Effects of greenhouse gases on Earth, Venus, and Mars: Beyond the one-blanket model^a)

I present a discussion of the effect of increasing carbon dioxide on planetary climate, at a level suitable for insertion as a module into an upper-level Physics course. The treatment includes two key ingredients that are often missing from more elementary discussions, yet are amenable to analytic methods: First, that convection implies a dependence of surface temperature on the height of the outermost infrared-thick layer; and second, that increasing the level of CO₂ closes spectral windows of absorption. These themes are applicable not only to an industrializing Earth but also to our neighboring planets.