The Washington PostDemocracy Dies in Darkness

Fossil fuel emissions are making carbon dating more difficult

Radiocarbon dating revealed that the Turin Shroud, the sacred linen cloth believed by Christians to be the burial garment of Christ, was actually less than 1000 years old. But the “revolutionary” technique may now be threatened by fossil fuel emissions. (Claudio Papi/Reuters)

Nearly three decades ago scientists were granted access to one of the world’s most mystifying and sacred objects: the Shroud of Turin. The ancient rectangle of linen, with its strange stains in the shape of a tortured body, had long been venerated as the burial garment of Christ. But the shroud’s origins were murky, and researchers had spent decades poring over the piece of fabric debating whether the story of its background could be true.

In 1988, thanks to a technique called radiocarbon dating, they had an answer: The shroud dated back to sometime between 1260 and 1390 — old, but not old enough to have been buried with Jesus.

“The Carbon-14 Bombshell,” National Geographic called the news, referring to the radioactive isotope that’s used for the dating process.

Carbon dating had never been, and likely never again will be, quite so glamorous — or so controversial. And, thanks to atmospheric changes caused by the burning of fossil fuels, it could become even more complicated.

That’s according to a new study in the Proceedings of the National Academy of Sciences published Monday. Physicist Heather D. Graven of Imperial College London found that carbon emissions from fossil fuels are artificially raising the carbon age of the atmosphere, making objects today seem older to a carbon dater. By 2050, new clothes could have the same radiocarbon date as something that’s ten centuries old.

The carbon dating process works by measuring amounts of carbon-14, an isotope (or variation) of ordinary carbon. Carbon-14 is produced in the atmosphere and ingested by plants during photosynthesis, which means that it eventually makes its way into most organic materials, be they bone fragments or a wooden desk.

Since carbon-14 is radioactive, it decays at a predictable rate every year starting as soon as the organism that contains it dies. This means that scientists can use the amount of carbon-14 left in an object to figure out how long ago it stopped living.

The development of this process in the 1940s was so influential it’s been called the “Radiocarbon Revolution.” For the first time, archaeologists were able to directly date objects, rather than simply calling them “older” or “younger” based on where they had been found.

When we burn fossil fuels like coal, we’re actually burning very ancient organic material — stuff that has lost all its carbon-14 over the course of millennia. The resulting infusion of “dead” carbon into the atmosphere in the past 125 years has artificially diluted the proportion of carbon-14. This means that plants and animals living today contain relatively less carbon-14 than those of centuries past.

Graven expects that the change will start impacting the carbon dating process by 2020.

“If we did any current measurements on new products, they will end up having the same fraction of radiocarbon to total carbon as something that’s lost it over time due to decay,” she told the BBC.

This shift won’t render carbon dating obsolete — it’s long been known that atmospheric carbon can fluctuate, and scientists are able to re-calibrate their estimates based on modern levels. But it does make the process more complex and less reliable for dating relatively young objects. If emissions continue at their current rates, Graven believes that carbon dating won’t be able to provide a definitive age for anything less than 2,000 years old.

That’s not great news for forensic scientists, as Gizmodo points out, since most investigators “might really like to know if human remains buried in the woods are a couple of decades old or a few centuries.” But researchers have other dating tools at their disposal — radiometric techniques using other isotopes, a process called “amino acid racemization” that looks at how a dead organism’s amino acids have changed.

Meanwhile, the more humans emit, the more skewed radiocarbon dates are becoming. By 2100, a newly made object would have a carbon date indistinguishable from something made 2,000 years earlier. It’s a strange example of how burning fuels is changing the way the present looks in relation to the past down to the atomic level.