Carbon-dating is a widely used technique that allows us to gauge the age of archaeological samples from up to 60,000 years ago. But it’s not a straightforward method.

It relies on a radioactive version of carbon called carbon-14, which is formed in the atmosphere and is taken up by plants (and whatever eats the plants). Once these die, the carbon-14 in their bodies decays away at a steady, predictable rate. By measuring it, we can calculate how old an ancient sample is.

But there’s a catch. The levels of carbon-14 in the atmosphere vary from year to year, so scientists need some way of assessing these fluctuations to correct their estimates. They need long-running timetables, where each year in the past several