Example of a two-stock system

A two-stock system is a system where one stock can be renewed and is constrained by a non-renewable stock. For example, in the agricultural industry, the renewable stock is the crops that can be continuously grown and harvested, while the non-renewable stock is the farmland itself which has physical limits and cannot be indefinitely expanded.

In the context of energy, the renewable stock could be solar or wind energy that is continuously available, while the non-renewable stock is the technology and infrastructure used to collect and store that energy.

Any physical system like this must have at least one reinforcing loop that drives growth and a balancing loop that constrains it. No physical system can grow forever and will eventually encounter limits, temporary or permanent.