A feedback is something that happens in response to a prior cause, and which then itself makes a change in what caused it. It is the modification or control of a process or system by its results or effects. The challenge in making climate models realistic is largely that of understanding these feedback processes and incorporating them in models. As the air warms, does water vapor increase? To what extent? How much does that affect the warming? Do ice and snow melt? How rapidly? Does that change the reflectivity of the Earth’s surface and affect the warming? Does the ocean circulation change? In what ways? How does that affect climate? Do the clouds change? Are they more plentiful? Less plentiful? Higher? Lower? Darker? Lighter? How do they feed back to the climate? We need quantitative answers to all these questions. For example, we are not asking the qualitative question of whether clouds warm us or cool us. We know that they do both. Rather, we are asking the hard, quantitative question: Which of those two events is dominant, the cloud’s contribution to the greenhouse effect (warming) or the cloud’s contribution to reflecting away sunlight (cooling)?