| The Earth and Venus are near each other in the | | | | the temperature to rise in the atmosphere |
| Solar System, and are similar in size, density, and | | | | (assuming no other effects intervened---a big "if" |
| composition. Based on our understanding of the | | | | in the realistic case since the atmosphere is |
| origin of the Solar System, we would expect that | | | | complicated). This would be a greenhouse effect. |
| their initial atmospheres would have been rather | | | | It would become a runaway greenhouse effect if |
| similar. Yet the present atmospheres of the two | | | | the rising temperature approached the boiling point |
| planets could hardly be much more different than | | | | of water, because then the oceans would begin |
| they are. How did this come to be? The reason is | | | | to convert to water vapor, the water vapor |
| thought to lie in what is termed the "Runaway | | | | would increase the effectiveness of heat trapping |
| Greenhouse Effect". | | | | and accelerate the greenhouse effect, this would |
| Sunlight falling on the surface of a planet is | | | | cause the temperature to rise further, thus |
| primarily in the visible part of the spectrum. | | | | causing the oceans to evaporate faster, etc., etc. |
| However, the reflection of light from the surface | | | | (This type of runaway is also called a "positive |
| tends to produce light of longer wavelength called | | | | feedback loop".) When the oceans were gone the |
| infrared (IR) radiation (also known as radiant heat; | | | | atmosphere would finally stabilize at a much higher |
| IR radiation is the heat that we sense being | | | | temperature and at much higher density, because |
| radiated from a hot surface like a hot piece of | | | | all the water would now be in the atmosphere. |
| metal). | | | | We can envision even a further runaway stage in |
| Now, because of their molecular structures, | | | | this scenario. Suppose the preceding runaway |
| certain gases like carbon dioxide and water vapor | | | | raised the temperature so high that chemical |
| (and many others) have the property that they | | | | reactions begin to occur that drive the carbon |
| are essentially transparent to visible light but | | | | dioxide from the rocks into the atmosphere (the |
| absorb IR radiation very strongly. Such | | | | process is called sublimation; a few hundred |
| compounds are sometimes termed greenhouse | | | | degrees Celsius would be sufficient). Then another |
| gases because, if they are present in a planetary | | | | runaway would occur as the carbon dioxide |
| atmosphere, they absorb the scattered IR | | | | feeding into the atmosphere would accelerate the |
| radiation and tend to raise the temperature of the | | | | heating, which would in turn accelerate the |
| atmosphere by trapping solar energy. (The | | | | transfer of carbon dioxide from the rocks to the |
| analogy with a real greenhouse is imperfect | | | | atmosphere. |
| because the mechanism by which a greenhouse | | | | The atmosphere would finally stablilize at a still |
| stays warm is different, but it is sufficiently good | | | | higher temperature and pressure after all the |
| that the name "(Planetary) Greenhouse Effect" is | | | | carbon dioxide had been driven from the rocks. In |
| now the common one for this phenomenon.) | | | | fact, we believe that if this sequence were to |
| The greenhouse effect occurs for all planetary | | | | take place on the Earth, the resulting temperature |
| atmospheres containing greenhouse gases, and is | | | | and pressure of the atmosphere left behind would |
| responsible for their being warmer than would be | | | | not be very different from that for present-day |
| the case otherwise. The greenhouse effect by | | | | Venus: the atmospheric termperature would be |
| itself could not account for the conditions that we | | | | hundreds of degrees Celsius and the pressure |
| find on Venus. However, under certain conditions | | | | would be maybe 100 times greater than it is |
| we believe the greenhouse effect can "run away". | | | | today. |
| For example, consider the case of a planet like | | | | Thus, we believe that in the case of Venus the |
| the Earth. The Earth has enormous amounts of | | | | initial solar heating kept oceans from forming, or |
| two greenhouse gases: water vapor and carbon | | | | kept them from staying around if they did form, |
| dioxide. However, for the Earth most of the | | | | and the subsequent lack of rainfall and failure of |
| water and carbon dioxide are not in the | | | | plant life to evolve kept the carbon dioxide in the |
| atmosphere. The water is mostly in the oceans, | | | | atmosphere rather than binding it in the rocks as |
| and the carbon dioxide is mostly bound chemically | | | | is the case for the Earth; thus, Venus has an |
| in rocks made from compounds that chemists call | | | | environmental disaster for an atmosphere. |
| carbonates (for example, limestone). | | | | The sobering warning for us is obvious: we have |
| Now suppose we increased the effectiveness of | | | | to be extremely concerned about processes such |
| greenhouse heating of the Earth's atmosphere, | | | | as burning of fossil fuels in large volumes that |
| for example by increasing the amount of solar | | | | might (we don't know for sure because the |
| radiation falling on it, or by increasing the | | | | scientific questions are complex) have the |
| concentration of greenhouse gases in the | | | | potential to trigger a runaway greenhouse effect |
| atmosphere (for example, by burning fossil fuels, | | | | and produce on the Earth atmospheric conditions |
| which produce water vapor and carbon dioxide as | | | | such as those found on Venus. |
| byproducts of burning). We would then expect | | | | |