A T-shirt made in 2050 could look exactly like one worn by William the Conqueror a thousand years earlier to someone using radiocarbon dating if emissions continue under a business-as-usual scenario. By 2100, a dead plant could be almost identical to the Dead Sea scrolls, which are more than 2,000 years old.
These well-known “aging” properties of atmospheric carbon were pinpointed for different emissions scenarios in a paper published in the Proceedings of the National Academy of Sciences yesterday. It describes how fossil fuel emissions will make radiocarbon dating, used to identify archaeological finds, poached ivory or even human corpses, less reliable.
As scrolls, plant-based paints or cotton shirts age over thousands of years, the radioactive carbon-14 that naturally appears in organic objects gradually decays. The amount of carbon-14 decreases relative to the amount of normal carbon. Radiocarbon dating seizes on that fraction, which decreases over time, to estimate age. A lower fraction indicates an older object.
The problem is that the fraction can decrease not only as carbon-14 decays but also as normal carbon increases. That is what is happening with the burning of fossil fuels, which are so old they do not contain any carbon-14. Nonradioactive carbon is now flooding the atmosphere, which creates a dilution effect.
Though this dilution effect is well-known, its precise scale under different emissions scenarios was not, until now. Heather Graven, the atmospheric scientist at the Imperial College London who wrote the paper, was surprised at how much emissions could “age” the atmosphere if pollution continues at its current rate.
“If you think of parts of the deep ocean that are quite old, that have been sequestered for thousands of years, in the business-as-usual scenario, then the atmosphere would have the same radiocarbon fraction as the oldest part of the ocean,” she said. “It really is kind of backward. It’s very, very low.”
The risk to researchers is that the old could become indistinguishable from the new if it is artificially aged by extra atmospheric carbon. The periods of history that archaeologists might confuse with the present-day change based on the different scenarios. In the best-case scenario, which would keep global warming under 2 degrees Celsius by the turn of the century, the extra aging effect would be null.
From high to low
The carbon fraction has already undergone a significant shift because of human activity in the past. Nuclear weapons testing in the 1950s and ‘60s created much more atmospheric radioactive carbon, rapidly increasing the famous ratio, according to NPR. It was the equivalent of making the atmosphere artificially younger.
When the fraction is decreasing, it’s hard to distinguish the new from the old; when the fraction is increasing rapidly, like in the past half-century, it’s much, much easier.
Physicists and bioengineers seized on this opportunity to study cell regeneration in plants and humans. A cell born after nuclear weapons testing would have a different, higher fraction of carbon-14 to normal carbon than one born several decades earlier. This indicator allows biologists to see which cells turn over and which cells remain the same.
For example, in 2009 scientists revealed in Science that about 0.3 to 1 percent of human cardiac muscle cells regenerated each year. They attributed the discovery to the carbon-14 produced during the Cold War.
Now, however, carbon emissions have risen to the point where they’ve countered the initial effect of nuclear weapons testing. Graven shows the present-day levels are close to preindustrial.
If the ratio were to remain constant, like in a low-emissions scenario, scientists wouldn’t be able to use it to measure the lives of individual cells, a technique that requires a rapidly changing indicator. And a decreasing fraction could start affecting radiocarbon dating by 2020, Graven added.
“Fossil fuel emissions are going to impact some of these neat applications, no matter what,” she said. “Now it’s a question of when it will happen and how low it will go.”
She hoped her models would fill that gap and help scientists adjust their calculus to different possible carbon scenarios.
“If they’re using radiocarbon in the future, they’ll know what to expect,” she said. “They can plan for these changes.”
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500