December 16, 2016, by Tim Radford
Skeleton identified as a Roman inhabitant of Winchester in England almost 2,000 years ago.
Image: Fred Dawson LRPS via Flickr
Emissions causing global warming will make radiocarbon dating much less accurate, but a German scientist has found a possible solution.
LONDON, 16 December, 2016 – Archaeologists can breathe again. The science of radiocarbon dating – which can confirm the date of a Stone Age burial or the pollen preserved in a dried-up lake – could be reliable for a while yet.
A German scientist reports that although the technique’s value looked like becoming a casualty of global warming, there is a way of checking whether the radiocarbon reading is right.
But first, the problem. Living things build their tissues, directly or indirectly, from atmospheric carbon dioxide.
This important greenhouse gas exists in two forms: an isotope called carbon-12 and a radioactive isotope called carbon-14, which decays at a predictable rate. Carbon-14 is formed in the atmosphere at a constant rate by assault from cosmic radiation.
To age a piece of contraband ivory, or medieval linen, or a body in a Roman graveyard, archaeologists measure the ratio of the two isotopes.
It has so far provided a clock accurate for up to 50,000 years. But as soon as humans started digging for coal and drilling for oil, they introduced a complication.
Fossil fuels were once living tissue, and the carbon dioxide emitted as they burn might be described as “pre-aged”, because all the carbon-14 has decayed.
The release of the gas from car exhausts and power station chimneys has already begun to alter the natural ratio in the atmosphere. Today’s plants will already seem artificially aged, according to the radiocarbon clock. This is known as the “Suess effect”, named after the Austrian physicist Hans Suess.
And a scientist pointed out in 2015 that this effect will become more pronounced. If the world continues to burn fossil fuels at the present prodigal rate, not only will sea levels rise and climates change, but forensic scientists will lose a valuable tool.
“A tree felled within the next century, when measured
using radiocarbon dating, will appear to be the same
age as wood that is several thousand years old”
Peter Köhler, a physicist at the Alfred Wegener Institute in Bremerhaven, Germany, now thinks he may have an answer.
Living tissue also incorporates the stable isotope carbon-13, and this could provide a guide as to the trustworthiness of any reading − even under the notorious “business-as-usual” scenario for greenhouse gas emissions.
“In 2150, new samples will appear to be the same age as 3,000-year-old carbon, and in extreme cases even the same as 4,300-year-old material,” says.
“This means that fresh samples − for example, of a tree felled within the next century − when measured using radiocarbon dating will appear to be the same age as wood that is several thousand years old.”
But he reports in Environmental Research Letters journal that the same Suess effect will also distort the ratio of carbon-13 to carbon-12. This at least offers a check on the likelihood of a false reading from radiocarbon dating..
“If my measurement shows a distorted 13C signal, then this also tells me that the 14C-based age has been affected by fossil carbon,” Dr Köhler says. “If, on the other hand, my 13C signal is within the expected range, then the fossil carbon has not had an effect and the 14C dating method shows the correct age.”
He used a climate change simulation to test his argument up to the year 2500. In all simulations, the carbon-13 could be used as a guide to reliability.
It may not be perfect. There might be problems reading the data from the deep oceans, where exchange between atmosphere and ocean is slow.
And if humans ever do successfully switch to growing biomass for energy, and at the same time learn to capture the carbon dioxide exhaust for underground storage, that too could create what Dr Köhler calls methodological difficulties for the archaeologists. – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.