The economy depends upon Earth as a source—extracting finite resources such as oil, clay, cobalt and copper, and harvesting renewable ones such as timber, crops, fish and fresh water. The economy likewise depends upon Earth as a sink for its wastes—such as greenhouse gas emissions, fertiliser run-off and throwaway plastics. Earth itself, however, is a closed system because almost no matter leaves or arrives on this planet: energy from the sun may flow through it, but materials can only cycle within it.
