What Are Fossil Fuels?

What Are Fossil Fuels?

The term “fossil fuels” is thrown about quite a lot these days. More often than not, it comes up in the context of environmental issues, Climate Change, or the so-called “energy crisis”. In addition to be a major source of pollution, humanity’s dependence on fossil fuels has led to a fair bit of anxiety in recent decades, and fueled demands for alternatives.

But just what are fossil fuels? While most people tend to think of gasoline and oil when they hear these words, it actually applies to many different kinds of energy sources that are derived from decomposed organic material. How humanity came to be so dependent on them, and what can we look to in order to replace them, are some of the biggest concerns facing us today.


Fossil fuels refers to energy sources that are formed as a result of the anaerobic decomposition of living matter that contains energy as a result of ancient photosynthesis. Typically, these organisms have been dead for millions of years, with some dating back as far as the Cryogenian Period (ca. 650 million years ago).

The Bryan Mound Strategic Petroleum Reserve, located in Brazoria Country, Texas. Credit: energy.gov

Fossil fuels contain high percentages of carbon and stored energy in their chemical bonds. They can take the form of petroleum, coal, natural gas, and other combustible, hydrocarbon compounds. Whereas petroleum and natural gas are formed by the decomposition of organisms, coal and methane are the results of the decomposition of terrestrial plants.

In the case of the former, it is believed that large quantities of phytoplankton and zooplankton settled on the bottoms of seas or lakes millions of years ago. Over the course of many millions of years, this organic matter mixed with mud and was buried under heavy layers of sediment. The resulting heat and pressure caused the organic matter to become chemically altered, eventually forming carbon compounds.

In the case of the latter, the source was dead plant matter that was covered in sediment during the Carboniferous period – i.e. the end of Devonian Period to the beginning of the Permian Period (ca. 300 and 350 million years ago). Over time, these deposits either solidified or became gaseous, creating coal fields, methane and natural gases.

Modern Uses:

Coal has been used since ancient times as a fuel, often in furnaces to melt metal ores. Unprocessed and unrefined oil has also been burned for centuries in lamps for the sake of lighting, and semi-solid hydrocarbons (like tar) were used for waterproofing (largely on the bottoms of boats and on docks) and for embalming.

Widespread use of fossil fuels as sources of energy began during the Industrial Revolution (18th – 19th century), where coal and oil began replacing animal sources (i.e. whale oil) to power steam engines. By the time of the Second Industrial Revolution (ca. 1870 – 1914), oil and coal began to be used to power electrical generators.

The invention of the internal combustion engine (i.e. automobiles) increased demands for oil exponentially, as did the development of aircraft. The petrochemical industry emerged concurrently, with petroleum being used to manufacture products ranging from plastics to feedstock. In addition, tar (a leftover product from petroleum extraction) became widely used in the construction of roads and highways.

Fossil fuels became central to modern manufacturing, industry and transportation because of how they produce significant amounts of energy per unit mass. As of 2015, according to the International Energy Agency (IEA) the world’s energy needs are still predominantly provided for by sources like coal (41.3%) and natural gas (21.7%), though oil has dropped to just 4.4%.

The fossil fuel industry also accounts for a major share of the global economy. In 2014, global coal consumption exceeded 3.8 billion metric tons, and accounted for US $46 billion in revenue in the US alone. In 2012, global oil and gas production reached over 75 million barrels per day, while the global revenue generated by the industry reached about US $1.247 trillion.

Countries of the world ranked in terms of their annual production of oil. Credit: Wikipedia Commons/Ali Zifan

The fossil fuel industry also enjoys a great deal of government protection and incentives worldwide. A 2014 report from the IEA indicated that the fossil fuel industry collects $550 billion a year in global government subsidies. However, a 2015 study by the International Monetary Fund (IMF) indicated that the real cost of these subsidies to governments worldwide is around US $5.3 trillion (or 6.5 % of global GDP).

Environmental Effects:

The connection between fossil fuels and air pollution in industrialized nations and major cities has been evident since the Industrial Revolution. Pollutants generated by the burning of coal and oil include carbon dioxide, carbon monoxide, nitrogen oxides, sulfur dioxide, volatile organic compounds and heavy metals, all of which have been linked to respiratory illnesses and increased risks of disease.

The burning of fossil fuels by humans is also the largest source of emissions of carbon dioxide (about 90%) worldwide, which is one of the main greenhouse gases that allows radiative forcing (aka. the Greenhouse Effect) to take place, and contributes to global warming.

In 2013, the National Oceanic and Atmospheric Administration announced that CO² levels in the upper atmosphere reached 400 parts per million (ppm) for the first time since measurements began in the 19th century. Based on the current rate at which emissions are growing, NASA estimates that carbon levels could reach between 550 to 800 ppm in the coming century.

If the former scenario is the case, NASA anticipates a rise of 2.5 °C (4.5 °F) in average global temperatures, which would be sustainable. However, should the latter scenario prove to be the case, global temperatures will rise by an average of 4.5 °C (8 °F), which would make life untenable for many parts of the planet. For this reason, alternatives are being sought out for development and widespread commercial adoption.


Due to the long-term effects of fossil fuel-use, scientists and researchers have been developing alternatives for over a century. These include concepts like hydroelectric power – which has existed since the late 19th century – where falling water is used to spin turbines and generate electricity.

Since the latter half of the 20th century, nuclear power has also been looked to as an alternative to coal and petroleum. Here, slow-fission reactors (which rely on uranium or the radioactive decay of other heavy elements)  are used to heat water, which in turn generates steam to spin turbines.

Since the mid-2oth century, several more methods have been proposed that range from the simple to the highly sophisticated. These include wind power, where changes in airflow pushes turbines; solar power, where photovoltaic cells convert the Sun’s energy (and sometimes heat) into electricity; geothermal power, which relies on steam tapped from the Earth’s crust to rotate turbines; and tidal power, where changes in the tides push turbines.

The spherical tokamak MAST at the Culham Centre for Fusion Energy (UK). Photo: CCFE

Alternative fuels are also being derived from biological sources, where plant and biological sources are used to replace gasoline. Hydrogen is also being developed as a power source, ranging from hydrogen fuel cells to water being used to powering internal combustion and electric engines. Fusion power is also being developed, where atoms of hydrogen are fused inside reactors to generate clean, abundant energy.

By the middle of the 21st century, fossil fuels are expected to have become obsolete, or at least declined significantly in terms of their use. But from a historical standpoint, they have been associated with the largest and most prolonged explosions in human growth. Whether humanity will survive the long-term effects of this growth – which has included an intense amount of fossil fuel burning and greenhouse gas emissions – remains to be seen.

We have written many articles about fossil fuels for Universe Today. Here’s What is an Enhanced Greenhouse Effect?, Gases in the Atmosphere, What Causes Air Pollution?, What if We Burn Everything?, What is Alternative Energy?, and “Climate Change is Now More Certain Than Ever,” New Report Says

If you’d like more info on Fossil Fuels, check out NASA’s Earth Observatory. And here’s a link to NASA’s Article on Safeguarding our Atmosphere.

Astronomy Cast also has some episodes that are relevant to the subject. Here’s Episode 51: Earth and Episode 308: Climate Change.


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