Two huge factors affect the global radiation balance, shortwave radiation (coming in from the Sun) and longwave radiation (given off from the Earth). We'll cover short wave radiation first.
The Short WavesYou already understand that radiation comes to the Earth. That light and energy is called incoming. The amount of energy given off by the Earth is usually the same amount. When light from the Sun gets to the Earth, it hits the ozone layer (absorbs UV), then hits water vapor and carbon dioxide (absorbs IR), and then a bunch of it scatters. Usually only about 60% of the energy makes it to the surface of the Earth. Most of that energy is radiation from the visible light range of the EM spectrum.
Some of that radiation is instantly reflected back out to space. The amount of radiation reflected by the surface of the Earth is called the albedo. The higher the albedo the more reflection it gives. A snow-covered field would have a very high albedo and dark soil in your garden has a very low albedo. The amount of reflection also changes with the angle of the Sun.
When the Sun is lower on the horizon (winter), there is more of a chance for the light to bounce off the surface and atmosphere. It is like a shiny window. If you are right in front of it, you can see in. If you are off to the side, it acts like a mirror and you see a reflection.
The Long WavesAs we said before, long wave radiation comes from the Earth. The Earth actually gives off radiation like the Sun. The wavelength of the Earth's energy is much longer. The amount of energy given off by the Earth is equal to the amount it receives from the Sun.
All of the outgoing energy added together (plants, animals, volcanoes) is the same as the amount of radiation absorbed by the Earth. If those amounts were not equal, the Earth would be heating up or cooling down. Emissivity is the ability for an object to give off energy. Scientists say that a black object has an emissivity of "1". Plants have an emissivity of .99 (almost 1). A rock has one of about .80.
Getting HotterThe atmosphere also plays a large role in how much energy gets away from the Earth. We talked about carbon dioxide absorbing infrared energy. What if carbon dioxide levels are increasing? That extra amount would absorb even more of the energy trying to get out. That extra absorption would cause the amount of IR radiation in the atmosphere to increase. The atmosphere would get hotter.
When the temperature of the atmosphere increases because there is more carbon dioxide, scientists call it the greenhouse effect. Smaller factors affect the amount of energy leaving the Earth. Factors like deforestation, the distance of the Earth from the Sun, and the amount of dust in the atmosphere will all change the outgoing energy amounts.
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- EM Spectrum
- Solar Radiation
- Global Geometry
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Earth’s Energy Balance (NASA Earth Observatory Video)
Useful Reference MaterialsEncyclopedia.com (Terrestrial Radiation):
Wikipedia (Radiant Energy):