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I’ve heard the Carbon-14 in the atmosphere proves that fossil fuels are the cause of climate change. How?

The mix of different types of carbon in an object gives clues to its history. The carbon from fossil fuels has a unique “fingerprint”—free of Carbon-14—that we can see changing the makeup of the atmosphere. 

 

October 17, 2024 

Today, we know the atmosphere has much more carbon dioxide (CO2) than it did in the recent past, and that the buildup of this heat-trapping gas is the main driver of climate change. We know this because we can directly sample the air and measure the amount of CO2 in it—and even find samples of much older air for comparison, frozen in Antarctic ice

It stands to reason that this CO2 comes from burning the fossil fuels coal, oil and gas. These fuels are composed mostly of carbon. That carbon is released into the air when they’re burned, and humanity has been burning massive amounts of fossil fuels over the same period the planet has been warming. But can we prove it? In fact, the carbon in our atmosphere shows telltale signs of originating from fossil fuels, thanks to one atom: “Carbon-14.”

Carbon comes in several forms, or “isotopes.” The most common by far is Carbon-12, which has six neutrons and is “stable,” meaning it doesn't degrade over time. Carbon-14, the rarest of carbon isotopes, has two extra neutrons, which make it unstable and radioactive. It takes roughly 5,700 years to break down, after which it turns into nitrogen.

That’s a short time in the history of the world, and if we weren’t getting extra Carbon-14 from somewhere, it would all long since have disappeared. But we do get new Carbon-14: tiny amounts of it are constantly made from nitrogen in our atmosphere, interacting with cosmic rays.

“Normally, you have an approximate steady state between the rate at which the cosmic rays form Carbon-14 and the rate at which it decays on Earth,” says Ed Boyle, a professor of ocean geochemistry at MIT who studies the evolution of Earth’s climate. In other words, unless something else is changing our atmosphere, the amount of Carbon-14 in the air should stay roughly the same.

These facts make Carbon-14 a useful “tracer” molecule, which researchers can use to learn the age of objects containing carbon, which includes almost every object on Earth. Consider a tree used to build an ancient house. When the tree was living, it took in Carbon-14 from the air—but after it was cut down, it stopped bringing in new Carbon-14. From that point on, the Carbon-14 in the wood breaks down at a predictable rate, half of it disappearing every 5,700 years. By measuring the ratio of Carbon-14 to Carbon-12 in the wood, an archaeologist can learn how old the house is. This tool is called radiocarbon dating, and in addition to wooden artifacts, it also helps scientists date bones, fossils and rocks.

In radiocarbon dating, you can think of Carbon-14 as a sort of gradually fading dye, added to a pool of clear water. The older the water, the less color it contains.

What if we tried to measure the atmosphere this way? You wouldn’t expect this to tell us much: unlike a dead tree, the atmosphere is constantly getting new Carbon-14. But over the last four decades, the ratio of Carbon-14 in the atmosphere has been falling anyway:1 our atmospheric “pool” keeps getting clearer. 

Since the atmosphere wouldn’t lose Carbon-14 simply with age, the pool must be clearing up another way: somehow, the air is gaining Carbon-12. In fact, the amount of carbon in the atmosphere is currently rising by over 4.5 billion tons a year,2 with very little Carbon-14 in the mix.

And this is a huge clue to where the carbon is coming from. If it came from plants, or ocean circulation, or soils, we would expect it to contain some Carbon-14, since these sources all interact regularly with the atmosphere and can incorporate new carbon molecules. But carbon from fossil fuels looks quite different. Fossil fuels have spent millions of years buried deep underground, and any Carbon-14 they did contain decayed away a long time ago. Burning them today releases mostly “clear” Carbon-12, and no “dyed” Carbon-14 at all.3 

“It's basically just pouring more water into the pool, but the new water doesn't have Carbon-14 in it,” says Boyle. “Now, if you were to stop burning fossil fuels entirely, the balance would get restored eventually, but it would take centuries for that to happen.” 

(There is one wrinkle in the consistent downward trend in Carbon-14: nuclear weapons testing in the mid-20th Century released a huge amount of radioactive Carbon-14. The story we’ve told above controls for that sudden burst of Carbon-14, and its gradual fading away in the decades since.)

Carbon-14 has more to teach us about climate change, says Boyle. Because Carbon-14 can act as a tracer, ratios of carbon in the air and the ocean can help us understand the natural carbon cycle and how quickly carbon moves through it. Carbon moves at different speeds through different parts of the deep and surface oceans, and past, natural episodes of climate change have been hugely influenced by changes in ocean currents that caused more or less carbon to travel between sea and air. “Exchanges between those two are an important part of the climate system,” Boyle says. “We can use Carbon-14 to help estimate the rates at which these processes were happening in the past.”

 

Thank you to Peter Penoyer of Fort Collins, Colorado for the question.

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Footnotes

1 Graven, Heather, Ralph F. Keeling and Joeri Rogelj, "Changes to Carbon Isotopes in Atmospheric CO2 Over the Industrial Era and Into the Future." Global Biogeochemical Cycles, Volume 34, Issue 11, 2020, doi:10.1029/2019GB006170.

2 Friedlingstein, Pierre et. al., "Global Carbon Budget 2023." Earth System Science Data, Volume 15, Issue 12, 2023, doi:10.5194/essd-15-5301-2023

3 The story is a bit more complicated than this: fossil fuels also contain less of the isotope Carbon-13 than the atmosphere does, a sign that this carbon comes from plants and algae. (Which, indeed, are the raw ingredients of coal, oil and gas.) Together, the ratio of the three carbon isotopes make up a sample’s unique “isotopic fingerprint,” and the carbon entering the atmosphere today bears the telltale print of fossil fuels.