Greenhouse Effect

Key idea: Planetary surfaces are heated by radiation from the Sun. The warm planetary surfaces then emit infrared radiation of their own. The Greenhouse effect describes the condition where this infrared radiation is trapped and absorbed by gases and vapor in the atmosphere, and the atmosphere is then heated more than it would otherwise be without the Greenhouse effect.

Astronomical tidbit: The photograph at the left shows the planet Venus shrouded in clouds of sulphuric acid. These clouds permanently cover the entire planet, and make conventional photography of the surface of Venus impossible. Global cloud features are clearly visible in this photograph. The atmosphere of Venus is extremely thick and is composed primarily of carbon dioxide. The presence of carbon dioxide in such a high concentration contributes to a runaway Greenhouse effect on Venus, and the surface temperature there is nearly 900 Fahrenheit degrees. That is hot enough to melt lead!!! The photograph was taken at ultraviolet wavelengths with the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope on January 24, 1995. Some of the cloud features in the photograph were enhanced with false color.

You have learned that planets are illuminated with electromagnetic radiation from the Sun. Some of this radiation is reflected by clouds and particles within the planetary atmospheres, and some of the radiation is absorbed within the atmospheres. Of the radiation which reaches the surfaces of the planets (for those planets which have surfaces!), a small fraction is reflected. The remainder of the radiation is absorbed by and heats the planetary surfaces. The Sun radiates electromagnetic energy at all wavelengths, but the radiation is strongest in the visible light portion of the electromagnetic spectrum. Most of the radiation which is absorbed by and which heats the surface of our planet is visible light radiation.

Warm planetary surfaces also emit electromagnetic radiation. The wavelength of the radiation is longer than that of most of the incoming radiation, however, because the planetary surfaces are much cooler than the Sun. Radiation emitted by the planetary surfaces is predominantly in the infrared region of the electromagnetic spectrum. This infrared radiation may not be free to propagate back through the atmosphere. The Greenhouse effect describes the condition where this infrared radiation is trapped and absorbed by gases and vapor in the atmosphere. The atmosphere is then heated more than it would otherwise be without the Greenhouse effect. Surface temperatures may be substantially higher than they would otherwise be without Greenhouse heating.

Questions: Why do you suppose that this phenomenon where visible light radiation from the Sun passes through the atmosphere and heats the surface, but infrared radiation from the warm surface is trapped by the atmosphere is called the Greenhouse effect? Why does it get extremely hot inside a car which is sitting in the Sun with the windows rolled up???

The temperature on the surface of the Earth is about 60 Fahrenheit degrees higher than it would otherwise be without a Greenhouse effect. If the Earth were that much cooler, it would be impossible for life to sustain itself. The average temperature on Earth would be well below the freezing point of water. We owe our lives to the Greenhouse effect!!!

The most significant Greenhouse gases in Earth's atmosphere are water vapor and carbon dioxide. Some scientists are concerned that man may be releasing too much carbon dioxide into our atmosphere, and that the additional CO2 will contribute to global warming due to an increased Greenhouse effect on Earth. World leaders are now meeting regularly in an effort to slow the world-wide release of Greenhouse gases such as carbon dioxide into our atmosphere.

Question: What do you think the effects on Earth would be if the global temperatures began to rise??

The planet Venus has a runaway Greenhouse condition. The temperature on the surface of Venus is nearly 900 Fahrenheit degrees which is hot enough to melt lead. It is estimated that the surface temperature on Venus would actually be below 0 Fahrenheit without the Greenhouse effect!!! The runaway condition results because the atmosphere of Venus is composed of very dense carbon dioxide. Carbon dioxide is a Greenhouse gas because it is very effective at absorbing the infrared emissions from the surface without letting them radiate away into space.

The atmosphere of Mars is also predominantly carbon dioxide, but Mars does not have a significant Greenhouse condition because the atmosphere is so thin that it cannot absorb much of the infrared energy which is emitted by the surface. The surface pressure on Mars is about 100 times less than it is on Earth, which is itself about 90 times less than it is on Venus. It is estimated that the Greenhouse effect on Mars warms the atmosphere at the surface by less than 10 degrees Fahrenheit.

Astronomical tidbit: This picture of Earth was taken by the crew of Apollo 17 in December, 1972. If not for the Greenhouse effect, Earth would look very different than it does in this photograph. The Greenhouse effect warms the Earth by approximately 60 degrees Fahrenheit, and maintains the average temperature on Earth above the freezing point for water. This allows us to maintain vast oceans and to support a wide variety of life forms. Notice in the photograph that huge masses of land appear to be extremely dry and arid even though most of our planet is covered with water. If global warming results from the man's continued release of tremendous amounts of Greenhouse gases into the atmosphere, more and more of Earth's surface may begin to look just as Africa and the Middle East do in this photograph.

Last updated: November 17, 1999

Joe Twicken / joe@nova.stanford.edu
Rob Wigand