ulating in the atmosphere at a faster rate than ecosystems can remove them. Marine ecosystems have degraded due to pollution, to the point of creating zones that are no longer able to sustain life. Fully recycling non-renewable resources will greatly reduce the amount of polluting waste we produce.
Some of the wastes that can be regenerated by ecosystems are polluting because they are in a form that is harmful to the environment. In such cases, they should be transformed into benign substances before being released. Other wastes are discharged in the wrong place for ecosystems to have access to them. If the nutrients in human waste are from food that grows on land it would be necessary to return them to the productive land rather than release them to the oceans, unless the loop can be closed, for example by using seaweed as a fertiliser.
While renewable resources are potentially perpetual, their rate of production is limited. In some cases, we have developed neither the technologies to harness them nor the reservoirs and demand management necessary to compensate for their intermittent availability. This is particularly true for renewable energies.
Given the high energy density of fossil fuels and high extraction rates, we have had immense amounts of power available to us. In some cases, this has led to great profligacy and inefficiency, for example our motor vehicles have the equivalent power of fifty to five hundred horses, often just to move one person. The transition from non-renewable to renewable energies will have to be accompanied by significant gains in efficiency.
Some renewable resources are already at their limits of exploitation, for example many fisheries. Greater international cooperation will be required to manage them to avoid over-exploitation in the future. Land use is another management issue. Competing demands from urban development, food production and energy production from biofuels or solar collectors will require nations to make decisions on how land is allocated. Important questions will have to be addressed, such as: will land be set aside for natural ecosystems and wild animals?
On an individual level, if, for example, it is necessary to limit energy consumption, people may have to make choices about how they live and what they do. Perhaps deciding between using an electric car for personal transport for a year and flying half-way around the world for an annual holiday in a plane powered by biofuels.
Because of the need to allocate resources, it is not possible to state categorically that a specific part of a system is sustainable. For example, it is meaningless to say that a building is sustainable as this will depend on the interactions it has with other parts of society, such as land use, transport, utilities and other amenities. Indeed, the greatest efficiencies in resource utilisation may well come from an integrated design of a village or city. Even then, sustainability will probably depend on what happens in the rest of the country and ultimately the world.
Adaptability and resilience
While a system should avoid changing the environment to remain in equilibrium with it, some changes may be beyond the control of the system. For example, global warming has a momentum behind it that will take decades to slow down. On longer time scales, our current climate is probably an interglacial period between ice ages. During transitions from one set of stable environmental conditions to another, both the society and ecosystems must have the capability to adapt until a new equilibrium can be achieved.
Transient shocks also have the potential to disturb the equilibrium between a system and its environment. Examples include storms, earth quakes and tsunamis. To be resilient both society and ecosystems need to either absorb the shocks or quickly recover from them. Some ecosystems provide protection to both society and other ecosystems, for example coral reefs and mangroves protect coasts from storms. Such non-productive benefits are known as ecosystem services.
We have degraded some ecosystems by polluti