What caused this bewildering argument by cons? It happened one way so it can never happen a different way?

This is what they like to say: that the world has at times been hotter and colder long ago without humans' help, so that there is no possible chance that humans can be responsible. Can anyone explain to me this bizarre logic? Do they think it's not possible for a vandal to light their house on fire if it had caught on fire by lightening in the past?