Combustion of fossil fuels is the single largest driver of global warming, responsible for the majority of the carbon dioxide humans add to the atmosphere each year. When coal, oil, or natural gas burns, carbon in the fuel combines with oxygen to produce CO2, a greenhouse gas that traps heat in the atmosphere. Atmospheric CO2 has now reached about 430 parts per million, up from roughly 280 ppm before the industrial era, and that increase has pushed global average temperatures approximately 1.1°C (2°F) above pre-industrial levels.
How Burning Fuel Creates Greenhouse Gases
Fossil fuels are mostly carbon and hydrogen. Combustion is the chemical reaction that breaks those molecules apart and combines them with oxygen. The carbon becomes CO2, and the hydrogen becomes water vapor. Both are greenhouse gases, but CO2 is the far bigger concern because it persists in the atmosphere for centuries. In the United States, CO2 from combustion and related processes accounts for about 80% of all human-caused greenhouse gas emissions.
Combustion also produces smaller quantities of nitrous oxide, a greenhouse gas roughly 270 times more potent than CO2 per molecule over a 100-year span. In conventional industrial boilers burning coal or oil, about 14% of the nitrogen in the fuel converts to nitrous oxide. That’s a relatively small share of total emissions, but it adds up across millions of power plants, factories, and vehicles worldwide.
Which Sectors Burn the Most
Three sectors dominate global combustion-related emissions. Electricity and heat production is the largest, accounting for 34% of global greenhouse gas emissions in 2019, almost entirely from burning coal, natural gas, and oil in power plants. Industry follows at 24%, where fossil fuels are burned on-site to power manufacturing, refining, and chemical processes. Transportation contributes 15%, covering everything from cars and trucks to ships and aircraft. Aviation emissions alone surged roughly 5.5% in the most recent reporting year as global air travel hit record levels.
Together, these three sectors represent nearly three-quarters of all global greenhouse gas emissions, and combustion is the core process behind almost all of it.
Not All Fuels Burn the Same
Different fossil fuels release very different amounts of CO2 for the same amount of energy. Coal is the most carbon-intensive: burning anthracite coal produces about 104 kg of CO2 per million BTUs of energy, while bituminous coal (the most common type) produces about 93 kg. Petroleum products like gasoline and diesel fall in the middle, releasing around 70 to 75 kg of CO2 per million BTUs. Natural gas is the cleanest-burning fossil fuel at roughly 53 kg, nearly half the emissions of coal for the same energy output.
This difference is why switching power plants from coal to natural gas reduces emissions in the short term, even though it still involves combustion. But “lower” is not “low.” Natural gas combustion still adds billions of tons of CO2 to the atmosphere annually.
Black Carbon: The Second-Largest Warming Agent
CO2 isn’t the only warming product of combustion. Incomplete burning, where fuel doesn’t fully react with oxygen, produces black carbon, the dark soot particles you see in diesel exhaust or wood smoke. Black carbon is now considered the second-largest contributor to climate warming after CO2. The particles absorb sunlight directly, heating the surrounding air. When they settle on snow or ice, they darken the surface and accelerate melting by reducing the amount of sunlight reflected back to space. Black carbon also disrupts cloud formation in ways that reduce the cooling effect clouds normally provide.
Unlike CO2, black carbon stays in the atmosphere for only days to weeks. That means reducing soot emissions from diesel engines, cookstoves, and industrial processes could produce rapid cooling benefits, even as the longer-term work of cutting CO2 continues.
Biomass Burning Is Not Carbon Neutral
Wood and other plant-based fuels are often promoted as climate-friendly alternatives because trees absorb CO2 as they grow, theoretically offsetting what’s released when they burn. The reality is more complicated. Wood pellets and coal release roughly the same amount of CO2 per unit of energy, because while wood has less carbon per kilogram, it also contains less energy (17 megajoules per kilogram versus 29 for hard coal). On an energy basis, the carbon intensity is nearly identical.
The critical question is how fast forests regrow. If coal power plants switch to burning wood pellets, the “carbon debt” from harvesting and burning those trees takes an estimated 21 years of forest regrowth to pay back. If natural gas is replaced with biomass, the debt isn’t repaid even after 90 years. Secondary forests recovering from harvest regain only about 24% of original forest carbon stocks after 20 years. If burning outpaces regrowth, or if the harvested land isn’t replanted, biomass combustion can actually produce higher long-term atmospheric CO2 concentrations than fossil fuels.
How Combustion Triggers Additional Warming
The warming caused by combustion doesn’t stop with the gases that come out of a smokestack or tailpipe. Rising temperatures set off feedback loops that release additional greenhouse gases from natural sources. The most significant involves permafrost, the permanently frozen ground across Arctic regions that stores enormous quantities of carbon. As combustion-driven warming thaws permafrost, microbes begin breaking down that ancient organic material and releasing methane, a greenhouse gas about 80 times more potent than CO2 over a 20-year period.
High-latitude methane emissions currently total roughly 35 to 45 teragrams per year. Modeling studies project this could increase to 41 to 70 teragrams per year as permafrost thaws. Deep permafrost layers, some containing carbon tens of thousands of years old, could contribute an additional 14 teragrams of methane annually if warming continues, representing a 40% increase over the current natural methane source from these regions. This creates a self-reinforcing cycle: combustion warms the planet, warming releases more greenhouse gases from permafrost, and those gases cause further warming.
The Scale of the Problem
Since the mid-1800s, the widespread adoption of combustion, first in steam engines and factories, then in automobiles and power plants, has increased atmospheric CO2 by more than 50%. The current concentration of about 430 ppm is rising by roughly 3 ppm per year, based on the most recent weekly measurements from NOAA’s Global Monitoring Laboratory. The 1.1°C of warming this has produced so far may sound modest, but it represents a massive increase in the total heat energy circulating through oceans, ice sheets, and the atmosphere.
Combustion remains so central to global warming because it is so deeply embedded in modern life. Generating electricity, manufacturing goods, moving people and freight, and heating buildings all rely overwhelmingly on burning fossil fuels. Reducing emissions means either replacing combustion with other energy sources like wind, solar, and nuclear power, or capturing the CO2 before it reaches the atmosphere. Both approaches are scaling up, but combustion still supplies the vast majority of the world’s energy.