Human activities, such as the burning of fossil fuels and forests, have caused the current increase in atmospheric carbon dioxide levels. Here are the results from two independent lines of research that provide evidence supporting this claim.

• As carbon dioxide in the air has increased, oxygen has decreased.
• The atmosphere’s carbon dioxide increasingly contains Carbon-12.

More Carbon Dioxide in the Air, Less Oxygen.

Measurements have shown that as carbon dioxide (CO₂) in the air has increased, oxygen (O₂) has decreased.

These measurements are consistent with the CO₂ coming from the burning of fossil fuels or forests, because when you burn these fuels, you combine the carbon in the fuel with oxygen in the air, producing CO₂; i.e., you increase the amount of CO₂ in the atmosphere and decrease the amount of oxygen, with the oxygen becoming part of the carbon dioxide.

The measurement also challenges other possible explanations for the increase in atmospheric carbon dioxide. You might think that the increase in carbon dioxide has come from volcanic eruptions. However, volcanoes might explain the increase in atmospheric CO₂, but they would not explain the observed decrease in oxygen.

Burning leaves a fingerprint: an increase in atmospheric carbon dioxide alongside a decrease on oxygen.

More carbon-12 in atmospheric carbon dioxide

Measurements have also shown that the atmosphere’s carbon dioxide (CO₂) increasingly contains the isotope carbon-12.

• There are two types (isotopes) of carbon atoms to consider here: Carbon-12 and Carbon-13. A Carbon-13 atom has one extra neutron in its nucleus compared to Carbon-12, which makes it heavier.

• In photosynthesis:
  • Plants take in CO₂ and water.
  • The carbon from the carbon dioxide goes into making wood.
  • The oxygen goes into the atmosphere.
• A measure of how much Carbon-12 you have in a sample is the percentage of carbon-12 to total carbon: the “Carbon-12 percentage”.
• Call CO₂ containing Carbon-12, light CO₂.
• Call CO₂ containing Carbon-13, heavy CO₂.
• Plants take light CO₂ more readily than heavy CO₂, so plants have a higher Carbon-12 percentage than the air.
• The “Carbon-12 percentage” in fossil fuels is also higher than in the air because fossil fuels come from ancient forests.
• Burning plants or fossil fuels releases CO₂ into the atmosphere, and that CO₂ has a high “Carbon-12 percentage”.
• So, this burning leaves a fingerprint: an increase in the “Carbon-12 percentage” in the CO₂ in the air.
• Measurements show that the Carbon-12 percentage in atmospheric CO2 has changed over time.
• The Carbon-12 in the air began to increase around 1850, at the beginning of the Industrial Age, and it continues to grow.

This logic means that the observed increase in Carbon-12 in atmospheric carbon dioxide supports the idea that the increase in atmospheric carbon dioxide is due to the burning of plants and fossil fuels during the industrial era.

Burning leaves a fingerprint: an increase in atmospheric carbon dioxide containing Carbon-12.

Conclusion

So here we have two sets of measurements coming from two independent lines of research:

• As carbon dioxide in the air has increased, oxygen has decreased.
• The atmosphere’s carbon dioxide increasingly contains Carbon-12 atoms.

Both of these independent lines of research support the theory that the increase in atmospheric carbon dioxide comes from the human burning of fossil fuels and forests rather than from other causes, such as volcanic eruptions.

Sources

Debunking the myth that the carbon dioxide increase is natural. (Skeptical Science)

How do we know that reincreases in cent carbon dioases are due to human activities? (Real Climate: 2004)

Loaded Feb 2019; Modified 6 July 2026.