> But maybe there is a third variable or combination of variables that is driving CO2 levels? Is that really so unreasonable.
I see. It's really inconvenient that all this additional CO2 is just megatons of identical molecules and comes without any discernible label that might tell where it came from......
...except that they actually do! Because photosynthesis favors the lighter C12 atom over C13, all plant material (and hence, all animal material, and all fossil fuel) have slightly lower concentration of C13, compared to other non-biological source of carbon (say, volcanic gas). If the increase in atmospheric CO2 is actually due to fossil fuels, we must witness a drop in the ratio of C13. And that's exactly what we see!
I see. It's really inconvenient that all this additional CO2 is just megatons of identical molecules and comes without any discernible label that might tell where it came from......
...except that they actually do! Because photosynthesis favors the lighter C12 atom over C13, all plant material (and hence, all animal material, and all fossil fuel) have slightly lower concentration of C13, compared to other non-biological source of carbon (say, volcanic gas). If the increase in atmospheric CO2 is actually due to fossil fuels, we must witness a drop in the ratio of C13. And that's exactly what we see!
https://www.skepticalscience.com/human-co2-smaller-than-natu...