reliable if you repeat it and get the same or a similar answer over and over again

So is your point that we can't replicate it because we only have one Earth?

But we can observe it elsewhere and replicate it in a smaller scale. We understand the greenhouse effect. There is nothing all that unique about the Earth as a gas holding vessel.

Venus is an observation of the greenhouse effect with an atmosphere of mostly thick carbon dioxide. And the same effect can be replicated in simple experiments by putting jars of with different contents in the sun or under a lamp. Iterations of this run the gamut from a young child science experiment to legitimate university research. It is very easily replicated one commonly done.

So basically what I'm saying is that we understand the underlying physical explanation of it very, very well. We can easily measure carbon dioxide levels in the atmosphere. We can easily see that it is increasing and we can compare that to what we have demonstrated countless times with greenhouse gases in other vessels.

To deny the greenhouse effect would require an explanation as to why it seemingly works different on Earth than on literally every other vessel.