As you have just learned, the temperature of earth has been relatively constant over time. You have also learned that this is possible because as the planet absorbs solar radiation, it also emits longwave radiation into space. Despite this radiative equilibrium, some longwave radiation remains trapped beneath the atmosphere of the earth and warms the surface of the earth. Watch the next movie to find out what causes this phenomenon called the “greenhouse effect.”
Earth’s greenhouse effect greatly affects climate. See if you can describe how carbon dioxide and water vapor cause the Earth’s greenhouse effect. Also, see if you can describe how the greenhouse effect maintains the surface temperature of earth.
Scientists have long known that the presence of an atmosphere keeps the surface of the planet warmer than it would be without an atmosphere.
In fact, without an atmosphere, the surface of the earth would be about 30 degrees Celsius cooler than it is now!
In order to understand why, we need to consider the surface of the earth and its atmosphere separately.
Because there is a radiative equilibrium, we know that the planet emits enough longwave radiation into space to equal the solar radiation absorbed by it. However, most of the longwave radiation that is emitted by the planet into space is emitted by the atmosphere, rather than the surface of the earth beneath it.
Although the surface of the earth does emit longwave radiation, only about 10% of this radiation passes through the atmosphere and escapes into space.
The rest is absorbed by clouds and by greenhouse gases in the atmosphere.
There are many greenhouse gases, but the most abundant greenhouse gases are water vapor and carbon dioxide. Shortwave radiation from the sun passes through greenhouse gases, but longwave radiation is absorbed by them.
Greenhouse gases absorb longwave radiation that is emitted by the surface of the earth.
Subsequently, they re-emit the energy as longwave radiation in all directions. About half of the re- emitted longwave radiation does escape into space, and contributes to the planet’s radiative equilibrium.
About half of the longwave radiation emitted by the gases is directed back toward the surface of the earth. As a result, a continual exchange of longwave radiation takes place between the surface of the earth and the atmosphere above it.
The longwave radiation contained in this exchange causes the warming effect known as the greenhouse effect.
This phenomenon is known as the greenhouse effect because, like the glass on a greenhouse, the atmosphere traps some of the energy beneath it.
The greenhouse effect produces a warmer climate on earth than would be possible without an atmosphere.
In summary, water vapor and carbon dioxide allow solar energy to penetrate earth’s atmosphere but prevent much of the longwave radiation emitted by the surface of the earth from escaping to space.
This trapping of longwave energy is called the greenhouse effect and enhances the surface temperature of the earth.