There's a wide gulf between, "CO2 has no impact" and, "Humanity is responsible for climate and weather". Your experiment would be better formulated as such: 1

There's a wide gulf between, "CO2 has no impact" and, "Humanity is responsible for climate and weather". Your experiment would be better formulated as such: 1) Establishing the optimal climate and weather. 2) Create two entire solar systems. 3) Place humans on the Earth of one solar system* 4) Observe the climate of both Earths 5) Decide which one best meets your standard of the ideal climate As with any scientific experiment, you would want to conduct it multiple times to ensure the results are reproducible. Measure the deviation of each climate and count the number of experiments to determine your degree of certainty. *3. Make sure humans don't appear (naturally?) on your non-human bearing earth. The question of if humans are a part of nature and therefore part of the ecosystem will arise. If humans are part of nature, you may then ask if any changes they may or may not contribute are therefore natural.