A fossil fuel is a carbon-rich energy source formed from the remains of ancient plants and animals that were buried underground for millions of years. The three main types are coal, oil (petroleum), and natural gas. When burned, the chemical energy stored in their molecular bonds is released as heat, which is why they power everything from cars to power plants. They currently supply the majority of the world’s energy, but burning them produces carbon dioxide, the primary driver of climate change.
How Fossil Fuels Form
The process starts with living things. Hundreds of millions of years ago, plants grew in vast swamps while tiny organisms called plankton filled the oceans. When these organisms died, some of them settled into environments where they didn’t fully decompose, often because they were quickly buried under layers of sediment in low-oxygen conditions like swamp floors or ocean beds.
Over millions of years, more sediment piled on top, pushing the organic material deeper underground. As it sank further, it was exposed to increasing heat from the Earth’s interior and immense pressure from the weight above. These conditions slowly transformed the carbon-rich organic matter into the fuels we extract today. The type of fuel that formed depended on what the original material was, how deep it was buried, and how much heat it was exposed to.
The Three Types
Coal
Coal comes from ancient land plants, primarily from dense forests and swamps that existed long before dinosaurs. As layers of dead plant material were buried and compressed, they gradually hardened into rock. Coal exists in several grades, from lignite (the softest, lowest-energy form) to anthracite (the hardest and most carbon-dense). It’s a solid, mined from the ground, and has historically been the dominant fuel for generating electricity.
Oil (Petroleum)
Oil formed primarily from marine plankton and other tiny sea organisms. As sediment accumulated on top of these remains over millennia, heat from deep in the Earth transformed them into a liquid mixture of hydrocarbons. Crude oil collects in underground reservoirs, trapped within tiny spaces in sedimentary rock. It can also be found near the surface in tar sands. Once refined, crude oil becomes gasoline, diesel, jet fuel, and thousands of other products.
Natural Gas
Natural gas forms through a similar process to oil but requires higher temperatures, which means it typically comes from organic material buried even deeper. It’s mostly methane, the simplest hydrocarbon, and exists as a gas underground. It’s often found alongside oil deposits and is used for heating, cooking, and electricity generation.
Why Burning Them Releases Energy
Fossil fuels are made of hydrocarbons, molecules built from carbon and hydrogen atoms. These atoms are held together by chemical bonds that store energy. When you burn a fossil fuel, oxygen reacts with those carbon and hydrogen atoms, breaking the original bonds and forming new ones: carbon dioxide and water. The new bonds hold less energy than the original ones, and the difference is released as heat. That’s the energy that spins turbines, moves pistons, and heats buildings.
This is why fossil fuels are so energy-dense. Millions of years of geological pressure essentially concentrated the sun’s energy (originally captured by those ancient plants and plankton through photosynthesis) into compact chemical packages. A gallon of gasoline contains enough energy to move a two-ton vehicle for 25 or 30 miles, which is a remarkable amount of power from a small volume of liquid.
How They’re Extracted
Coal is typically mined, either from the surface (strip mining) or from deep underground shafts. Oil and natural gas require drilling. Conventional wells tap into underground reservoirs where the fuel has pooled in porous rock, and the natural pressure underground often pushes the oil or gas to the surface.
When fuel is trapped in dense rock like shale, extraction gets more complex. Hydraulic fracturing, commonly called fracking, pumps high-pressure water mixed with sand into the rock to crack it open. The sand holds the fractures open so oil or gas can flow out. Paired with horizontal drilling, which allows a single well to reach sideways through a rock layer for long distances, fracking has unlocked enormous reserves, particularly in the United States, where these techniques operate at a scale not seen elsewhere in the world. Offshore drilling extends extraction to the ocean floor, with some platforms operating in extremely deep water.
Uses Beyond Fuel
Burning for energy is the most visible use of fossil fuels, but it’s far from the only one. Petrochemicals derived from oil and natural gas are the building blocks for over 6,000 everyday products. Plastics are the most obvious example, but the list extends to fertilizers, detergents, synthetic fabrics, adhesives, pharmaceuticals, cosmetics, and electronics. Your toothbrush, contact lenses, phone case, shampoo, and the asphalt on your street all trace back to petroleum. Even some components in wind turbine blades are made from petrochemicals.
This dual role as both an energy source and an industrial raw material is part of what makes fossil fuels so deeply embedded in modern life. Replacing them as a fuel is one challenge; replacing the materials they provide is another entirely.
Environmental Cost
The fundamental environmental problem with fossil fuels is that burning them releases carbon dioxide, a greenhouse gas that traps heat in the atmosphere. Not all fossil fuels are equal in this regard. Coal is the worst offender, releasing roughly 210 to 229 pounds of CO2 per million BTUs of energy, depending on the type. Oil-based fuels like gasoline and diesel produce around 155 to 165 pounds per million BTUs. Natural gas is the cleanest of the three, emitting about 117 pounds per million BTUs, roughly half of what coal produces for the same amount of energy.
Beyond carbon dioxide, fossil fuel combustion releases other pollutants, including nitrogen oxides and particulate matter, which contribute to smog and respiratory problems. Methane, the main component of natural gas, is itself a potent greenhouse gas, and leaks during extraction and transport add to the climate impact. Extraction also carries local environmental risks: water contamination from fracking, habitat destruction from mining, and oil spills from offshore drilling and pipeline failures.
How Long Reserves Will Last
At current consumption rates, estimates suggest the world has roughly 50 to 55 years of proven oil and natural gas reserves remaining, with coal lasting somewhat longer, perhaps 70 years or more. These numbers shift constantly as new reserves are discovered, extraction technology improves, and consumption patterns change. If natural gas production were ramped up to compensate for declining oil, for instance, gas reserves would be depleted faster.
These projections don’t mean the world will simply “run out” on a specific date. What happens in practice is that remaining reserves become progressively harder and more expensive to extract, which drives up costs and accelerates the economic case for alternatives. Governments worldwide spent $620 billion subsidizing fossil fuel consumption in 2023 alone, according to the International Energy Agency, down from a record-breaking $1 trillion in 2022 during the global energy crisis. Those subsidies, concentrated in emerging and developing economies, keep fossil fuels artificially affordable and shape how quickly the transition to other energy sources unfolds.