FIGURE 3.1 Determination of surface temperature response to a radiative forcing Δ F, in terms of the top-of-atmosphere energy budget. The top-of-atmosphere energy budget is the net of outgoing infrared radiation minus incoming solar radiation. The budget is zero when the system is in equilibrium. In these graphs, the budget is expressed schematically as a function of surface temperature. Equilibrium surface temperature is determined by the point where the line crosses the horizontal axis. If the system starts in equilibrium, but CO2 is increased so that the line is shifted downward by an amount Δ F (via reduction in outgoing infrared), then the intersection point shifts to warmer values by an amount Δ T. When the slope of the energy budget line is smaller, a given Δ F causes greater warming, as indicated by the pair of lines with reduced slope. This connects the slope of the energy budget line with climate sensitivity. The slope of the line is like the stiffness of a spring, and the radiative forcing Δ F is like the force with which one tugs on the spring. When the spring is not very stiff (e.g., a spring made of thin rubber bands) a given force will make the spring stretch to a great length—analogous to a large warming. If the spring is very stiff (e.g., a heavy steel garage door spring) the same force will cause hardly any stretching at all—analogous to low climate sensitivity. A spring with no stiffness at all would represent a very special case, demanding a specific physical explanation, just as would a case of zero slope of the energy budget line, which corresponds to infinite climate sensitivity.
to be closer to the Sun than Earth is, in order to make up for the lack of a greenhouse effect. The same greenhouse effect that keeps Earth from freezing over in its actual orbit reduces the temperature at which Earth radiates to space, reduces the slope characterizing the black-body feedback, and hence increases the sensitivity. Taking into account the greenhouse effect