Systems Thinking
For some two centuries we have known that the Earth is a closed system with finite resources. As planetary explorers completed the task of mapping the lands and waters, people slowly grew to understand that there are no "new" resources. We have only one Earth. All of our activities are but a small part of this larger system. Viewing our human systems as operating within the larger ecosystem is crucial for achieving a sustainable relationship with the environment, and assuring our own species' continued survival on the planet. 1
Each natural resource used by human beings—food, water, wood, iron, phosphorous, oil and hundreds of others—is limited by both its sources and its sinks. 2 Resources should not be removed faster than they can be renewed nor disposed of more quickly than they can be absorbed. Although environmentalists used to be concerned primarily about running out of sources, today more people are concerned about running out of sinks. Global warming, the ozone hole, and conflicts over the international shipment of hazardous waste are all problems that have arisen from our attempts to dispose of resources faster than the environment can absorb them.
Systems thinking requires us to understand that while there is only one Earth, it is composed of a multitude of subsystems all interacting with each other. A variety of models have been developed to explain the Earth's subsystems. When measuring our progress toward sustainable development, these models provide useful frameworks for choosing indicators. The differences between the models show the specific perspectives which groups bring to sustainable development and embody their differing values.
These subsystems are connected together by intricate feedback loops. The science of complexity suggests that in some systems a very small occurrence can produce unpredictable and sometimes drastic results by triggering a series of increasingly significant events. 3 We have seen that emissions in the North have thinned the protective ozone layer over Antarctica, increasing rates of skin cancer in the South. Financial crises in Asia have threatened the economies of other countries around the world. And ethnic violence in Central Africa has led to refugee migrations that are overwhelming the support systems of nearby regions, triggering further crises and migrations.
We have learned that the consequences of decisions made in one part of the world quickly affect us all.
1. Carl Folke, Monica Hammer, Robert Costanza and AnnMari Jansson. "Investing in natural capital - Why, what and how?" In AnnMari Jansson, Monica Hammer, Carl Folke, and Robert Costanza. Investing in natural capital: The ecological economics approach to sustainability, Washington D.C.: Island Press, 1994. p. 4.
2. Donella H. Meadows, Dennis L. Meadows, Jorgen Randers. Beyond the limits, Toronto: McClelland & Stewart Inc., 1992.
3. Anthony Clayton, Nicholas J. Radcliffe. Sustainability: A systems approach, London: Earthscan Publications Ltd., 1996. | 
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