Summary and Final Tasks

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The sun sends out energy continuously. Plants have figured out how to store that energy for later use, by combining water and carbon dioxide to make more plants. Almost all the other living things on Earth survive by “burning” these plants to get the stored energy.

Usually, plants are burned soon after they die, but occasionally some plants are buried without oxygen and survive for much longer. Time and the Earth’s heat combine to “cook” these old, buried plants, making fossil fuels. We rely on oil—primarily from “slimy” plants (algae, and similar water plants), coal—primarily from “woody” plants, and gas from both.

Most of the coal is found in the rocks where it formed, but most of the oil and gas we are using had migrated upward through spaces in the rock and then been trapped in geologically special places before reaching the surface. Recently, we have begun “fracking” to get oil and gas still trapped in the rocks where they formed. We are also using bitumen from “tar sands”, the leftovers from where oil seeped all the way to the surface and the more-fluid parts were burned by bacteria or else evaporated. We’re trying to learn how to use “oil shale” containing dead plants that would make oil if they were cooked more. And we’re thinking about the possibility of using natural gas that has formed clathrate ice in cold places beneath the sea floor.

The known reserves of these fossil fuels—the ones we’re sure we can use—will be gone in a few decades at the current rate of use. The total resource—including the fuels we think we’ll discover as we search harder, and we think we’ll learn how to use as we invent new ways—would last a few centuries at the current rate of use, but that might drop to less than a century if population and use per person continue to rise. And, sharp increases in price and other problems are likely to start well before the fossil fuels become really scarce.

Nature will make new fossil fuels, but not nearly fast enough to help us. We are burning our way through a “bank account” of fossil fuels supplied by nature, with no income to replace what we use. And, as we will see in the next lessons, our fossil-fuel burning is releasing carbon dioxide that is accumulating in the air and changing the climate.

Reminder - Complete all of the Module 3 tasks!

You have reached the end of Module 3! Double-check the Module Roadmap to make sure you have completed all of the activities listed there before you begin Module 4.


References and Further Reading

Alley, R.B., Earth: The Operators’ Manual, 2011

Organic Origins of Petroleum, United States Geological Survey Energy Resources Program

Schweinfurth, S.P., 2003, Coal—A Complex Natural Resource, United States Geological Survey Circular 1143