evidence: Commenters on my blog also often claim that since the concentration of CO2 in the atmosphere is so low compared to that of water vapor, also a greenhouse gas, it could not possibly play the role of a thermostat. But here, too, rigorous research shows otherwise. [...] The answer involves different characteristics of greenhouse gases. When the atmosphere cools enough, water vapor condenses and rains out. By contrast, carbon dioxide, methane and other greenhouse gases do not — they are non-condensing. The researchers found that without these non-condensing greenhouse gases — CO2 foremost among them — there would be nothing to prevent the atmosphere from cooling enough to cause water vapor to rain out. And since it is such a potent greenhouse gas, if water vapor were to rain out, the result would be very dramatic cooling. In this way, CO2 may not be as potent a greenhouse gas as water vapor, but it is actually more important.