Reading 3

THE GREENHOUSE EFFECT

1                                  In the nineteenth century, scientists discovered that certain gases in Earth’s atmosphere behave like the panes of glass in a greenhouse. These gases admit visible radiation from the sun but prevent the escape of infrared radiation from Earth’s surface. Because of their radiative properties, the action of these atmospheric gases is known as the greenhouse effect, and the gases are known as greenhouse gases. Water vapor is the principal greenhouse gas. Others are carbon dioxide, ozone, methane, and nitrous oxide.

2                                  Window glass is relatively transparent to visible radiation but slows the transmission of infrared radiation. Plant greenhouses are designed to take advantage of this property of glass by being constructed almost entirely of glass panes. Visible radiation—light from the sun— enters the greenhouse, and is absorbed by the dark plants. Thus, light energy is converted to heat. The plants use some of the heat and re–radiate the rest as infrared radiation, most of which cannot escape the greenhouse because it is trapped by the solid glass. The atmosphere inside the greenhouse therefore becomes heated, and the temperature can rise well above that of the outside air. Similarly, greenhouse gases in Earth’s atmosphere keep infrared radiation from escaping into space, thereby keeping the planet warm.

3                                  However, the greenhouse analogy is not completely accurate because the trapping of infrared radiation by glass is only part of the reason that most plant greenhouses retain internal heat. Greenhouses cut heat loss mainly by acting as a shelter from the wind, thereby reducing heat loss due to conduction and convection. As a rule, the thinner the greenhouse glass and the stronger the external wind speed, the more important the shelter effect is. Still, the greenhouse analogy remains relevant in most discussions of radiation balance in Earth’s atmospheric system.

4                                  The effect of a greenhouse gas can be seen by comparing the typical summer weather of the American Southwest with that of the coast along the Gulf of Mexico. Both areas are at about the same latitude and therefore receive about the same intensity of solar radiation. In both places, afternoon temperatures typically exceed 30 degrees Celsius. At night, however, air temperatures often differ remarkably due to the absence or presence of the leading greenhouse gas, water vapor. In the desert Southwest, there is less water vapor in the air to impede the escape of infrared radiation; therefore, heat is readily lost to space. Air temperatures on the surface of the Southwest desert may fall below 15 degrees Celsius. In contrast, along the more humid Gulf Coast, infrared radiation does not escape to space as readily, and minimum temperatures may fall only into the 20s Celsius.

5                                  Clouds produce a greenhouse effect because they are composed of radiation–absorbing water droplets or ice crystals. Nights are usually warmer when the sky is cloud–covered than when the sky is clear. However, clouds can affect climate in two opposing ways. On the one hand, clouds warm the planet’s surface by absorbing and re–radiating infrared radiation; on the other hand, they cool the surface by reflecting solar radiation away from Earth. Analyses of satellite measurements of radiation indicate that clouds have a net cooling effect on global climate. Thus, a more extensive cloud cover would tend to cool the planet.

6                                  The greenhouse effect also operates on other planets. On both Mars and Venus, the principal atmospheric gas, carbon dioxide, is also the main greenhouse gas. Earth has an abundance of plants to absorb carbon dioxide, but Mars and Venus do not possess living organisms. Consequently, Earth’s two closest neighbors have extremely high concentrations of atmospheric carbon dioxide. The atmosphere of Mars is considerably thinner than the atmosphere on Earth, so its greenhouse effect raises the average surface temperature by only about 10 degrees Celsius. In contrast, the atmosphere of Venus is about 90 times denser than Earth’s, and its greenhouse warming is estimated at 523 degrees Celsius. The hot, thick, cloud–filled atmosphere shrouding Venus is composed of 97 percent carbon dioxide.

7                                  Some scientists believe that Venus used to be similar to Earth, with liquid water on the surface. Then, billions of years ago, Venus started to heat up. Eventually, all its surface water evaporated into the atmosphere, and planetary warming became self–sustaining and unstoppable. Venus provides a warning for what could happen on Earth if the greenhouse effect continued unchecked and a high percentage of surface water became water vapor. If that happened, global warming would reach the point of no return, as it did on Venus.

Glossary:

conduction: the transmission of heat through a heat–conveying substance convection: the transfer of heat by the movement of air currents