The carbon-14 already present begins to decay at a known rate: the half-life of carbon-14 is approximately 5,730 years.

By measuring the ratio of carbon-14 to carbon-12 in an ancient sample and comparing it to the ratio in a living organism, researchers can calculate how much time has elapsed since the sample’s carbon was last renewed — since the organism died.

The elegant simplicity of the method masks several complications that became apparent as the science matured.

The first complication is calibration.

Carbon-14 production in the atmosphere is not perfectly constant over time — it varies with solar activity and other factors.