Sample being removed from human femur bone for carbon dating using accelerator mass spectrometry (AMS).

A small sample is removed from a piece of bone for carbon dating.Credit: James King-Holmes/SPL

The researchers who track the ever-rising levels of atmospheric carbon dioxide have charted a landmark moment. As of 2021, the burning of fossil fuels has officially shifted the composition of carbon isotopes in the air of the Northern Hemisphere enough to cancel out a useful signal from nuclear-weapons testing1.

This could cause problems for valuable carbon-dating techniques. Modern items now look like objects from the early twentieth century in terms of radiocarbon dating, says Heather Graven, a chemical physicist at Imperial College London who has been charting this effect for years. The trend “could soon make it difficult to tell if something is 1,000 years old or modern”, says Paula Reimer, a radiocarbon-dating specialist at Queen’s University Belfast, UK. Although there are usually other clues to an object’s provenance, “there are often stray finds without that information”, says Reimer, such as unidentified human remains that might come from a historic burial site or from a person who died recently.

Altered carbon: Line chart showing the proportion of radioactive carbon-14 in atmospheric carbon dioxide since 1950.

Source: Ref 1.

The development also means that forensic scientists will no longer be able to use radiocarbon fingerprints to pinpoint the ages of materials such as ivory, antiques and wine. “If you’re working in forensics or detecting fakes, this is a really sad moment,” says Tom Higham, an archaeologist at the University of Vienna.

Time machine

Carbon-dating techniques rely on the fact that there are several isotopes of carbon in the air. Stable carbon-12 is the most common. But there is also a small amount of radioactive carbon-14, which is generated mainly when cosmic rays interact with the atmosphere. The proportion of carbon-14 varies naturally over time.

Living things absorb both types of carbon. After they die, the relative amounts of the two isotopes start to change as the radioactive carbon-14 decays with a half-life of 5,700 years. By measuring how much carbon-14 is left in an object, researchers can date organic materials, such as wood, fabric or bone, that are up to about 55,000 years old. Typically, the smaller the ratio of carbon-14, the older the material.

Between 1952 and 1962, the testing of nuclear weapons released a spike of ‘bomb carbon’ that quickly doubled the amount of carbon-14 in the air. Since then, that carbon-14 has been slowly absorbed by living things and the ocean. At the same time, the burning of fossil fuels has been rapidly releasing CO2 that does not contain carbon-14.

As of 2021, these two effects have officially cancelled each other out in the Northern Hemisphere (see ‘Altered carbon’). This means that the carbon-14 ratio in modern materials is now the same as in those from pre-industrial times. And because fossil fuels are still being burnt, the proportion of carbon-14 in the air will lessen further, mimicking conditions even further in the past. By 2050, Graven predicts1, the carbon-14 ratio will be similar to what it was in the Middle Ages (between the fifth and fifteenth centuries).

When there are wiggles and spikes in the proportion of carbon-14 in the air over time, radiocarbon dating can’t always distinguish one date from another. This is true for the period of 800 bc to 400 bc, for example: “You literally can’t date anything [precisely] within that 400 years,” says Higham. The phenomenon of fossil fuels cancelling out bomb carbon provides yet another opportunity for radiocarbon confusion.

Bye-bye bomb curve

For modern objects that are just decades old, the decay of carbon-14 is negligible. But the rapid spike in carbon-14 released by nuclear weapons has created a diagnostic ‘bomb curve’ of carbon-14 levels. “It’s the silver lining of bomb testing,” says Higham. This means that the amount of carbon-14 in an object can provide a precise time stamp for organic materials formed between around 1960 and 2020. Higham has used it to detect forged whisky and date Chinese tea, among other things; the technique has been used on everything from groundwater to human cells2.

Researchers have long known that the end of this technique was coming, but increasing CO2 emissions have accelerated the process. In the coming decades, as fossil-fuel use wanes and the bomb curve flattens, the carbon-14 value will no longer be diagnostic of a date. “It’s such a shame,” says Higham.

“This wildlife forensics tool; the window is closing on its effectiveness,” says palaeoecologist Kevin Uno at Columbia University in New York City, who has used the bomb curve to date ivory samples and study elephant poaching3. “It’s kind of depressing.”

The demise of the bomb curve means that researchers will increasingly have to rely on other techniques or isotopes to do their dating, including a third type of carbon, carbon-13. “There may be some other radionuclide we can use,” says Uno.