A fossil fuel is a fuel formed by natural processes, such as anaerobic decomposition of buried dead organisms, containing organic molecules originating in ancient photosynthesis that release energy in combustion. Such organisms and their resulting fossil fuels typically have an age of millions of years, and sometimes more than 650 million years.

Fossil fuels contain high percentages of carbon and include petroleum, coal, and natural gas. Commonly-used derivatives of fossil fuels include kerosene and propane. Fossil fuels range from volatile materials with low carbon-to-hydrogen ratios (like methane), to liquids (like petroleum), to nonvolatile materials composed of almost pure carbon, like anthracite coal. Methane can be found in hydrocarbon fields alone, associated with oil, or in the form of methane clathrates. Although peat shares many characteristics with fossil fuels, including its decayed organic structure and release of greenhouse gas upon usage, whether it can be considered a true fossil fuel is debated.

As of 2018, the world's main primary energy sources consisted of petroleum (34%), coal (27%), and natural gas (24%), amounting to an 85% share for fossil fuels in primary energy consumption in the world. Non-fossil sources included nuclear (4.4%), hydroelectric (6.8%), and other renewables (4.0%, including geothermal, solar, tidal, wind, wood, and waste). The share of renewables (including traditional biomass) in the world's total final energy consumption was 18% in 2018.

Most air pollution deaths are due to fossil fuel combustion products: it is estimated that this pollution costs over 3% of global GDP, and that fossil fuel phase-out would save 3.6 million lives each year.

The use of fossil fuels causes serious environmental damage. The burning of fossil fuels produces around 35 billion tonnes (35 gigatonnes) of carbon dioxide (CO2) per year. Natural processes on Earth can only absorb a small part of this amount, therefore there is a net increase of many billion tonnes of atmospheric carbon dioxide per year. CO2 is a greenhouse gas that increases radiative forcing and contributes to global warming and ocean acidification. A global movement towards the generation of low-carbon sustainable energy is underway to help reduce global greenhouse gas emissions.

 

Note:   The above text is excerpted from the Wikipedia article Fossil fuel, which has been released under the GNU Free Documentation License.