In the simple model of the climate derived in Chapter 4, the temperature of a planet is set by the number of atmospheric “layers,” the albedo, and the solar constant. I said there that the number of layers was determined by the abundance of greenhouse gases in the atmosphere, but I did not define what a greenhouse gas is, or which components of our atmosphere are greenhouse gases. In this chapter, I address these questions and discuss in detail one of our atmosphere's most important greenhouse gases, namely carbon dioxide.

Carbon dioxide, or CO2, is the primary greenhouse gas emitted by human activities, and policies to control modern climate change frequently focus on reducing our emissions of this gas. Understanding carbon dioxide requires more than just knowing how much of it humans are dumping into the atmosphere. It also requires an understanding of the carbon cycle – how carbon moves between the atmosphere, ocean, land biosphere, and rocks on the Earth. This will help us understand what happens to carbon dioxide after it is emitted into the atmosphere, which in turn will help us understand the future trajectory of our climate.