There several independent lines of inquiry that link human activity to current climate change. Here are two.
Oxygen in the air decreases as carbon dioxide increases.
Burning things containing carbon, like petrol and wood, removes oxygen from the air, combining the oxygen with the carbon to make carbon dioxide. And this is what the scientists are seeing when they measure the composition of our atmosphere. The amount of oxygen in the air decreases as carbon dioxide level increases. This supports the idea that the increase in carbon dioxide in the air is due to the burning of carbon. It also challenges the idea that the extra carbon dioxide is coming from other sources, like volcanic eruptions.
Humans are changing the carbon dioxide in our air.
Scientists are analysing the nature of carbon dioxide in the air. And this has been changing in a way that is consistent with the idea that the increase in carbon dioxide is coming from the burning of fossil fuels and forests. This burning changes the proportions of the isotopes of carbon in atmospheric carbon dioxide.
There are two types (or isotopes) of carbon to consider here:
• There is carbon-12, call it “light carbon”, and
• There is carbon-13, call it “heavy carbon”.
The proportion of the isotopes
Now, the weight of plants is mainly the carbon that comes from the carbon dioxide the plant takes in during photosynthesis. And plants prefer absorbing carbon dioxide containing “light carbon”, rather than that containing “heavy carbon”. So the carbon in plants has a lower heavy carbon ratio than that in carbon dioxide in the air. And, as fossil fuels originate from ancient forests, the “heavy carbon ratio” in fossil fuels is similar to the ratio in plants.
So burning leaves a fingerprint: an increase in carbon dioxide with a low heavy carbon ratio.
And scientists have determined how this heavy carbon ratio in atmospheric carbon has changed over time. The heavy carbon ratio began decreasing in about 1850, at the beginning of the industrial age and it continues to decrease.
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.
How do we know that recent carbon dioxide increases are due to human activities? (Real Climate: 2004)