Researchers have pinned the age the Earth to around 4.54 billion years old. The earliest evidence of the genus Homo dates back to 2.8 million years ago and the oldest artwork was created about 40,000 years ago. All these dates come from radiometric dating — a process that looks at different isotopes in samples. Since some isotopes decay faster than others, the ratio between isotopes can provide a date. Most samples from early human history are dated using carbon isotopes, but that method has a problem, reports Adrienne LaFrance for The Atlantic, and that problem is getting worse.
Carbon atoms in the air end up in all sorts of organic material: plants draw in carbon dioxide, animals and humans eat the plants and the carbon ends up building tissues, including the isotope carbon 14 which is unstable. As soon as a plant or animal dies, it no longer incorporates new carbon 14, and the atoms already present start to decay to the nonradioactive isotopes carbon-12 and carbon-12. Older remains have less carbon 14 as a result.
But the fossil fuels that humans are burning and the subsequent carbon dioxide they are releasing, will skew the radiocarbon age of any new organic material created today. Fossil fuels come from old organic material that has already depleted its carbon 14 and as a result new organic material appears older than it is. New carbon-14 is created by cosmic rays bombarding the atmosphere, but that process isn’t keeping up with emissions.
"With fossil-fuel emissions increasing at current rates, within the next 20 to 30 years it will be difficult to distinguish newly produced materials from historical artifacts several hundred years old using radiocarbon dating techniques," researcher Heather Graven told Liz Kalaugher for Environmental Research Web. Graven published work in Proceedings of the National Academy of Sciences that explains that by the year 2100, with forecasted emissions, the atmosphere will have a radiocarbon age of 2,000 years old. LaFrance explains the implications for The Atlantic:
If Graven's calculations are correct, carbon dating as we know it today will no longer be reliable by the year 2030. Which means scientists won’t be able to use carbon dating to distinguish between new materials and artifacts that are hundreds or thousands of years old. (Carbon dating is already limited in scope because older artifacts have to be dated using other methods. For instance, Lucy, the 3.2-million-year-old human ancestor, was dated by scientists who studied the volcanic flows and ashes in deposits where her bones were found.)
Instead, researchers will need to look for other clues to figure out if finds are modern or ancient. They will have lost an important tool.