are made out of fossils. While this may seem simple enough in terms of explaining how fossil fuels came about, the actual process is far more complicated and -for lack of a better word- time-intensive. Fossil fuels were formed about 360-300 million years ago, during the Carboniferous Period which takes its name from the fact that carbon is the main element in fossil fluels¹.
Throughout a period of literally millions of years, even before the dinosaurs, a number of fossil fuels formed all in a very similar way: coal, (crude) oil and natural gas. Their formation was facilitated by the fact that the planet had a completely different environment and climate. Hundreds of millions of years ago, the Earth was covered with wide, shallow seas and swampy forests. In these seas and forests grew a number of plants, algae, and plankton. They absorbed sunlight and created energy through the process of photosynthesis. When they perished, they drifted to the bottom of the sea or lake.
Over time (i.e. millions of years), the dead plants were crushed under the seabed. As more rocks and sediment piled on top of them, this created high heat and increasing pressure underground. It is this combination of factors, that eventually helped the fossils of ancients plants turn into fossil fuels. As a result, today, there are sizeable underground reservoirs of fossil fuels over the world which we have been exploiting to power our energy needs. Fossil fuels have also generated a number of very important by-products. For example, the US is dependent on coal to meet its electricity demand, but coal by-products are important for making cement, plastic and other items. The same goes for oil which is primarily converted into gasoline but which is also used to make a myriad of other products from nail polish and crayons to water pipes and shoes¹.
No matter how vital for meeting our energy needs, it is reasonable to expect that despite improvements in technologies for identifying reservoirs or how much better we become at using efficiently the fossil fuels found in those reservoirs, there is a limited amount of fossil fuels that is available. This is discounting the fact that some reservoirs might not be accessible to us due to a lack of technology or because other social or environmental concerns are prohibiting their exploitation. As such fossil fuels are a finite resource; if we continue relying on fossil fuels for powering our energy needs there will be a time when fossil fuels run out.
While fossil fuels are a reliable form of energy, by their very nature they can only be used once. For example, oil companies may extract petroleum 24 hours a day, seven days a week, 365 days a year over several years; successful oil sites produce oil for about 30 years or longer¹. But once extracted, processed and used, there is no way to re-use this finite resource. It literally turns into thin air – or more precisely highly polluting air. Over the past 20 years, nearly three-fourths of human-caused emissions came from the burning of fossil fuels².
From a theoretical point of view, one could argue that since fossil fuels were made using a natural process, they can be replenished. Considering, however, that the average age of the gasoline in your car’s fuel tank is about 70 million years³, it is easy to realise that we consume such great quantities of fossil fuels that the Earth could not possibly replenish reservoirs given how many millions of years it takes for fossil fuels to be created. So, practically speaking, fossil fuels are non-renewable resources. They are finite and we use them at a rate that very much surpasses how fast nature can produce them.
As the example of fossil fuels and the rate at which they can be produced illustrates, renewability can be somewhat of a relevant concept. Whether something is renewable or not depends on how much of it is used and at what rate. As such, it is important to highlight that while all fossil fuels are non-renewable, not all non-renewable sources of energy are fossil fuels⁴. The most typical example here is uranium used for producing nuclear energy. Uranium ore, a solid, is mined and converted to a fuel used at nuclear power plants. Uranium is not a fossil fuel, but it is classified as a non-renewable energy.
Similarly, other forms of energy can be renewable if their production adheres to some environmental standards or non-renewable if it doesn’t. Biomass, is a good example illustrating this. Biomass can be considered a renewable energy source as is it relies on biomass feedstocks. These feedstocks are plants that are processed and burned to create electricity and can include crops such as corn or soy, as well as wood. If these biomass feedstocks are not replanted as fast as they are used them, then biomass energy becomes a non-renewable energy source¹.
Other forms of energy such as solar, tidal or hydro are considered renewable because they rely on resources that can be used repeatedly because they are replaced naturally. The fact that we are harnessing their power does not mean that these resources are diminished in any way.