Just a side point that I think is relevant to this discussion: It's almost impossible to accurately measure the massive amounts of GHGs from any single source, much less from ALL of them. This is not to say that they shouldn't be approximated and they could end up being pretty accurate percentage-wise, but the exact measurements are assuredly wrong just because the system we are trying to measure is so HUGE and dynamic. Science is never settled, but this is currently the best guess based on other observed principles.