The Ecological Footprint applies principles of ecology to human society to create a framework for mapping society's metabolism. In a generalized ecosystem model, primary producers fix energy from sunlight through photosynthesis. This energy is then available for consumers, who use this primary production for growth and maintenance activities. All material ingested by consumers, however, eventually returns to the biosphere as waste products, where it is broken down and recycled back into the raw materials for primary production.
This model, where every input eventually turns into waste, holds for any organism that exists in a natural environment, including human society. The human economy takes high-quality matter and energy as inputs from the environment and returns these in degraded form as material waste and heat. Societies consume resources in order to maintain themselves. Between resource intake and waste discharge, matter accumulates in these systems, leading to increased body mass in the case of animals, or an accumulation of material stocks in societies.
From a human perspective, the ability of the biosphere to absorb wastes and regenerate resources is known as the regenerative capacity of the planet. Although the Earth's regenerative capacity is robust, it can be eroded in three significant ways, according to The Natural Step:
1. Natural cycles can be overwhelmed by harvesting renewably generated resources, such as trees or fish, faster than they can be replenished. Direct physical interference, such as the paving over of green surfaces or farming practices that cause soil erosion, can also damage the underlying capital that creates these resources, further reducing the Earth's total regenerative capacity.
2. Substances produced by society that are persistent and do not readily break down, and which ecosystems have not developed abilities to assimilate, can compromise regenerative capacity as they accumulate in the biosphere. Examples include synthetic chemicals such as DDT and PCBs. Many of these man-made substances have no natural analogs, and the biosphere as a whole has not evolved efficient means to break down and re-assimilate these products on human timescales.
3. Substances normally buried deep with the Earth's crust can be extracted, refined, and introduced into the biosphere at quantities that ecosystems are not able to assimilate. Examples include heavy metals, radioactive elements, minerals, and mined carbon. Because there are few natural cycles that can return these substances to the crust within human time spans, these substances systematically accumulate in the biosphere.
The fundamental insight of the metabolism model is that, in the long term, the biosphere must be able to turn wastes back into resources faster than the human society turns the resources into waste. If the extraction of resources becomes too large, or if nature's regenerative capacity is compromised, biological capital will be systematically degraded and wastes will accumulate.
While in the past, most 'environmental' problems arose from poor management of some local aspects of society's metabolism, such as careless waste disposal, smokestacks, or overuse of a river basin, now the very size of society's global metabolism that has become the overarching concern. While local overuse of the biosphere has a long history (e.g., overfishing, deforestation, soil erosion), the global human economy has now become so large, relative to the regenerative capacity of planet Earth, that it is now for the first time in human history confronting global limits.
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