Societies before the Industrial Revolution were dependent on the annual cycle of plant photosynthesis for both heat and mechanical energy. The quantity of energy available each year was therefore limited, and economic growth was necessarily constrained. In the Industrial Revolution, energy usage increased massively and output rose accordingly. The energy source continued to be plant photosynthesis, but accumulated over a geological age in the form of coal. This poses a problem for the future. Fossil fuels are a depleting stock, whereas in pre-industrial time the energy source, though limited, was renewed each year.