What does sustainability mean?

The term sustainability originally comes from forestry; one should only fell as many trees as will regrow through new plantations, keeping tree populations and yields constant. The concepts of weak and strong sustainability provide different answers to the question of what it means to maintain a sustainable stock. 

Weak sustainability is applied in environmental economics and is based on the principle of interchangeability; natural capital (natural resources) can be replaced by physical capital (e.g. machines or material infrastructure) and human capital (e.g. knowledge). The three areas of environment, society and economy exist separately and interact through the exchange of resources. Physical capital is denoted by the economic sphere, human capital by the social sphere and natural capital by the ecological sphere. Sustainability means keeping the total value of the capital stock (the sum of the three types of capital) constant and increasing it where possible. Natural, physical and human capital are comparable and mutually substitutable, i.e. interchangeable, by means of one measure, namely money. In order to carry out this exchange, methods of comparison are needed, for example a cost-benefit analysis. 

Markets, in which the three forms of capital are traded, can be created. This leads to commodification, meaning that free goods, like air and water, which are foundational for life, are turned into commodities, which can be traded like any other good. It is therefore not seen as problematic if natural capital is shrinking today as regions turn into deserts, as long as at the same time physical capital is increased, for example by building roads. Due to interchangeability, environmental damage can be compensated financially. Those who fly can ‘offset’ the emissions caused with compensation payments, for example into reforestation projects.

The key concept of weak sustainability is optimization – the neoclassical concept of the best possible allocation of scarce resources. In order to allocate resources optimally, externalities have to be considered and calculated. Externalities are caused by actors without them bearing the resultant costs. For example, when a company emits polluted air from a chimney without installing filters or paying compensation to those negatively affected. If externalities are not included in the price, the optimal market outcome does not correspond to the optimal social outcome, which results in market failure due to false price signals. The internalisation of external effects, such as monetary compensation for environmental damage, is therefore the central instrument in the concept of weak sustainability: By means of ‘right prices’, environmental burdens which have been externalised up to now are internalised, i.e. included in prices. Examples are levies or taxes on polluted water or air as well as emission certificates. Weak sustainability follows the polluter-pays principle: Whoever generates ecological and social costs should also bear them. However, what the ‘right’ price for the extinction of a species or degradation of ecosystems should be is not so clear.

Strong sustainability is at the heart of the debates in ecological economics, which go beyond discussing an optimal allocation of resources. Strong sustainability is based on the principle of embeddedness: the economy is a subsystem, embedded in society and the biophysical sphere. Strong sustainability assumes that economic and social life is based on irreplaceable, interwoven ecosystems that must be preserved. Economic activities are confronted with ecological limits. The substitutability of nature with other types of capital is limited. Instead of the idea of optimisation, strong sustainability requires a holistic and systemic view of social-ecological systems and a reasonable deliberation between alternatives. From this point of view, the three areas of environment, social affairs and economy are in many respects incommensurable, meaning not comparable with a measure, and therefore not mutually interchangeable. 

In the understanding of strong sustainability, nature is not a stock of resources (capital), but a complex ecosystem that provides humankind with vital functions. Nature has an intrinsic value because there are qualitative differences between produced capital and nature; the former is reproducible (e.g. new bridges can be built), the destruction of nature is often irreversible. ‘The fish in an aquarium can be made into a fish soup, but fish soup cannot be made into fish for an aquarium’.

Strong sustainability is based on the precautionary principle: possible damage or pollution to the environment that could become dangerous for people must be avoided or reduced, even if there is not certainty that it will occur.