Comment Re:They already have (Score 1) 667
Our control Earth is history. We can see that this Earth killed most macroscopic surface life a few times in history, and we have evidence for why that happened. We can see that it once would not have supported our sort of life. We can see how its atmosphere developed and how ecological networks have formed. We also have a pretty good understanding of gases and their behavior, and we can measure the gases in the atmosphere and the gases emitted from our civilization.
We can readily disprove theories of lucky socks and umbrellas causing rain. We can't, however, explain how any atmosphere would be able to tolerate inputs of the sort our civilization produces without some change.