Emissions threaten age of uncertainty for carbon dating

New study warns that rising CO2 levels from the burning of fossil fuels will undermine the precision with which once-living things can now be scientifically dated.

Climate change driven by increasing levels of atmospheric carbon dioxide will not just damage the health of the planet. A UK scientist now warns that it will also make life increasingly difficult for archaeologists, forensic scientists, art experts, fraud and forgery detectives, and people who detect ivory poachers.

That is because the swelling volume of carbon pumped into the atmosphere from factory and power station chimneys and motor and airline exhausts is beginning to artificially “age” the planet’s atmosphere and bedevil attempts to use the technology known as carbon dating.

If emissions continue under the now-notorious “business as usual” scenario, then by 2050 a brand-new cotton shirt will have the same radiocarbon-dating age as the cloak worn by William the Conqueror when he invaded Britain in 1066.

But if, on the other hand, the world’s governments do move swiftly to curb fossil fuel emissions, then by 2050 a brand-new cotton shirt will seem only 100 years old.

Given current emissions trends, fossil fuel emission-driven artificial ‘ageing’ of the atmosphere is likely to occur much faster and with a larger magnitude than previously expected.

Heather Graven, lecturer in climate physics and Earth observation at Imperial College London

Forensic scientists exhuming a skeleton, Egyptologists investigating an ancient tomb, and fraud detectives concerned with suspected forgeries of Renaissance paintings could still possibly make allowances for that.

Natural ratio

Radiocarbon dating is a 70-year-old technique now used with increasing precision to date anything once alive from the last 50,000 years. It exploits the natural ratio of two isotopes of carbon in the atmosphere.

Plants, and the animals that eat them, absorb radioactive carbon-14 and stable carbon-12 from the atmosphere in proportions which – except during the atmospheric nuclear weapons tests in the 1950s and 1960s − have not changed much from the Ice Ages to the beginning of the Industrial Revolution.

When the tree dies or the animal becomes old bones, the carbon-14 decays at a predictable rate, and the ratio that remains in the laboratory sample is a measure of the specimen’s age.

But Heather Graven, a lecturer in climate physics and Earth observation at Imperial College London, reports in the US journal Proceedings of the National Academy of Sciences that by 2020, as the fossil fuel emissions mount up, the fraction of carbon-14 in the atmosphere could drop to such a level that carbon-dating could become increasingly uncertain.

Fossil fuels are reservoirs of carbon from plants and algae that died so long ago that all the carbon-14 has decayed. When carbon dioxide exhausts from combustion engines reach the atmosphere, they increase the levels of non-radioactive carbon, artificially ageing the atmosphere and, accordingly the new growths that exploit the atmospheric carbon.

Dr Graven warns that now, from the point of view of an archaeologist using radiocarbon dating, the planet’s atmosphere is ageing at the rate of 30 or so years for every year of international inaction.

If there are no steps to reduce emissions, then by 2050 the atmosphere will have a signature of what carbon ratios were 1,000 years ago. By 2100, just one human lifetime away, the atmospheric clock will have been turned back to the era of Imperial Rome.

That means that a freshly-dead dung beetle that falls into an Egyptian tomb dating from the reign of Cleopatra would have the same radiocarbon age as the scarab that was trapped in the tomb under the sarcophagus 20 centuries ago.

“Given current emissions trends, fossil fuel emission-driven artificial ‘ageing’ of the atmosphere is likely to occur much faster and with a larger magnitude than previously expected,” Dr Graven concludes.

“This finding has strong and, as yet, unrecognised implications for many applications of radiocarbon in various fields, and it implies that radiocarbon dating may no longer provide definitive ages for samples up to 2,000 years old.”

Historic sources

It also could, for instance, mean that border officials may not be able to distinguish museum collection ivory from illegally-poached elephant tusks, that fraud officers will not be able to confirm the age of costly single malt whisky or vintage claret, and that Jewish and Christian scholars may no longer be able to date important historic sources such as the Dead Sea Scrolls, or resolve doubts about the much-contested provenance of iconic relics such as the Shroud of Turin.

Dr Graven, who uses radiocarbon technology to study the global carbon cycle, told Climate News Network: “I was inspired by how many innovative applications there are for radiocarbon in diverse fields. This made me realise that fossil fuel emissions are likely to have an impact on these various uses for radiocarbon.

“By quantifying the potential changes over this century with model simulations, this study could help other scientists who use radiocarbon to prepare for forthcoming changes.”

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