Fossil fuel

A fossil fuel is a hydrocarbon-rich fuel, chiefly coal, oil, and natural gas, formed from the remains of ancient plants and marine organisms buried and transformed by heat and pressure over millions of years. Burning a fossil fuel releases the chemical energy stored in that ancient organic matter, which is why fossil fuels have powered most of the industrial world’s electricity, heating, and transportation since the nineteenth century.

How fossil fuels form

Coal forms mainly from plant material that accumulated in ancient swamps and bogs, where low-oxygen, waterlogged conditions slowed decay and allowed partly decomposed plant matter to compress into peat, then over geological time into lignite, bituminous coal, and eventually anthracite as heat and pressure increased. Oil and natural gas form differently, mostly from the remains of marine microorganisms, especially plankton and algae, that settled into ocean or lake sediment, were buried under successive layers of rock, and were gradually converted by heat and pressure into kerogen and then into liquid petroleum and gaseous hydrocarbons. Because both processes require very specific geological conditions sustained over millions of years, fossil fuel deposits are concentrated in particular sedimentary basins rather than distributed evenly across the planet.

Types and primary uses

Coal is burned mainly to generate electricity and to produce coke for steelmaking, and it remains especially important in several large, rapidly industrializing economies even as its global share has slipped in recent years. Oil is refined into gasoline, diesel, jet fuel, and a wide range of petrochemical feedstocks used to make plastics, fertilizers, and other industrial materials, which makes it central to transportation in particular. Natural gas is burned for home heating, electricity generation, and industrial processes, and it is increasingly shipped internationally as liquefied natural gas, which has turned what was once a largely regional, pipeline-bound fuel into a more globally traded commodity.

Current scale of fossil fuel use

Fossil fuels still dominate the global energy system. The Energy Institute’s 2025 Statistical Review of World Energy, covering full-year 2024 data, found fossil fuels accounted for nearly 87 percent of global energy consumption. Natural gas alone supplied about 25 percent of global energy and accounted for roughly 33 percent of the total increase in global energy supply during 2024, the largest contribution of any single fuel that year, reflecting both rising demand and continued expansion of gas infrastructure even as renewable capacity also grows quickly elsewhere.

A brief history of fossil fuel use

People burned coal and peat on a small scale for centuries before the Industrial Revolution, but coal became central to the global economy only once steam engines, and later coal-fired electricity generation, made it practical to convert coal’s chemical energy into mechanical and electrical power at scale starting in the late eighteenth and nineteenth centuries. The modern oil industry traces to commercial drilling that began in the mid-nineteenth century, and oil’s importance accelerated through the twentieth century alongside the rise of the automobile and aviation, both of which depend on liquid fuels refined from petroleum. Natural gas was historically flared off or ignored as a byproduct of oil drilling because it was difficult to transport, and it only became a major fuel in its own right once pipeline networks, and later liquefied natural gas shipping, made moving gas over long distances practical and profitable.

Environmental and climate impact

Burning fossil fuels releases carbon dioxide that had been locked away in the ground for millions of years, adding it to the atmosphere far faster than natural processes can remove it, which is the primary driver of the enhanced greenhouse effect behind modern climate change. Fossil fuel combustion also releases particulates, sulfur dioxide, and nitrogen oxides that contribute directly to air pollution and associated health problems in addition to their climate effects. Coal is generally the most carbon-intensive of the three fuels per unit of energy produced, followed by oil, with natural gas typically the least carbon-intensive, though natural gas extraction and transport can leak methane, itself a potent greenhouse gas, which complicates simple comparisons between the three fuels’ overall climate impact.

Reserves, depletion, and the energy transition

Energy analysts distinguish between a resource, the total estimated quantity of a fuel in the ground, and a proven reserve, the portion that can be extracted profitably with current technology and prices. Both figures shift over time as prices, technology, and exploration change, which is one reason predictions that the world would soon run out of oil have repeatedly been pushed back for decades rather than fossil fuels simply running dry. The more immediate pressure on fossil fuels now comes less from physical scarcity than from climate policy and the falling cost of renewable energy and storage, which together raise the prospect of large fossil fuel reserves becoming stranded assets, fuel that remains in the ground because extracting and burning it is no longer economically or politically viable.

FAQs

Are fossil fuels actually running out? Known reserves have grown over time as exploration and extraction technology improved, even as cumulative production has risen, so outright physical exhaustion has repeatedly been pushed further into the future; today the bigger constraint on fossil fuel use is climate policy and competition from cheaper renewable energy rather than running out of the resource itself.

Which fossil fuel produces the least carbon dioxide per unit of energy? Natural gas is generally the least carbon-intensive of the three major fossil fuels when burned, though methane leaks during its extraction and transport can offset some of that advantage depending on how tightly a given supply chain controls those leaks.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 1, 2026Researched and drafted with AI assistanceReport an issue