Let’s consider this question about the source of carbon dioxide. So we know carbon dioxide levels are rising–even the climate contrarians, skeptics, and deniers, everyone agrees with that. But you could argue, and indeed some of them do argue, that the carbon dioxide is not coming from burning fossil fuels and changes in land use, that that carbon dioxide is coming from volcanoes. Is there something we can do to test that hypothesis that would tell us if that hypothesis were false? And the answer is yes. Volcanoes do emit carbon dioxide, but it has a different carbon isotope signature than carbon dioxide that you get when you burn fossil fuels. Organic life tends to prefer light carbon, carbon-12, so when organic molecules are forming, they preferentially absorb carbon-12 out of the atmosphere and tend to leave carbon-13 behind. So when you have biomass that becomes fossil fuels, those fossil fuels are depleted in carbon-13.
So if you burn those fossil fuels, you should be returning carbon dioxide to the atmosphere that is low in carbon-13. So we have a test of the theory, a test that could enable us to falsify the volcanic theory. If the carbon in the atmosphere was mostly coming from volcanoes, the ratio of carbon-13 to carbon-12 in the atmosphere should not change. But if it were coming from fossil fuels then the ratio should change, it should decline. So scientists have done that test. They have measured the carbon isotopic composition of the CO2 in the atmosphere, and here’s the result. This is a really beautiful graph. This is one of my favorite graphs of all the things I ever talk about. So here’s… on the right, there’s carbon dioxide increasing since the Industrial Revolution, and here’s the carbon isotopic signature. You can see that the carbon-13 value is falling and it’s practically a mirror image, so as the CO2 rises, the carbon-13 falls, and that tells us that the volcanic hypothesis is false.
So if anybody tells you that the carbon dioxide in the atmosphere is coming from volcanoes, they’re either confused, ignorant, or lying..