 In the simplest climate model there is no atmosphere. Therefore, radiation is absorbed only by Earth's surface, and the atmosphere's emissivity is zero. And that solar radiation energy, which is just the difference between the incoming solar radiation energy and the reflected solar radiation energy, equals Earth's infrared radiation energy outgoing to space. Let's represent that amount of energy with a single arrow. At the Earth's surface, and at all levels above, there is one arrow coming down and one arrow going up to maintain radiative equilibrium. Consider next to more realistic climate model, one that has two atmospheric layers that do not absorb the incoming solar radiation, but do strongly absorb infrared radiation. Since they are good absorbers in the infrared, they are also good emitters in the infrared. The radiative equilibrium at each level, the number of arrows which represent units of radiation energy, must be equal. Starting at the top of the atmosphere, the upper layer must emit one arrow of infrared radiation up to balance the solar visible radiation energy coming down. At the interface between the upper and lower layers, there is one arrow of solar radiation energy going down, and the upper layer is emitting one arrow of infrared radiation down because if it is emitting one up, then it must also emit one down, since we are assuming that the layer has a uniform temperature. That puts two down arrows at the interface between the upper layer and the lower layer. To balance these two, the lower layer must be emitting two infrared arrows up. And since the lower layer also has a uniform temperature, it must also be emitting two arrows down to Earth's surface. With one solar and two infrared arrows down to Earth's surface, Earth's surface must emit three arrows of infrared radiation energy up. To emit that much infrared, Earth's surface must be at a higher temperature, since its irradiance is proportional to its temperature to the fourth power.