Venus and Earth are about the same size and so close that they are frequently called the “twin planets” of our solar system. Yet, Venus is so hot that lead will melt on its surface! A runaway greenhouse effect makes Venus this hot. The greenhouse effect occurs when the atmosphere of a planet acts much like the glass in a greenhouse. Like the greenhouse glass, the atmosphere allows visible solar energy to pass through, but it also prevents some energy from radiating back out into space.
The greenhouse effect insures that the surface of a planet is much warmer than interplanetary space because the atmosphere traps heat in the same way a greenhouse traps heat. Certain gases, called greenhouse gases, tend to reflect radiant energy from the Earth back to the Earth’s surface, improving the atmosphere’s ability to trap heat. All greenhouse gases are trace gases existing in small amounts in our atmosphere. Greenhouse gases include carbon dioxide, methane, nitrous oxide, some chlorofluorocarbons, and water vapor.
We know that the greenhouse effect is necessary for survival. Without it, the Earth would be so cold that life as we know it couldn’t exist. However, scientists still have questions that must be answered. What kinds and amounts of green- house gases are necessary for survival? Are the amounts of greenhouse gases increasing, decreasing, or remaining the same? To answer these questions, scientists monitor the amounts of greenhouse gases in the Earth’s atmosphere.
The atmospheric gas most responsible for the warming effect on both Venus and Earth is carbon dioxide (CO2). On both planets, a primary source of CO2 is volcanic eruptions. The difference between these two planets is that on Venus, 97% of the atmosphere is CO2, whereas on Earth, muc h less than one percent of the atmosphere is CO2. Why is there so much less CO2 on Earth? The carbon cycle holds the answer.
In the natural cycle of carbon, plants take in CO2 and give off oxygen (O2), whereas animals take in O2 and emit CO2. Further, CO2 dissolved in seawater is used by plants during photosynthesis and by other seawater organisms, such as clams and coral, to produce calcium carbonate (CaCO3) shells. These processes help control the amount of CO2 in our atmosphere.
Human beings complicate the natural carbon cycle because they increase the amount of CO2 in Earth’s atmosphere by burning fossil fuels. Driving automo- biles, heating buildings, and producing consumer goods all add to the concen- tration of CO2 in Earth’s atmosphere.
Methane (CH4) is another greenhouse gas. It is produced in swamps, bogs, and rice paddies, as well as in the intestinal tracts of most animals, including cattle, sheep, and humans. Coal, oil, and gas exploration also contribute to the accu- mulation of CH4 in the atmosphere. However, CH4 concentrations are much less than CO2 concentrations.
Nitrous oxide (N2O), or “laughing gas,” is another greenhouse gas accumulating in the atmosphere, although not as fast as CH4. Fertilizer decomposition, indus- trial processes that use nitric acid, and small amounts from automobile emis- sions all contribute to increasing atmospheric N2O.