Understanding CO2 Emissions Sources

So how do we know that the rise of CO2 in the atmosphere that we see

comes from our burning of fossil fuels and not from something that the volcanoes have done?

Well, the first step in the problem is just bookkeeping.

We measure how much CO2 comes out of the volcanoes.

We measure how much CO2 comes out of our smokestacks and tailpipes.

The natural source is small.

Humans are putting out 50 to 100 times more CO2 than the natural volcanic source.

We can then ask the air whether our bookkeeping is right, and the air says that it is.

Volcanoes make CO2 by melting rocks to release the CO2.

They don't burn, and they don't use oxygen.

But burning fossil fuels does use oxygen when it makes CO2.

We see that the rise in CO2 goes with a fall of oxygen,

which says that the rising CO2 comes from burning something.

We can then ask the carbon in the rising CO2 where it came from.

Carbon comes in three flavors.

The lightweight, carbon-12, which is especially common in plants.

The medium weight, carbon-13, which is a little more common in the gases coming out of volcanoes.

And the heavy weight, carbon-14.

It's radioactive and decays almost entirely after about 50,000 years,

which is why you won't find it in very old things like dinosaur bones or fossil fuels.

We see a rise in carbon-12, which comes from plants.

We don't see a rise of carbon-13, so the CO2 isn't coming from the volcanoes.

And we don't see a rise in carbon-14, so the CO2 can't be coming from recently living plants.

And so the atmosphere says that the rising CO2 comes from burning of plants that have been dead a long time.

That is fossil fuels.

The CO2 is coming from our fossil fuels. It's us.