Neil DeGrasse Tyson actually covered this point in Cosmos, but I think it bears repeating since deniers keep missing it. It’s called the
isotopic signature in the CO2. It’s how we know the carbon dioxide to be of our own creation:
THE HUMAN CAUSED GLOBAL WARMING FINGERPRINT
How do we know the increase in CO2 is human caused? There is an isotopic signature, like a fingerprint. CO2 that comes from natural sources has a low carbon-14 ratio. The pre-industrial atmospheric levels of CO2 were around 280ppm (parts per million). As of 2010 the amount is 390ppm. The extra 100ppm does not have the carbon-14 signature. The only other possible source that can account for the extra 100ppm is human industrial emissions of fossil fuels.
When protons from GCRs (Galactic Cosmic Rays) collide with the nitrogen-14 (seven protons plus seven neutrons in the nucleus) in the air, carbon-14 is created (in addition to other isotopes such as beryllium-10) through a nuclear reaction:14N + p → 14C + nThis means that carbon with a low isotope carbon-14 ratio must come from deep in the ground, out of reach of cosmic rays.Furthermore, the ratio of O2 to N2 has diminished. This is expected from the increased combustion of fossil fuels, in which O2 combines with C to form CO2. The oceans have also become more acidic, leading to an increase in CO2 levels in both the atmosphere and the oceans.
They want you to think this is some kind of weird hunch out of nowhere, or some baseless thing from scientists trying to make millions off of a scare hoax. But there are no millions to be made, and the evidence is so freakin’ clear and absolute that it’s insane to ignore it.
Since carbon-14 is created during exposure in the atmosphere, low isotope carbon-14 has not been subjected to atmospheric exposure and therefore must have come from underground. In other words, we dug it up, burned it, and now low isotope carbon-14 is in the atmosphere. That is how the two origins of carbon can be clearly identified, by the isotopic signature.
Said another way, low isotope carbon-14 is from fossil fuels, while the rest of the carbon has been in the natural cycle for longer periods. Longer expose times increase the C-14 in the molecule. Carbon that is stored and released through natural processes such as the normal seasonal carbon breathing that occurs in plant life has a longer atmospheric lifetime.
Yep, the deniers are insane, and ignoring it, and making billions off of doing so. And wanting you to do the same while thinking we’re the ones getting rich off a hoax. #x