Human causation

Here are two independent lines of research suggesting that human activity has caused the current climate change.

As carbon dioxide in the air increases, oxygen decreases.

Scientists have observed that oxygen decreases as carbon dioxide (CO2) in the air increases. This is what would happen if the increase in CO2 came from burning fossil fuels or forests because burning things containing carbon removes oxygen from the air, combining oxygen with carbon to produce CO2.

This observation suggests that burning plants or fossil fuels has caused the increase in atmospheric CO2. It also challenges the idea that CO2 comes from other sources, like volcanic eruptions.

The light carbon in atmospheric carbon dioxide is increasing

Scientists observe that the atmosphere’s carbon dioxide (CO2) increasingly contains light carbon: the isotope carbon-12. This increase in “light carbon” is evidence that humans have caused the current global heating.

There are two types (isotopes) of carbon to consider here: carbon-12, call it “light carbon”, and carbon-13, call it “heavy carbon”. A measure of how much light carbon you have in a sample is the percentage of light carbon to total carbon: the “light carbon percentage”,

  • In photosynthesis, plants take in CO2 and water, use the carbon to build themselves up, and release oxygen.
  • Plants more readily take in CO2 containing light carbon.
  • The “light carbon percentage” in plants is higher than the percentage in the air.
  • The “light carbon percentage” in fossil fuels is also higher than the percentage in the air as fossil fuels come from ancient forests.
  • Burning plants or fossil fuels releases CO2 into the atmosphere, and that CO2 has a high “light carbon percentage”.
  • So, this burning leaves a fingerprint: an increase in the “light carbon percentage” of atmospheric CO2.
  • Scientists have determined that light carbon in atmospheric CO2 has changed over time. The light carbon began increasing in about 1850, at the beginning of the industrial age, and it continues to grow.

Burning leaves a fingerprint: an increase in atmospheric carbon dioxide containing light carbon.

The observed increase in light carbon in atmospheric CO2 supports the idea that the increase in CO2 in the air is due to the burning of plants and fossil fuels during the industrial era.


Both these independent lines of research support the theory that the increase in carbon dioxide in the atmosphere comes from the human burning of fossil fuels and forests rather than from the oceans or volcanos releasing carbon dioxide.


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

How do we know that recent carbon dioxide increases are due to human activities? (Real Climate: 2004)

Updated 5 Aug 2021; Checked 4 Oct 2022.