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.
When you burn carbon, you take oxygen from the air and combine it with the carbon to made carbon dioxide. So, if the increase of carbon dioxide in the air is due to the burning of carbon, then we would expect to see the amount of oxygen in the air decrease as carbon dioxide level increases. And this is what measurements of these levels show.
The carbon dioxide level decreases during the sunny northern months. Then it increases more during darker months, increasing year on year. The oxygen level follows this cycle only decreasing as carbon dioxide increases and vice versa.
Humans are changing the carbon dioxide in our air.
Humans continue to burn large amounts of fossil fuels and forests, and the burning is changing the nature of carbon dioxide in the air. That is, it is changing the proportions of the isotopes of carbon in atmosphec 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: 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.
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)