The short answer

Carbon dating estimates the age of organic matter by comparing the ratio of carbon-12, a stable atom, to carbon-14, an unstable atom which decays with a half-life of 5,730 years.

The long answer

How in the world can scientists take things like a bone, a wood chip, or a tooth and declare its ancient age?

The answer comes from radiocarbon dating, or carbon dating for short. Carbon dating is a method used to date biological material (that is, things that have once lived) up to around 50,000 years old.

The hidden-in-plain-sight secret to carbon dating is ... carbon. Here's how carbon dating works:

Carbon-14 is formed

It all begins in Earth's upper atmosphere where cosmic rays (high-energy particles from space) are continuously shining down on our planet. These cosmic rays cause all sorts of chemical and physical interactions, one of which forms carbon-14.

All carbon is not created equal. Carbon has three isotopes (atoms of the same element that have the same number of protons but a different number of neutrons): carbon-12, carbon-13, and carbon-14.

Carbon-12 and carbon-13 are stable isotopes, while carbon-14 is unstable and radioactive, meaning it decays naturally over time.

Carbon-14 is absorbed by all living things

All living organisms absorb carbon, including carbon-14, from the atmosphere.

Plants take in carbon in the form of CO₂ during photosynthesis, animals eat plants, and animals eat plant-eating animals. When living things die, they stop absorbing carbon.

Carbon-14 decays over time

The ratio of these carbon-14 to carbon-12 in the air and in all living organisms is nearly constant at any given time. But after things die, the ratio begins to change because carbon-14 is unstable. Carbon dating works by comparing the ratios of carbon-12 (stable) to carbon-14 (unstable).

Carbon-14 decays at a half life of 5,730 years. That means after 5,730 years, there will be half as much carbon-14; after 11,460 years, there will be a quarter as much; after 17,190 years, there will be an eighth as much, and so on. But the amount of carbon-12 will remain the same.

By analyzing the ratio of carbon-12 and carbon-14, scientists can estimate the age of a formerly living thing pretty closely.

🧠 Bonus brain points

What are the limitations of carbon dating?

Carbon dating isn't perfect. Here are three drawbacks to carbon dating:

#1 It can only be used to date previously living things.

Since carbon dating relies on the act of living things absorbing carbon, it can only be used to date organic material. Inorganic material like rocks, metals, salts, and water cannot be dated using carbon dating.

#2 It can only be used to date things up to ~50,000 years old.

Carbon-14's half life is 5,730 years. That means organic material older than 40,000 years is extremely difficult to date since the amount of carbon-14 left by that point is so small. After 10 half-lives or 57,360 years, there is less than 0.1% of the amount of original carbon-14 left, making it nearly impossible to date objects older than this age.

#3 The ratio of carbon-12 to carbon-14 is impacted by special events.

The ratio of carbon-12 to carbon-14 in the atmosphere isn't exactly the same over time. It can be influenced by various factors, such as solar activity, changes in Earth's magnetic field, volcanic eruptions, and human activities, like nuclear bomb testing and increased CO₂ production from pollution. These fluctuations impact the accuracy of carbon dating.

However, scientists can account for these fluctuations by calibrating with samples dated by other methods. For instance, tree rings can be counted, and their radiocarbon content can be measured. This allows the creation of a "calibration curve," which accounts for these variations. I guess future scientists will need to make a new calibration curve to deal with our modern human habits of nuclear bombing and polluting. ¯\_(ツ)_/¯

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Sources

Blakemore, E. (2019, July 12). Radiocarbon helps date ancient objects—but it’s not perfect. National Geographic. https://www.nationalgeographic.com/culture/article/radiocarbon-dating-explained

Brain, M. (2023, October 20). How Carbon Dating Works. HowStuffWorks Science. https://science.howstuffworks.com/environmental/earth/geology/carbon-14.htm

BrainStuff - HowStuffWorks. (2015, June 25). How Carbon Dating Works. YouTube. https://www.youtube.com/watch?v=Kcuz1JiMk9k

National Oceanic & Atmospheric Administration. (2005, October 1). The Technical Details: Radioactive Decay. Global Monitoring Laboratory. https://gml.noaa.gov/ccgg/isotopes/decay.html

Smithsonian Institution. (2024, January 3). Dating. Smithsonian Institution. https://humanorigins.si.edu/evidence/dating

U.S. Department of Energy. (n.d.). Isotope Basics. NIDC: National Isotope Development Center. https://www.isotopes.gov/isotope-basics

University of Chicago. (n.d.). Carbon-14 dating, explained. University of Chicago News. https://news.uchicago.edu/explainer/what-is-carbon-14-dating

Wood, R. (2012, November 27). Explainer: what is radiocarbon dating and how does it work?. The Conversation. https://theconversation.com/explainer-what-is-radiocarbon-dating-and-how-does-it-work-9690