Earth is covered with a blanket of gas made mainly of nitrogen, oxygen, carbon dioxide, and water vapor. This blanket is essential to life on Earth, for (at least) two reasons:
1. We breathe the air, taking in the oxygen and breathing out carbon dioxide. Similarly, plants use sunlight together with the carbon dioxide in the atmosphere to photosynthesize and create organic compounds.
2. The atmosphere absorbs the infrared or longwave radiation emitted by Earth's surface and re-emits it back to Earth, so warming the surface up to a habitable temperature.
Because this is a book on physical science, not biology, we will consider only this second effect, which is similar to the effect of glass in a greenhouse and for that reason is called the greenhouse effect.
Let us, then, consider the path of solar and infrared radiation through the atmosphere, as illustrated in figure 1.3. If the sky is clear, then most of the solar radiation incident at the top of the atmosphere goes through the atmosphere to the surface. If the sky is cloudy, then roughly half of the solar radiation is reflected back to space, with the rest either absorbed in the cloud or passing through to Earth's surface. When the solar radiation reaches the surface, a fraction is reflected and returns to space, and the rest of the radiation is absorbed, warming the surface. All told, combining the effects of the reflection by clouds and at Earth's surface, the fraction of solar radiation reflected back to space—that is, the planetary albedo—is about 0.3. The albedo of clouds themselves
is higher than this, typically about 0.5, but it can be as high as 0.9 for thick clouds; the albedo of the surface is on average about 0.1 but is much higher if the surface is covered with fresh snow or ice.
The surface is warmed by the solar radiation absorbed and so emits radiation upward, but because the surface temperature of Earth is so much less than that of the sun, the radiation emitted has a much longer wavelength—it is infrared radiation. Now, the atmosphere is not transparent to infrared radiation in the same way that it is to solar radiation; rather, it contains greenhouse gases—mainly carbon dioxide and water vapor—that absorb the infrared radiation as it passes through the atmosphere. Naturally enough, this absorption warms the atmosphere, which then re-emits infrared radiation, some of it downward, where it is absorbed at Earth's surface. Thus, and look again at figure 1.3, the total downward radiation at the surface is much larger than it would be if Earth had no atmosphere. Consequently, the surface is much warmer than it would be were there no atmosphere, and this phenomenon is known as the greenhouse effect.
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