A fossil fuel is a fuel formed by natural, geological processes. The principal source of fossil fuels is the anaerobic decomposition of buried dead organisms, containing organic molecules originating in ancient photosynthesis. The transitions from these source organic materials to the resulting fossil fuels typically requires a geological process of millions of years, and sometimes more than 650 million years. During this process the materials transform into several high-carbon minerals, including petroleum, coal, and fossil or natural gas. Humans extract these transformed materials through mining and drilling and frequently use them for energy through combustion.
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When extracted, fossil fuels can also be transformed into other chemicals or derivatives by the refining and chemical industries. Commonly-used refined fossil fuels include kerosene, gasoline and propane, and common chemicals, include most plastics and agricultural chemicals like fertilizers and pesticides. 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.
Fossil fuels cause serious environmental damage and direct negative consequences on local communities at every stage in their use: extraction, transportation and consumption of the fuels. Most significantly, the burning of fossil fuels produces around 35 billion tonnes (35 gigatonnes) of carbon dioxide (CO2) per year, or about 89% of all carbon dioxide (CO
2) emissions. Natural processes on Earth (mostly through absorption by the ocean) 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, thus fossil fuels are the main source of greenhouse gas emissions causing global warming and ocean acidification. Additionally, 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.
Recognition of the climate crisis, pollution and other negative impacts caused by fossil fuels, has lead to a widespread policy transition and activist movement focused on ending their use in favor of renewables. However, because the fossil fuel industry is so important to the global economy and historically heavily subsidized this transition is expected to have significant economic impacts. Many stakeholders argue that this changes needs to be a just transition and create policy that addresses the stranded assets of the fossil fuel industry. International policy, in the form of Sustainable Development Goal 7: Affordable and Clean Energy, Sustainable Development Goal 13: Climate Action and the Paris Climate Agreement, are designed to facilitate this transition at a global level. In 2021, the International Energy Agency, concluded that no new fossil fuel extraction projects could be opened, if the global economy and society wants to avoid the worst impacts of climate change and meet international goals for climate change mitigation.