I'm not a scientist but it seems that I picked up the notion somewhere that in order for a theory to become an established scientific principle or law, others, elsewhere, acting independently, must have been able to replicate experiments that support or "prove" the theory.
That means we'll have to find another planet very similar to Earth, build and operate a lot of coal fired, carbon belching factories and hydrocarbon consuming automobiles, etc, operate them for about a century or two while recording atmospheric carbon concentrations at specific intervals and see if we get the same results.
Of course, by the time we get that done we'll only have a sample of 2 so we'll have to go elsewhere in the universe and try again and again until we've had a sufficient number of "positive" tests that prove the theory.
To my limited knowledge, that has not yet been done.