Why Do Fossil Fuels Have So Much Carbon Stored in Them?

Mankind's crucial fossil fuels—coal, natural gas and crude oil—all unfortunately contain large amounts of carbon. But why is that and where does it come from?

When fossil fuels are burned they produce carbon dioxide (CO2)—a greenhouse gas that traps heat in the atmosphere, contributing to global warming and climate change. Carbon dioxide is not the only greenhouse gas in the atmosphere, but it's the most abundant and important one, accounting for about 66 percent of the warming effect on the climate, figures from the World Meteorological Organization show.

Since the Industrial Revolution, atmospheric concentrations of CO2 have been rising significantly as a result of human activities. While there are natural sources of carbon dioxide emissions—such as naturally occurring wildfires, volcanoes and decomposing biomass—which dwarf human-related emissions, our activities have thrown the Earth's natural carbon cycle out of balance.

Natural carbon sinks like plant photosynthesis and absorption into the ocean remove roughly the same amount of CO2 from the atmosphere as is emitted by natural sources. But as humans have produced more and more carbon dioxide, concentrations of the gas in the atmosphere have been rising faster than they can be removed by natural sinks.

Around 90 percent of human-produced CO2 emissions are the result of the burning of fossil fuels, with the remainder linked to deforestation and other land use changes, as well as certain industrial processes, such as cement manufacturing.

Why Do Fossil Fuels Have So Much Carbon Stored in Them?

Fossil fuels are generally formed when organic material like plankton (in the case of oil and gas) and terrestrial plants (in the case of coal) are buried and decompose either in marine environments or in swamp- or peat-bog like environments, respectively. This process creates carbon-rich deposits that are extracted and burned for energy.

"Once you bury these organic materials and start to heat them up and add pressure to them, you convert these originally organic materials to hydrocarbons like oil and gas or solid materials like coal over probably millions of years," Simon Jowitt, an associate professor in the Department of Geoscience at the University of Nevada, Las Vegas, told Newsweek.

The carbon in fossil fuels was originally carbon dioxide in the air that was absorbed by plants and animals.

"The hydrocarbons are the part that could not escape back into the air," Robert McLachlan, a professor in the School of Mathematical and Computational Sciences at Massey University, New Zealand, and author of the Planetary Ecology climate change blog, told Newsweek.

The living creatures or plants that were eventually transformed into fossil fuels by geological processes contained carbon in the form of organic molecules originally created by photosynthesis—or in the case of zooplankton (tiny, free-floating aquatic animals), from the consumption of the phytoplankton (microscopic, free-floating, aquatic, plant-like organisms) that generated their organic molecules by photosynthesis.

What's the Most Abundant Fossil Fuel?

Coal is the most abundant fossil fuel in the world—and indeed in the United States—by some distance. At current rates of use, there are at least 300 years of coal left on Earth, perhaps even more, Jowitt said.

"Oil and natural gas reserves are lower but are still somewhat plentiful, especially if they can be extracted by fracking, etcetera," he said.

But it is worth noting that the quantity of fossil fuels in known reserves is not fixed and can fluctuate over time.

"We find more sources of coal, oil, and gas every year, and the estimates are also sensitive to the price of fossil fuels," Paul Kempler, a research assistant professor at the Oregon Center for Electrochemistry, told Newsweek.

Fossil fuels store their energy in the chemical bonds that hold the hydrocarbon molecules together.

"That energy can stay trapped like that right up until the moment the hydrocarbons are burned with oxygen (O2) and broken into CO2 and water (H2O) while releasing heat," Chulsung Bae, a professor in the Department of Chemistry and Chemical Biology and the Department of Chemical & Biological Engineering at Rensselaer Polytechnic Institute, told Newsweek.

This energy originally came from the sun and was used by plants and animals for their own needs. Coal has a lower energy density than crude oil, petroleum, and fuels like diesel—all of which have lower energy densities than natural gas and methane.

"It's like comparing the calories in say broccoli to butter; you can have a pound of each, but the chemistry of the butter is better at storing energy than the chemistry of the broccoli," Jowitt said.

How Much Fossil Fuel Is Burned Every Day in the World?

In 2021, the world consumed nearly 97 million barrels of oil per day, figures from BP's Statistical Review of World Energy published this year indicate.

The report also shows that the world consumed around 4 trillion cubic meters of natural gas per year, or around 11 billion cubic meters per day.

Meanwhile, data from the International Energy Agency indicates that humans get through around 8 billion tons of coal per year, or nearly 22 million tons per day.

All fossil fuels contain carbon, an abundant chemical element referred to by the symbol "C." Natural gas, for example, contains a lot of methane, which has the chemical formula CH4—in other words it contains one carbon atom and four hydrogen atoms.

"When you burn these fossil fuels, you essentially combine these elements with oxygen—remember you can only generally combust or burn materials when oxygen is present," Jowitt said. "This splits the methane apart and forms new molecules, in this case CO2 and H2O, or water. Hence the combustion of methane, or any fossil fuel, generates carbon dioxide."

This process also produces a significant amount of heat, as well as impurities: gases like sulfur, carbon monoxide if combustion is not complete (as is common with coal), nitrogen oxides or NOx (which can be very harmful to humans in that it can cause asthma attacks), and particulates (which can cause respiratory issues).

Sulfur and nitrogen in the atmosphere can also combine with oxygen and hydrogen to form sulfuric or nitric acid, generating acid rain.

"All in all, not very good, and coal is particularly bad at containing these other contaminants," Jowitt said.