​Study: Rising Fossil Fuel Emissions Will Screw With Carbon Dating

There are many reasons why we need to cut down fossil fuel emissions: pollution, climate extremes, irreparable damage to our natural world. Now there's another to add to that list: our emissions are messing with radiocarbon dating.

A study published in PNAS on Monday claims that increasing fossil fuel emissions will throw off analyses of carbon isotopes, which are used in radiocarbon dating—a method of measuring the age of an object by analysing how much radioactive decay it has undergone, also known simply as carbon dating.

Author Heather Graven, a lecturer in climate physics and Earth observation at Imperial College, writes that, "Given current emissions trends, fossil fuel emission-driven artificial 'aging' of the atmosphere is likely to occur much faster and with a larger magnitude than previously expected" and that her findings imply "that radiocarbon dating may no longer provide definitive ages for samples up to 2,000 [years] old."

By 2050, it may be impossible to rely just on radiocarbon dating to tell apart a brand new t-shirt and a thousand-year-old robe.

Graven explained how fossil fuel emissions could throw measurements off. "Radiocarbon is a radioactive form of carbon and it decays over thousands of years, so when we measure radiocarbon we're actually measuring a fraction of radiocarbon to total carbon," she said. "After radioactive decay, this fraction can decrease, and that's what's used in radiocarbon dating." It follows that the less radiocarbon is left in a sample, the older it is.

"But the fraction can also decrease if carbon is added that doesn't have any radiocarbon in it," Graven continued.

As fossil fuels are millions of years old, they don't have any radiocarbon left—it's already decayed. So their emissions are now effectively diluting the amount of radiocarbon in the atmosphere to begin with, and the more we pump out, the larger the discrepancy. What that means is that a new object could end up with a similar fraction of radiocarbon as a much older object that has gone through some decay.

It's not a totally new problem—the comparative amounts of radiocarbon in the air were also affected by the industrial revolution, when fossil fuel emissions increased. After that, human actions had the opposite effect on the levels of radiocarbon in the atmosphere: nuclear weapons testing in the 50s created higher levels of atmospheric radiocarbon.

Now, fossil fuels are taking us back. "Soon, the fraction of radiocarbon will drop below the normal preindustrial level," said Graven.

Her paper focuses on the next century; by the year 2050 she suggests that if we continue consuming fossil fuels at the same level of growth, levels of radiocarbon in the atmosphere will appear the same as after 1,000 years of radioactive decay; by 2100 the atmosphere could "age" 2,000 years.

To go back to the t-shirt example, the cotton used to produce it would come from a plant that takes in carbon from the atmosphere owing to photosynthesis. Given the much lower proportion of radiocarbon in the atmosphere when that happens, it starts off with less radiocarbon. This could then be confused with the older garment, which started off with more but has lost a lot through radioactive decay.

"What's going to happen is actually, because the fraction of radiocarbon in the atmosphere will become depleted, it's going to be hard to distinguish whether something is new or it's old," said Graven. "It might have the same fraction of radiocarbon as something that's undergone decay."

Graven based her findings on models that simulated emissions scenarios developed for the Intergovernmental Panel on Climate Change (IPCC).

Radiocarbon dating is used to help archaeologists tell how old their finds are, and detect fraudulent artworks or illegal ivory.

However, rapidly decreasing carbon emissions and stabilizing the fraction of radiocarbon in the atmosphere would have a knock-on effect on other tools that actually use the fact that these levels are changing so quickly to make their analyses. For example, one technique uses the decrease in radiocarbon to date the birth of cells in the human body. If fossil fuel emissions were rapidly reduced and the amount of radiocarbon in the atmosphere stabilized, that would be harder.

"Either way, if we rapidly reduce emissions or if we continue to increase emissions, it looks like some of these uses of radiocarbon would be affected," said Graven.

But given all the other benefits of cutting down on fossil fuel emissions, it's clear which sacrifice is preferable.