The title of this post is a purposeful play on the title of a 2010 paper by Lacis, Schmidt, Rind, and Ruedy, entitled Atmospheric CO2: Principal control knob governing Earth?s temperature.

In that paper, the authors claimed that the existence of CO2 is what provides enough warming to keep the Earth from becoming an ice planet. They also claim that, because CO2 is ?non-condensing? (whereas water vapor, Earth?s most abundant greenhouse gas, does condense) this gives it special status as some sort of primary control knob governing Earth temperatures.

That latter argument has never quite convinced me of anything?both CO2 and water vapor have various sources and sinks, and just because water vapor goes through a phase change and CO2 doesn?t is, in my mind, irrelevant. Yes, the CO2 source/sink processes act more slowly than the water vapor ones (evaporation and precipitation) do, but on the long time scales of Earth?s history, who cares? (more…)