BACKGROUND SUPPLEMENT, ACTIVITY C9
What is the Greenhouse Effect?
If you've ever been in a greenhouse, you probably noticed that it was warmer inside than it was outside. The "greenhouse effect" refers the fact that temperatures we experience on Earth are warmer than they would be if the atmosphere didn't exist. Textbook authors and others commonly explain the atmospheric greenhouse effect using the example of a real greenhouse, explaining that glass is transparent to shortwave radiation (sunlight), but not to longwave radiation emitted by the greenhouse's contents. Sunlight gets in and warms everything up, but the glass prevents the outgoing longwave radiation from leaving, so everything stays warmer than it would be without the glass. Actually, greenhouses maintain their temperatures by keeping the air inside them from blowing away--that is, by supressing convection. The thin panes of glass have very little thermal mass and contribute almost nothing to the overall temperature in the greenhouse. The analogy may be bad for understanding real greenhouses, but it captures the essential workings of the atmospheric greenhouse effect. o
understand how it works, you need to know a few basic facts. First, all objects that are warmer than absolute zero (which is to say, all
objects) emit radiation. How much an object emits depends on its temperature, which you can think of as a measure of how much energy the object contains--how "energetic" it is. Second, all objects either absorb or reflect electromagnetic radiation when it reaches them. When an object absorbs radiation (energy), it heats up (there are exceptions--evaporating water and photosynthesis, for example--where the absorbed energy is stored as mechanical or chemical energy). And the more heats up, the more radiation it emits. Third, all electromagnetic energy (e.g., X-rays, visible light, radio waves) travels at the same rate. This means that if the amount of energy an
BACKGROUND SUPPLEMENT, ACTIVITY C9 obect has to radiate increases, something has to change, and that something is its wavelength: hotter objects radiate at shorter wavelengths than cooler objects. The sun is a very hot object that emits shortwave radiation. When this radiation reaches Earth's surface, it is either reflected or absorbed, depending on the nature of the surface and the angle the sunlight strikes it. We'll ignore reflected radiation for now. Absorbed radiation heats up Earth's surface. Because the surface is relatively cool, it emits longwave radiation. In this diagram, shortwave solar radiation is shown in yellow, longwave radiation in red (again, we're ignoring reflection):
Note that there is no atmosphere in this diagram, and the longwave radiation escapes directly to space. The amount of energy leaving is exactly the same amount as came from the sun. Thus, on an Earth with no atmosphere, it would be very hot during the day, and very cold at night. This is exactly the case on the moon, which has no atmosphere. But Earth does have an atmosphere, which allows most of the solar energy that isn't reflected back to space to pass through to the surface. As in diagram 1, this energy heats the surface, which in turn emits longwave radiation upwards. But with an atmosphere, not all of the energy escapes directly to space. Earth's atmosphere contains certain gases, called the "greenhouse gasses" (mainly water vapor [H20], carbon dioxide [CO2], and ozone [O3]--note that these are all molecules comprised of three atoms), that
BACKGROUND SUPPLEMENT, ACTIVITY C9 absorb longwave radiation emited from the surface, heat up, and emit longwave radiation themselves, like this:
(Notice that the greenhouse gas molecule is emiting radiation both upwards to space and downwards toward the surface.) Gas molecules are small and are relatively inefficient at absorbing and radiating radiation, but there are a lot of them. Each one acts as a heat source, and part of what we experience as warm temperatures comes from these gas molecules. In fact, Earth's surface gets about twice as much energy from the atmosphere as it does from the sun. This is because the sun, while very hot, only covers a small part of the daytime sky, whereas the atmosphere is all around us all the time. As a result of having two heat sources--the sun and the atmosphere-temperatures at the surface are warmer (about 60°F, 34°C) than they would be without an atmosphere. This warming is what we call the greenhouse effect. Contrary to popular belief, the greenhouse effect is a perfectly natural and healthy process. In fact,without the greenhouse effect, Earth's surface would be too cold for life to exist as we know it. However, human activities are increasing the amount of greenhouse gases in our atmosphere at a rate that many scientists believe will cause Earth's surface temperatures to increase. This concern arises from the fact that we are adding more radiation absorbers and emitters to the atmosphere.
BACKGROUND SUPPLEMENT, ACTIVITY C9