You can easily reproduce that an atmosphere higher in CO2 gets warmer without having a second Earth, you don't need a full planet to test a single atmospheric e

You can easily reproduce that an atmosphere higher in CO2 gets warmer without having a second Earth, you don't need a full planet to test a single atmospheric effect. What you can't do is quantify that to a meaningful measure in the context of Earth. We have no way of knowing if X more CO2 will cause Y warming, or if 2X CO2 will cause 2Y warming or 10Y or .5Y. We can be very certain tat more CO2 means warmer, but how much or how bad isn't clear.