As in the case of the notion that smoking cigarettes can cause cancer, and that Aids is spread by unprotected sex, I'd be inclined to go with the mass of scientists who believe human activity is a significant factor contributing to a general global warning, rather than with the occasional exception who thinks those ideas are all nonsense.

It's too important to gamble on the possibility they are all wrong.

If there are natural causes making for global warming, which is of course quite possible, that doesn't make it any less important to do what we can to avoid making things still worse.