Slide 7: The atmosphere II: energy budget
According to the Stefan-Boltzmann equations, the energy hitting the top of the atmosphere is the inversely proportional to the square of the distance from the sun. At 150 million kilometres, the top of the atmosphere receives 1368Wm-2 (as measured by satellites).
If the earth were a perfect "black body", it would absorb the radiation, and then re-radiate it at a longer wavelength, as shown on the dotted lines.
However, the atmosphere absorbs much of the radiation, and the earth's hypothetical surface temperature of 255K is considerably lower than the actual 288K.
This is the greenhouse effect.
The atmospheric gases and aerosols also scatter much of the incoming radiation, reducing the amount that actually hits the earth.
This diagram shows the energy fluxes within the atmosphere. Notice that although 30% of incoming radiation is reflected directly into space, only 4% of long wave radiation from the surface leaves the planet directly. The remainder is absorbed by the atmosphere, and either re-absorbed by the ground or by atmospheric gases.
Eventually this energy is lost to space, but the net effect over time is to increase the energy near the surface of the planet, until an equilibrium is reached.