I understand the theory I'm using here ... it's just a theory ... nothing more ... you asked about belief and that's a philosophical question ...
Either way, we can measure man's contribution to atmospheric carbon dioxide ...
"However, significant Mesozoic and Cenozoic coal deposits formed after lignin-digesting fungi had become well established, and fungal degradation of lignin may have already evolved by the end of the Devonian, even if the specific enzymes used by basidiomycetes had not."
-- Wikipedia's article
"Carboniferous"
Yes, I can poke holes in my theories ... that's good science ... and there's other theories ... most explain the build up of carbon-12 in our atmosphere as human caused ... including Dr Golds' I believe ...