Sustainable development is defined as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. However, this does not provide a very useful prescription of what we should do now for three reasons. First, because it is in the satisfaction of our wants rather than meeting our needs that we are compromising the ability of future generations to meet theirs. Second, because we do not know how future generations will want to meet their needs. Third, because their ability to meet their needs has already been compromised by past development. How then do we define and aim for sustainability?

Equilibrium

To gain a better understanding of sustainability, it is useful to start from first principles. Past, current and future generations form a system which we call society. In general, a system comprises processes that transform inputs received from the environment into outputs transferred back to the environment. Most physical systems utilise processes to perform activities, in such cases, we call the inputs resources and the outputs wastes. In order for a system to be sustainable, it is necessary to predict what will happen to it in the future. This is only possible with certainty when neither the system nor its environment fundamentally change over time, so this is clearly an ideal situation for a sustainable system to aim for.

There are only two ways for a system to remove a resource from its environment without fundamentally altering it over time. The first is if the environment contains a perpetual source of that resource and an infinite sink for the corresponding waste. Sunlight is a perpetual resource that will continue until the sun expires in some 5 billion years time. Outer space is an infinite sink but gravity means that only waste heat can easily be transferred to it.

The other way is if the environment precisely balances the system by transforming the system’s waste into the corresponding resources. These tend to be complicated environmental systems, or ecosystems. For example, plants convert waste carbon dioxide from animals’ respiration into oxygen and carbohydrates. They use sunlight as energy for this process and water which, for terrestrial plants, is likely to be recycled from the oceans as rain, a process that is also driven by sunlight. Resources that are either perpetual or produced by ecosystems are called renewable.

Closing the loops

While the sun shines all the time, every place on earth only receives sunlight for a specific number of hours each day, depending on the time of year. The intermittent nature of many renewable resources can be smoothed out by reservoirs. For example, the atmosphere is a reservoir of both carbon dioxide and oxygen while lakes and aquifers store rain water. Obviously where such reservoirs are not available in the environment, systems can create their own to smooth out daily and seasonal changes in renewable resources.

Aquifers can collect and store water over thousands of years. Likewise fossil fuels are reservoirs of biomass that has accumulated for millions of years that have undergone geological processes. Geological processes are also responsible for creating mineral deposits, often over even longer time scales.

Modern human society has acted as though ancient aquifers, fossil fuels and mineral deposits were perpetual resources. To achieve equilibrium with the environment it will be necessary to reduce extraction rates to what can be naturally be replenished or stop extracting the resources entirely. The latter simply means closing the all the loops between wastes and resources by totally recycling those that cannot be regenerated by ecosystems, i.e. non-renewable resources. If they cannot be recycled then substitutes must be found based either non-renewable resources that can be recycled by the system or renewable resources that can be recycled by the environment.

Likewise, we have treated the atmosphere and the oceans as infinite sinks for our waste. Greenhouse gases, particularly from burning fossil fuels, are accumulating 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.

Allocating resources

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.

Restoring ecosystems

We have degraded some ecosystems by polluting them with wastes, others by over-exploitation, others still by destructive actions. This has reduced their productive capacity, their adaptability, their resilience and the quality of their services. Hence, we have already reduced the probability of sustainability for future generations. Therefore, as society moves towards equilibrium with the environment, we should also seek to restore we have caused to become degraded.

Conclusions

A sustainable society is one that is in equilibrium with its environment having closed the loops between its inputs and outputs. It differentiates between renewable and non-renewable resources, ensuring that ecosystems can supply or recycle the former while recycling the latter itself. It avoids making changes to the environment but where changes are outside its control, it ensures that both the systems within society and the ecosystems within the environment can adapt to those changes until a new equilibrium is attained. Likewise, it ensures that both are resilient to transient shocks.

Our society may have a long way to go and sustainability may not be certain, but restoring the productivity, adaptability and resilience of the ecosystems it has degraded will only help. The harnessing of renewable resources and using them more efficiently are areas of where improvement is vital. Finding new ways to recycle non-renewable resources or replacing them with renewable substitutes are opportunities for innovation. Clearly greater cooperation in managing ecosystems and allocating resources is going to become increasingly important. Not everything needs to be done immediately but some actions are more urgent than others and they need to be identified and priorities set.

By looking at their inputs and outputs, it can be simple to determine which parts of a system are not sustainable, but it is often meaningless to say that one part is sustainable without considering the whole. Therefore, it may be impossible to achieve sustainable development but development towards sustainability can be and should be our goal.