Explain how the greenhouse effect works and why it is important.

Understanding how the atmosphere is heated requires an understanding of how atmospheric gases interact with the short-wavelength incoming solar radiation and the long-wavelength outgoing radiation emitted by Earth. Figure 2.20 shows that the majority of solar radiation is emitted in wavelengths shorter than 2.5 micrometers—shortwave radiation. By contrast, most radiation from Earth’s surface is emitted at wavelengths between 2.5 and 30 micrometers—longwave radiation.

Heating the Atmosphere

When a gas molecule absorbs radiation, the energy is transformed into kinetic energy, which is detectable as a rise in temperature (sensible heat). For example, the absorption of UV energy by oxygen molecules in the stratosphere accounts for the high temperatures experienced there.

The lower part of Figure 2.20 gives the absorptivity of the principal atmospheric gases. The only significant absorbers of incoming solar radiation are water vapor, oxygen, and ozone, which account for most of the solar energy absorbed directly by the atmosphere. Oxygen removes most of the shorter-wavelength UV radiation high in the atmosphere, and ozone absorbs UV rays in the stratosphere between 10 and 50 kilometers (6 and 30 miles). If most UV radiation were not absorbed before it reaches Earth’s surface, human life would not be possible because UV energy disrupts our genetic code.

At the bottom of Figure 2.20, you can see that for the atmosphere as a whole, none of the gases are effective absorbers of visible radiation with wavelengths between 0.3 and 0.7 micrometer. This visible light band constitutes about 43 percent of the energy radiated by the Sun. Because the atmosphere is a poor absorber of visible radiation, most of this energy is transmitted to Earth’s surface. Thus, we say that the atmosphere is nearly transparent to incoming solar radiation. Solar energy is not an effective “heater” of Earth’s atmosphere.

The atmosphere is generally a relatively efficient absorber of longwave (infrared) radiation emitted by Earth (see the bottom right of Figure 2.20). Water vapor and carbon dioxide are the principal absorbing gases, with water vapor absorbing about 60 percent of this terrestrial radiation. Therefore, water vapor, more than any other gas, accounts for the warm temperatures of the lower troposphere, where it is most highly concentrated.

Although the atmosphere is an effective absorber of most radiation emitted by Earth’s surface, it is quite transparent to the band of radiation between 8 and 12 micrometers. Notice in Figure 2.20 (lower right) that the gases in the atmosphere (mainly CO2, and H2O) absorb minimal energy in these wavelengths. Because the atmosphere is transparent to radiation between 8 and 12 micrometers, much as window glass is transparent to visible light, this band is called the atmospheric window. Although other “atmospheric windows” exist, the one located between 8 and 12 micrometers is the most significant because it is located where Earth’s radiation is most intense. Thus, the atmospheric window is important because it allows longwave radiation from Earth’s surface to pass directly to space without being absorbed. In addition, the atmospheric window allows satellites to detect outgoing longwave radiation and monitor what’s happening at the surface and in the atmosphere.

By contrast, clouds that are composed of tiny liquid droplets (not water vapor) are excellent absorbers of the energy in the atmospheric window. Clouds absorb outgoing longwave radiation and radiate much of this energy back to Earth’s surface. Thus, clouds serve a purpose similar to window blinds because they effectively block the atmospheric window and lower the rate at which Earth’s surface cools. This explains why nighttime temperatures remain higher on cloudy nights than on clear nights.

Because the atmosphere is largely transparent to solar (shortwave) radiation but more absorptive of terrestrial (longwave) radiation, the atmosphere is heated from the ground up. This explains the general drop in temperature with increased altitude in the troposphere. The farther from the “radiator” (Earth’s surface), the colder it gets. On average, the temperature drops 6.5°C for each kilometer (3.5°F per 1000 feet) increase in altitude, a figure known as the normal lapse rate (see Chapter 1). The fact that the atmosphere does not acquire the bulk of its energy directly from the Sun but is heated by Earth’s surface is of utmost importance to the dynamics of the weather.

The Greenhouse Effect

Research of “airless” planetary bodies such as the Moon have led scientists to determine that if Earth had no atmosphere, it would have an average surface temperature below freezing. Fortunately, Earth’s atmosphere “recycles” some of the outgoing radiation, which makes our planet habitable.

As discussed earlier, clear skies are largely transparent to incoming shortwave solar radiation and transmit much of the shortwave radiation to Earth’s surface. This radiation is absorbed at the surface and eventually reradiated skyward as longwave terrestrial radiation. Gases that absorb longwave radiation are called greenhouse gases. Two atmospheric gases, water vapor and carbon dioxide, absorb a significant portion of the longwave radiation emitted by Earth’s surface. As Earth’s radiation heats these absorptive gases, the temperature of the atmosphere increases. The atmosphere, in turn, radiates some of this energy out to space, but more important, it radiates an equivalent amount back toward Earth’s surface, where it further warms the lower atmosphere. This process is known as the greenhouse effect. Without this complicated game of “pass the hot potato,” Earth’s average temperature would be −18°C (0°F) rather than the current temperature of 15°C (59°F) (Figure 2.21).

When you think of the greenhouse effect, you probably think of the greenhouses that are used for growing plants. The glass in a greenhouse allows shortwave solar radiation to enter and be absorbed by the objects inside. These objects, in turn, radiate energy, but at longer wavelengths that warm the air inside the greenhouse. Unlike the atmosphere, the glass ceiling prevents convection and traps the heat inside the greenhouse. Despite this difference, the term greenhouse effect is still used to describe atmospheric heating.

Media reports frequently and erroneously identify the greenhouse effect as the “villain” in the global warming problem. However, the greenhouse effect and global warming are different concepts. Without the greenhouse effect, Earth would be uninhabitable. Scientists have mounting evidence that human activities (particularly the release of carbon dioxide into the atmosphere) are responsible for a rise in global temperatures (see Chapter 14). Thus, humans are compounding the effects of an otherwise natural process (the greenhouse effect). It is incorrect to equate the greenhouse phenomenon, which makes life possible, with global warming—which involves undesirable changes to our atmosphere and is caused mainly by human activities.

• atmospheric window: Terrestrial radiation between 8 and 11 micrometers in length to which the troposphere is transparent.

• greenhouse effect: The transmission of shortwave solar radiation by the atmosphere coupled with the selective absorption of longer-wavelength terrestrial radiation, especially by water vapor and carbon dioxide, resulting in warming of the atmosphere.

• greenhouse gases: Gases in the atmosphere that absorb infrared (longwave) radiation emitted by Earth.

• Radiant energy absorbed at Earth’s surface is eventually reradiated skyward. Because Earth has a much lower surface temperature than the Sun, its radiation is in the form of longwave infrared radiation. Because atmospheric gases, primarily water vapor and carbon dioxide, are more efficient absorbers of terrestrial (longwave) radiation, the atmosphere is heated primarily from the ground up.

• The greenhouse effect refers to the selective absorption of terrestrial radiation by atmospheric gases, mainly water vapor and carbon dioxide, that causes Earth’s average temperature to be warmer than it would be otherwise.

• The greenhouse effect is a natural phenomenon that makes Earth habitable. It is often, but inaccurately, portrayed as the “villain” of global warming, but human activities that release greenhouse gases (primarily carbon dioxide) into the atmosphere actually cause climate change.