Water vapour vs CO2: a furphy. No matter how much water vapour you have in the atmosphere, adding CO2 will still increase the temperature. Why? Because CO2 absorbs infrared at wavelengths that go through water vapour like it's not there. Those wavelengths happen to be very close to the peak of Earth's blackbody curve, so CO2 has a disproportionate effect.
300 vs 200ppm: Take a nice big glass of water. Add two drops of food colouring (you choose the colour). Now add a third drop. Record your observations. For extra credit, put a glass of clean water, a glass with two drops and a glass with 3 drops in front of a nice powerful light source, and record the temperature of each over time.
