On Earth, two elements, nitrogen (N2) and oxygen (O2), make up almost 99% of the volume of clean, dry air. Most of the remaining 1% is accounted for by the inert gaseous element, argon (Ar). Argon and the tiny percentage of remaining gases are referred to as trace gases. Certain trace atmospheric gases help to heat up our planet because they appear transparent to incoming visible (shortwave) light/radiation but act as a barrier to outgoing infrared (longwave) radiation. These special trace gases are often referred to as greenhouse gases which are heat-trapping gases that radiate the heat back to the Earth's surface, to another greenhouse gas molecule, or out to space.
The major greenhouse gases are carbon dioxide (CO2), water vapor (H2O), methane (CH4), and nitrous oxide (N2O). These greenhouse gas molecules all are made of three or more atoms. The atoms are held together loosely enough that they vibrate when they absorb heat. Eventually, the vibrating molecules release the radiation, which will likely be absorbed by another greenhouse gas molecule. This process keeps heat near the Earth’s surface and contributes to the greenhouse effect.
Plants, animals, and humans are able to live and thrive on Earth because the conditions in our atmosphere are just right. Earth’s atmosphere acts like a blanket, keeping the temperatures from being too hot or too cold for living organisms. Without the greenhouse effect, Earth’s average temperature would be roughly -19° C (-2° F), well below freezing, instead of the current comfortable 15° C (59° F).
We can evaluate the effect of greenhouse gases by comparing Earth with its nearest planetary neighbors, Venus and Mars. These planets either have too much greenhouse effect or too little to be able to sustain life as we know it. The differences between the three planets have been termed the "Goldilocks Principle". The "Goldilocks Principle" refers to the fact that Venus is too hot, Mars is too cold, but Earth is just right.
Mars and Venus have essentially the same types and percentages of gases in their atmosphere. However, they have very different atmospheric densities.
- Venus has an extremely dense atmosphere and many more greenhouse gases than Earth. The concentration of CO2 is responsible for a "runaway" greenhouse effect and a very high annual average surface temperature of 464° C (867° F).
- Mars has a very thin atmosphere, and therefore very small amounts of greenhouse gases compared to Earth; so even though the atmosphere of Mars is mostly CO2, there is so little of it that it does not warm Mars very much and the annual average surface temperatures are quite low at around -63° C (-82° F).
- Mars is much further away from the Sun than is Venus.