Fossil fuels — concentrated energy, dispersed cost
Coal, oil and gas from the deposit to the atmosphere. Theory, formulas, worked examples and applications.
Millions of years, used in minutes
What "fossil energy" really means
Coal, oil and natural gas are, in essence, ancient solar energy transformed. Millions of years ago, living organisms — plants, plankton, other microorganisms — captured solar energy through photosynthesis and stored it as organic carbon. When those organisms died and were buried under successive layers of sediment, geological heat and pressure slowly transformed them into highly concentrated carbon deposits: coal if the original material was mainly terrestrial plant matter, oil and gas if it was mainly marine organic matter. That concentration — energy captured over millions of years, compressed into a small volume — is exactly what made the industrial revolution possible: steam engines, combustion engines, large-scale industrial heat and mass transport, all powered by a fuel that could be extracted, stored and transported with relative ease.
The same concentration that gives the advantage causes the problem
That same energy density is why burning a relatively small mass of fossil fuel releases an amount of carbon that had been completely out of the atmospheric carbon cycle for millions of years. Photosynthesis and other natural processes take geological timescales to fix that carbon again; industrial activity releases it in a matter of seconds when burned. That mismatch of timescales — not any inherently "bad" property of carbon itself — is the physical reason massive fossil fuel use alters the atmosphere's composition faster than natural systems can compensate.
Before the flame: mines, wells and an entire logistics chain
Before any of the three fuels gets burned, there is an extensive logistics chain with its own environmental cost: underground or open-pit mines for coal; drilling, often thousands of meters deep both on land and under the sea, for oil and gas; refineries that turn crude into usable products; pipelines, trains and ships that carry the fuel often across entire continents. Every link in that chain has its own risk of leak or accident: oil spills on land or at sea, natural-gas leaks — mainly methane — at wells, compressors and pipelines, and coal dust released during transport.
What remains after combustion
Burning any of the three fuels releases CO₂ as the main product, but also local pollutants — particles, nitrogen and sulfur oxides, in different proportions depending on the fuel —, waste heat that any thermal plant has to dissipate, and in coal's case, solid ash in quantities far greater than any other generation technology. In natural gas specifically, the unburned methane itself that leaks before reaching the point of combustion — during extraction, processing or transport — matters as much as or more than the CO₂ from final combustion, because methane is a far more potent greenhouse gas while it remains in the atmosphere, even though it breaks down faster than CO₂.
The operational advantage that explains why they still dominate
Fossil fuels can be stored and transported with an ease that no other primary energy source fully matches: they can be stacked in a warehouse, loaded onto a ship, or kept in a tank for months without significant degradation, allowing electricity or heat to be generated exactly when needed, without depending on weather or the instantaneous availability of the resource. That same operational advantage has a systemic counterpart: a continuous fuel supply that must keep being extracted, bought and burned indefinitely, with direct exposure to international price volatility, to the local pollution it causes, and to a climate accumulation that does not stop while they keep being burned at this rate.
«Cleaner» does not mean clean
Gas usually emits less CO₂ per unit of electricity at the plant than coal, but the outcome depends on efficiency and leaks. Replacing highly polluting coal can reduce harm; building fossil infrastructure for decades can lock out alternatives.
Prudence prioritizes efficiency, electrification where the full cycle actually improves, strict methane control, and retiring the most polluting facilities. For difficult industrial uses, also compare renewable heat, electricity, low-carbon hydrogen and material reduction.
The numbers, with their source
The three fossil fuels, side by side
| Fuel | Life-cycle CO₂eq (g/kWh) | Deaths per TWh | Distinctive trait |
|---|---|---|---|
| Coal | ~820 (740–910) | ~25 | The dirtiest per unit of energy by any measure: carbon, particulates, mercury and other heavy metals. |
| Oil | ~650–750 | ~18 | Most of its emissions and deaths do not come from electricity generation, but from road and air transport. |
| Natural gas (combined cycle) | ~490 (410–650) | ~2.8 | Less CO₂ and local pollution at the plant, but methane leaked before combustion is a very potent greenhouse gas. |
Sources: NREL — Life Cycle Assessment Harmonization; Our World in Data — Safest and cleanest sources of energy.
The health cost that never appears on the bill
The World Health Organization estimates that air pollution — ambient and household combined, with fossil fuel and biomass combustion as main causes — is associated with around 7 million premature deaths a year worldwide. That figure does not distinguish between generating electricity, heating a home, or driving a vehicle: it is the cumulative effect of burning carbon and hydrocarbons near where people live, and it is why "deaths per TWh" for coal and oil are orders of magnitude higher than for any low-carbon source.
Source: WHO — Air pollution.
Why «less than coal» is not the same as «low carbon»
Replacing a coal plant with a combined-cycle gas plant cuts direct-combustion CO₂ to a little less than half, but that saving can disappear if methane leaks during extraction and transport are not controlled: methane warms the atmosphere many times more than CO₂ while it remains there (roughly 20-30 times more over 100 years, more than 80 times more over 20 years). The natural gas leaf develops this account with the leak rate measured by the IEA.
What this table is missing
None of these figures include the cost of restoring mines, sealing abandoned wells, or managing ash and sludge once a plant is closed — costs that rarely fall on whoever benefited from the cheap energy while the infrastructure was in use.
Sources
- The IPCC summarizes energy-system emissions and reduction pathways in AR6, WG3, chapter 6.
- The IEA maintains a global methane tracker.
- The WHO explains the health effects of air pollution.