollution, which inhibits reproduction in many vertebrate species. In the first instance: Merely to reduce coal consumption by the global coal-depletion rate would not suffice to avert a climatic catastrophe. The coal-depletion rate is small, climate impacts from coal combustion emissions are building quickly, and annual reductions in those emissions must occur at high rates if ecosystem-threatening consequences are to be avoided. Similarly, in the case of petrochemical pollution, merely to reduce the dispersion of plastics and other petrochemicals into the environment by the annual rate of depletion of oil and natural gas would not suffice to avert environmental harms on a scale potentially leading to the collapse of ecosystems and human societies.
If a reduction in emissions or other pollutants can be obtained without a reduction in consumption of nonrenewable resources—for example, by using technological means to capture polluting substances and sequester them harmlessly, or by curtailing the production of certain industrial chemicals—then a reduction in consumption of such resources need only occur at the depletion rate in order to achieve sustainability. However, society should be extremely skeptical and careful about claims that untested technologies can safely sequester polluting substances for very long periods of time.