Most of the global warming hypothesis is based upon simulation of the atmosphere. Simulation can become a self-fulfilling process. When on sets out to simulate a process, one tends to put things in the model that are consistent with the theory behind the process. If you believe that global warming exists you model the theories that support the process. If you believe carbon dioxide causes global warming, you put equations in that correleate warming to carbon dioxide and you get global warming.
As a most wise man once said: "Simulation is like masturbation. You do it enough and you begin to think it's better than the real thing."
Good observation !