Radiocarbon dating is an incredibly useful technique. It has been used to date objects from within approximately the last 60,000 years, revolutionising archaeology and finding uses in everything from detecting fake wine vintages to identifying illegal ivory.

Unfortunately, humanity's reliance on fossil fuel combustion may ultimately mean that our species can’t have nice things like handy dating techniques. We’re releasing ancient carbon—in which the carbon-14 has long since undergone radioactive decay—into the atmosphere at such a rate that living organisms are absorbing less carbon-14, making them look old in the eyes of carbon dating.

If we keep going at our current rate, we’ll hit a point in just 35 years where it’ll no longer be possible to tell the difference between modern objects and those that were on this fair Earth 1,000 years ago. In 85 years, we'll no longer be able to use radiocarbon dating to tell whether a sample is modern or from 2,000 years ago.

The best case scenario, writes Heather Graven in this week’s issue of PNAS, is that we implement ambitious plans to reduce fossil fuel combustion, and slow the carbon ageing of the atmosphere, avoiding the most extreme effects (and, you know, solving that whole climate change thing). Even in this scenario, though, there are fields that use carbon dating to look at quite recent changes, meaning that they would still be rendered useless by the changing atmosphere.

Burning coal gives the planet wrinkles

Carbon, like many elements, has a number of different isotopes, which are forms of the same element with different numbers of neutrons. Carbon-14 is a naturally occurring radioactive isotope that, over time, decays to a more, non-radioactive stable form. Cosmic rays convert nitrogen into carbon-14 so that, even though it’s decaying all the time, the atmosphere doesn’t run out of it.

While organisms are alive, they interact with the atmosphere and other organisms, absorbing the prevailing levels of carbon-14. When they die, this interaction cuts off, leaving their carbon-14 levels to decay over time without being replenished.

This is what radiocarbon dating relies on: we know the rate at which carbon-14 decays, so by looking at the levels of carbon-14 in a sample, we can tell when something was alive. This works for the remains of plants and animals, as well as anything containing organic matter, like human artefacts such as clothes that were made from plants.

Fossil fuels, of course, are very, very old organic matter—so old that they have practically no carbon-14 left. So when we burn them, we dilute the amount of atmospheric carbon-14, meaning that present-day organisms have lower levels of carbon-14 than we might otherwise expect.

Because dead organisms also have low levels of carbon-14, the result is that organisms living today, interacting with today’s atmosphere, will have the same levels as dead organisms from long ago.

The fallout

If we keep going at this rate, writes Graven, “by 2050, fresh organic material could have the same [carbon-14 levels] as samples from 1050, and thus be indistinguishable by radiocarbon dating.” By 2100, living organic material will have the same readings as items from 100 AD. That means, hypothetically, that any fraudster could stitch up a robe and claim it dates from the early days of Christianity, and radiocarbon dating wouldn’t be able to tell the difference.

For certain uses of radiocarbon dating, and certain samples, this isn’t a problem. If radiocarbon dating places an item at 2,000 years ago, and it was found alongside piles of artefacts that can be dated to that period by other methods, the evidence still stacks in the normal way. For findings without this kind of context, though, the loss of the technique will be a bigger issue.

Other uses of radiocarbon dating that have fewer options for complementary evidence could be more strongly affected, even if we manage to slow down our carbon emissions dramatically. With the most ambitious environmental policies we could envision, carbon-14 levels around the year 2100 would look like they did just before the Industrial Revolution. Scientists who use carbon dating at more fine-grained levels, like those studying the age of human cells, will still have a gap even if we put the brakes on our emissions really fast.

Ultimately, Graven writes, this means that we're going to need to reassess our dating methods no matter how successful our environmental policies are. Improving precision in radiocarbon dating itself, to enable detection of even the tiniest differences in levels of decay, is one step we could take.

Greater precision is unlikely to fill the gap by itself, though, meaning that many scientists will need to develop alternative measurement techniques just to keep doing what they're currently doing. The impacts of this, as Graven writes, will be far-reaching.

PNAS, 2015. DOI: 10.1073/pnas.1504467112 (About DOIs).