Fossil Fuels

All economic production requires low-entropy (available) energy and raw materials.. Our primary energy sources consist of finite flows of solar energy that strike the planet at a fixed rate over time over which we have no meaningful control, and finite stocks of fossil fuels and radioactive elements that can be used up as fast as our technologies permit. All raw materials come from nature.  The economic process converts these inputs into into economic products that contribute to human well-being, generating considerable waste emissions in the process. Furthermore, all economic products  eventually break down, fall apart and return to our ecosystems as waste.  Given the unbreakable physical link between resource inputs into the economic process and waste outputs, ecological economists account for both inputs and outputs with the term throughput.  Energy throughput can be used to recycle some material flows, but energy itself cannot be recycled.  

For our purposes, low entropy can be understood as organized and useful. For much more detail on this topic, see Georgescu-Roegen, N. (1971). The Entropy Law and the Economic Process. Cambridge, MA, Harvard University Press or Chapter 4 in Daly, H. E. and J. Farley (2011). Ecological Economics: Principles and Applications, 2nd ed. Washington, DC, Island Press.