After the sun’s energy enters the Earth’s atmosphere the heat is either absorbed—(taken in, reflected—hitting a barrier and bouncing off), or scattered—(hitting a barrier and bouncing off in many different directions). The heat left on Earth is equal to the amount of heat absorbed minus the amount of heat given off. If more energy comes into the system than goes out of the system, the planet warms.
If less energy goes into the system than goes out of the system, the planet cools. Replace the word “money” for “heat” and “on Earth” with “in your bank account” and you describe a household budget. Of course, Earth’s heat budget is a lot more complex than a simple household budget.
Heat at Earth’s Surface
About half of the solar radiation that strikes the top of the atmosphere is filtered out before it reaches the ground. This energy can be absorbed by atmospheric gases, reflected by clouds, or scattered. Scattering occurs when a light wave strikes a particle and bounces off in some other direction. About 3% of the energy that strikes the ground is reflected back into the atmosphere.
The rest is absorbed by rocks, soil, and water and then radiated back into the air as heat. These infrared wavelengths of energy can only be seen by infrared sensors.
The Heat Budget
Because solar energy continually enters Earth’s atmosphere and ground surface, is the planet getting hotter? The answer is no (although the next section contains an exception), because energy from Earth escapes into space through the top of the atmosphere. If the amount that exits is equal to the amount that comes in, then the average global temperature stays the same.
This means that the planet’s heat budget is in balance. What happens if more energy comes in than goes out? If more energy goes out than comes in? To say that the Earth’s heat budget is balanced ignores an important point. The amount of incoming solar energy is different at different latitudes. Where do you think the most solar energy ends up and why? Where does the least solar energy end up and why?