Experts who measure the content of carbon dioxide in the atmosphere marked a turning point: The strong combustion of fossil fuels has meant that the composition of the carbon isotopes in the air in the northern hemisphere has changed. Since 2021, the isotope relationships have shifted in such a way that the amount of 14c, which had also reached the atmosphere by nuclear waffle tests of the 1950s and 1960s, is now leveled by the high CO2 emissions.
The problem is that dating methods such as the radiocarbon method could become useless. Chemical physicist Heather Graven from Imperial College London has been observing this effect for several years. Modern objects would provide different 14C data, relegating the result to the early 20th century rather than to the end. "Soon it could be difficult to determine whether something is 1,000 years old or modern," says Paula Reimer, a specialist in radiocarbon dating at Queen's University in Belfast. Although the age or origin of objects can also be determined by the context of the find, this would not apply to scattered finds. Likewise, it would not be so easy to decide whether human remains are centuries old or come from a recently deceased person.
The development would also mean that forensic technicians would no longer be able to determine the age of materials such as ivory, wine or antiques. "Anyone who works in forensics or detects fakes is experiencing a really sad moment right now," says Tom Higham, an archaeologist at the University of Vienna.
How does the radioco method by bells operate?
Radiocarbon dating is based on the presence of different isotopes of carbon in the atmosphere. The most common is stable carbon-12, but in very small quantities also radioactive carbon-14, which is produced under the influence of cosmic rays. Over the past millennia, however, the proportion of 14C has fluctuated, and strong volcanic eruptions, for example, change the content.
Both types of carbon isotopes accumulate in living beings. Plants absorb them with the carbon dioxide of the air during photosynthesis. They then enter humans and animals via the food chain. When an organism dies, it stops absorbing carbon isotopes. From this point on, the stored 14C decays – with a half-life of about 5700 years. Since 12C remains stable and 14C decays, their ratio changes over time. This is exactly what experts measure to determine age: how much of both isotopes can be found in a sample reveals how much time has elapsed since the death of a living being to this day.
In this way, organic materials such as wood, textiles or bones can be dated. At least up to the age of approximately 55,000 years. In the case of older objects, the 14c quantities are too low for precise evidence. Because the lower the 14C content, the older the material is.
What role does the nuclear weapons effect play
A large amount of carbon has now been released in nuclear weapon tests between 1952 and 1962. With this a few years, the amount of carbon-14 doubled in the air-the so-called nuclear weapon effect. Since then, living beings and the oceans of this 14c have been stored. In parallel, a lot of CO2 has escaped into the atmosphere due to the combustion of fossil fuels. Coal and oil are several million years old, so there is no 14c in them. However, 12C escapes when burning, which in turn reduces the proportion of 14c in the air.
Since 2021, the following has now been the case: the two effects – nuclear weapons tests and carbon dioxide emissions – have cancelled each other out in the Northern Hemisphere. This means that the ratio of 12C and 14C approximately corresponds to the conditions of the pre-industrial period. And since fossil fuels are still being burned, the proportion of carbon-14 in the air will continue to decrease. The result: measurements provide increasingly higher ages. According to Graven, by the year 2050, the 14C share will be as similar as in the Middle Ages.
Previous fluctuations in the carbon-14 content in air are already causing difficulties. Because when dating the samples, the results cannot always be separated enough. Why is that? Due to the fluctuating concentration of the carbon isotope in the atmosphere, 14C dating must be calibrated. This uses wood samples that can also be dated with the help of dendrochronology, i.e. the counting of annual rings. So not only the absolute age is known through dendrochronology, but also the corresponding isotope relationship of that time.
But calibration doesn't always work. Around 800 to 400 BC, the so-called Hallstatt Plateau, "you literally can't date anything [exactly]," says Higham. The fact that fossil fuels are now also destroying the effect of nuclear weapons will cause additional problems.
How to Date Modern Objects Using Carbon-14
Because experts measure not the decay of carbon-14 in modern organic objects that were created between 1960 and 2020, but the amount of the isotope. In this way, Higham has tracked down fake whiskey and dated Chinese tea; The method works for groundwater or human cells.
Researchers have long known that the end of the 14C method is imminent. But rising CO2 emissions have accelerated the process. "It's a shame," says Higham. "A wildlife forensics tool is losing its effectiveness," adds paleoecologist Kevin Uno of Columbia University in New York. He used the method to date ivory samples and investigate elephant poaching. "It's kind of depressing."
But the search for new methods has long since begun. "Maybe there is another radionuclide that we can use," says Uno.
