The Atmospheric Greenhouse Effect

The atmospheric greenhouse effect is the result of electromagnetic energy from the Sun that is trapped in a planet’s atmosphere. Atmospheric gases freely transmit visible- -short wavelength- solar energy, warming the planet’s surface. The warmed surface tries to radiate the excess energy back into space, but because the planet is much colder than the Sun, it radiates at much longer, infrared (IR) wavelengths. But carbon dioxide and water vapor strongly absorb IR radiation, converting it to thermal (heat) energy. They subsequently re-radiate this thermal energy in all directions; some of the thermal energy continues into space, but much of it returns to the ground. The planetary surface receives thermal energy both from the Sun and from the atmosphere, and consequently it heats up.

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Nowhere in the solar system is the atmospheric greenhouse effect more dramatic than on Venus. Its opaque cloud cover and massive atmosphere of carbon dioxide and sulfur compounds raises Venus’s surface temperature by more than 400 K, to about 740 K (870’F). At such high temperatures, any remaining water and most carbon dioxide locked up in surface rocks would long ago have been driven into the atmosphere, further enhancing the atmospheric greenhouse effect.

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Why does the atmospheric greenhouse effect make the composition of Earth’s atmosphere so different from that of Venus? The answer lies in Earth’s location in the solar system. Consider early Earth and early Venus, each having about the same mass, but with Venus orbiting somewhat closer to the Sun than Earth does. Volcanoes and comet impacts poured out large amounts of carbon dioxide and water vapor to form early atmospheres on both planets. Most of Earth’s water quickly rained out of the atmosphere to fill vast ocean basins, but Venus was closer to the Sun, and its surface temperature was higher than Earth’s, so most of the rainwater on Venus immediately re-evaporated. Venus was left with a surface containing very little liquid water and an atmosphere filled with water vapor. The continuing buildup of both water vapor and carbon dioxide in the atmosphere of Venus then led to a runaway (out of control) atmospheric greenhouse effect that drove up the surface temperature of the planet. Ultimately, the surface of Venus became so hot that no liquid water could survive there.

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This early difference between a watery Earth and an arid Venus forever changed the ways that the two planets’ atmospheres and surfaces evolved. On Earth, water erosion caused by rain and rivers continually exposed fresh minerals, which then reacted chemically with atmospheric carbon dioxide to form solid carbonates (minerals containing metal combined with oxygen and carbon). This reaction removed some of the atmospheric carbon dioxide, burying it within Earth’s crust as a component of a carbonate rock called limestone. Later, the development of life in Earth’s saltwater oceans accelerated the removal of atmospheric carbon dioxide. Tiny sea creatures built their protective shells of carbonates, and as they died they built up massive beds of limestone on the ocean floors. As a result of water erosion and the various chemical reactions related to living organisms, all but a trace of Earth’s total inventory of carbon dioxide is now tied up in limestone beds. Earth’s particular location in the solar system seems to have spared it from the runaway atmospheric greenhouse effect. But what if Earth had formed a bit closer to the Sun? If all the carbon dioxide now in limestone beds had not been locked up by these reactions, Earth’s atmospheric composition would resemble that of Venus or Mars.

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Some scientists think that Venus once had as much water as Earth-as liquid oceans or as more water vapor than is measured today. If that’s true, what happened to all the water? One possibility is that water molecules high in Venus’ s atmosphere were broken apart into hydrogen and oxygen by solar ultraviolet radiation. Hydrogen atoms, being of very low mass, were quickly lost to space. Oxygen escaped more slowly, so some eventually migrated downward to the planet’s surface, where it could have been removed from the atmosphere by combining with surface minerals. The Venus Express spacecraft has measured hydrogen, and some oxygen, escaping from the upper levels of Venus’s atmosphere.

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?The atmospheric greenhouse effect is the result of electromagnetic energy from the Sun that is trapped in a planet’s atmosphere. Atmospheric gases freely transmit visible- -short wavelength- solar energy, warming the planet’s surface. The warmed surface tries to radiate the excess energy back into space, but because the planet is much colder than the Sun, it radiates at much longer, infrared (IR) wavelengths. But carbon dioxide and water vapor strongly absorb IR radiation, converting it to thermal (heat) energy. They?subsequently?re-radiate this thermal energy in all directions; some of the thermal energy continues into space, but much of it returns to the ground. The planetary surface receives thermal energy both from the Sun and from the atmosphere, and consequently it heats up.

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