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. Peat is also sometimes considered a fossil fuel.
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.
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. Compared with 2017, world energy-consumption grew at a rate of 2.9%, almost double its 10-year average of 1.5% per year, and the fastest since 2010.Although fossil fuels are continually formed by natural processes, they are generally classified as non-renewable resources because they take millions of years to form and known viable reserves are being depleted much faster than new ones are generated.Most air pollution deaths are due to fossil fuel combustion products, it is estimated to cost over 3% of global GDP, and fossil fuel phase-out would save 3.6 million lives each year.The use of fossil fuels raises serious environmental concerns.
The burning of fossil fuels produces around 35 billion tonnes (35 gigatonnes) of carbon dioxide (CO2) per year.
It is estimated that natural processes can only absorb a small part of that amount, so 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 renewable energy is underway to help reduce global greenhouse-gas emissions.