Earth's Energy Balance

The Earth's average temperature remains fairly constant from year to year. There are long term trends (like the ice ages), but no evidence of any dramatic temperature change from one year to the next. Therefore, the Earth must be releasing into space the same amount of energy that it receives from the sun. If this did not occur, the atmosphere would measurably warm or cool, depending on the amount of heat lost to space. So what happens to solar radiation once it enters the atmosphere, and how does energy get radiated into space? Well, we can think of the incoming solar radiation as being broken up into parts. The diagram below shows the average break down of solar radiation as it enters the atmosphere.

When we combine the break down of incoming radiation with the break down of outgoing radiation, we have the earth-atmosphere energy balance. The chart below displays the average break down of energy lost and gained at the earth's surface. The units used are relative to the amount of incoming radiation. Notice that the total energy lost at the earth's surface (lower left hand corner) is equal to the amount gained at the surface (lower right hand corner).

The net energy at the earth's surface can be thought of as the sum of the incoming energy minus the energy required to heat the air minus the energy to evaporate water. This simple equation is known as the energy budget equation. The energy used to heat the air is directed upward, away from the surface, and so is considered a subtraction. The same is true for the energy required to evaporate water. This energy, however, is in the form of a heat we can't feel. It is known as latent heat.