Because the only identifiable sources of increased thermal forcing are greenhouse gases, mainly co2. The additional c02 since pre-industrial are identifiable because they are of a different isotope than that which is present naturally.

Like most counter-arguments, this one is easily answered by the well established science. If you think the science is politically movitated or part of the hoax, please note that most of the science was hypothesized long before the current politization and can be found in old papers and textbooks.