Human causation

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

As carbon dioxide in the air increases, oxygen decreases.

Scientists observe that as carbon dioxide (CO2) in the air increases, the amount of oxygen decreases. This is what would happen if the increase in CO2 came from burning fossil fuels or forests. It’s because burning things containing carbon removes oxygen from the air and combines the oxygen with the carbon to produce CO2.

This observation supports the idea that the increase in CO2 in the air is due to the burning of carbon. It also challenges the idea that CO2 is coming from other sources, like volcanic eruptions

The light carbon in atmospheric carbon dioxide is increasing

Scientists observe that the CO2 in the atmosphere increasingly contains light carbon: the isotope carbon-12.

There are two types (isotopes) of carbon to consider here:
• There is carbon-12, call it “light carbon”, and
• There is carbon-13, call it “heavy carbon”.

The weight of plants is mainly the carbon that comes from the CO2 that the plant takes in during photosynthesis. As plants prefer absorbing CO2 containing “light carbon”, rather than that containing “heavy carbon”, the carbon in plants has a higher percentage of light carbon than that in the CO2 in the air. And, as fossil fuels originate from ancient forests, the percentage of light carbon in fossil fuels is similarly high.

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

Scientists have determined how 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 increase.

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 carbon.


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 being released from the oceans or volcanoes.


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 21 April 2019