"Oil and gas" redirects here. For other uses, see Oil and gas (disambiguation). Coal, one of the fossil fuels A fossil fuel is a fuel formed by natural processes, such as anaerobic decomposition of buried dead organisms, containing energy originating in ancient photosynthesis. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years. Fossil fuels contain high percentages of carbon and include petroleum, coal, and natural gas. Other commonly used derivatives 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 either alone, associated with oil, or in the form of methane clathrates. The theory that fossil fuels formed from the fossilized remains of dead plants by exposure to heat and pressure in the Earth's crust over millions of years was first introduced by Andreas Libavius “in his 1597 Alchemia [Alchymia]” and later by Mikhail Lomonosov “as early as 1757 and certainly by 1763”. The United States Energy Information Administration estimates that in 2007 the world's primary energy sources consisted of petroleum (36.0%), coal (27.4%), natural gas (23.0%), amounting to an 86.4% share for fossil fuels in primary energy consumption in the world. Non-fossil sources in 2006 included nuclear (8.5%), hydroelectric(6.3%), and others (geothermal, solar, tidal, wind, wood, waste) amounting to 0.9%. World energy consumption was growing at about 2.3% per year Although fossil fuels are continually being formed via natural processes, they are generally considered to be non-renewable resources because they take millions of years to form and the known viable reserves are being depleted much faster than new ones are being made. The use of fossil fuels raises serious environmental concerns. The burning of fossil fuels produces around 21.3 billion tonnes (21.3 gigatonnes) of carbon dioxide (CO2) per year. It is estimated that natural processes can only absorb about half of that amount, so there is a net increase of 10.65 billion tonnes of atmospheric carbon dioxide per year. Carbon dioxide is a greenhouse gas that increases radiative forcing and contributes to global warming. A global movement towards the generation of low-carbon renewable energy is underway to help reduce global greenhouse gas emissions.