 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.
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 