Chapter 4 described the primary factors that control our climate: the composition of our atmosphere (e.g., the amount of greenhouse gases in it), the solar constant, and the albedo. If we know how these factors are changing in the future, we can estimate how the climate will change in response.

Of these factors, the most important changes over the next century are expected to be in the composition of the atmosphere. This means that predicting future climate basically comes down to predicting the amount of greenhouse gas in our atmosphere. This in turn requires that we predict how much greenhouse gas will be emitted into the atmosphere each year from human activities. Such projections of greenhouse-gas emissions, known as emissions scenarios, form the backbone of our predictions of climate change over the coming century. In this chapter, I describe how they are constructed and what they tell us about our future climate.