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?
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 accum