The Radiation Balance
The Spectrum of Electromagnetic Radiation
The incoming energy from the Sun to Earth is mainly visible sunlight, called the “visible portion of the spectrum of electromagnetic radiation.” We perceive visible sunlight as colors from violet (short-wave radiation) to red (long-wave radiation). The sequence of colors seen in the rainbow represents the “spectrum” of this light, ordered according to wavelength. A relatively minor amount of energy leaves the sun as radiation with shorter wavelength (“ultraviolet”) and as radiation with longer wavelength (“infrared” or “heat radiation”).
Visible light (the colors of the rainbow) occupies the narrow part of the spectrum between the dashed lines in the first figure. The (invisible) light with wavelengths just shorter than violet is called “ultraviolet,” meaning “beyond violet.” It is largely absorbed in the atmosphere and only a modest amount of this light arrives at the surface of Earth. This is fortunate, because ultraviolet light, abbreviated “UV,” can cause damage to skin and to vertebrate retinas and interferes with photosynthesis in algae and plants. Protection from UV light is provided (among other things) through the ozone layer in the lower stratosphere (The ozone layer will be discussed in more detail later Section 2.6). The (invisible) light with wavelengths just longer than red is called “infrared,” meaning “below red” and often simply referred to as “IR.” IR is heat radiation coming from the Sun. Interestingly, some organisms, especially some insects, can see UV and navigate by it, while some snakes have adapted to see in IR. (See the Glossary for more reading on electromagnetic radiation under Spectrum.)
An idealized spectrum
of electromagnetic radiation
from the Sun, with a temperature
of 6000 °C (~11,000 °F) , and the much cooler Earth, with a surface temperature of 15 °C (~60 °F). Wavelength
along the horizontal axis (measured in microns where 1000 microns = 1mm) plotted against the amount of radiation emitted on the vertical axis. Note how the Sun emits the most radiation in the visible range (with some overlap in the UV and IR) while the Earth emits mostly infrared radiation
The Sun’s Radiation
All objects (unless they have a temperature of zero degrees Kelvin) radiate energy. The temperature of an object determines the type of radiation it emits. Hence, every star radiates energy with wavelengths corresponding to its surface temperature: a cooler star would radiate a more reddish light, a hotter one a more bluish one. Reddish and bluish stars can be seen readily in the night sky. Most of the light emitted by our star, the Sun, is yellowish. By measuring the light received from the Sun we know that its radiation corresponds to a surface temperature of about 6000°C (or 6300 K where “K” is the symbol for units of Kelvin – see the Units Glossary for more about the Kelvin scale). The organisms on Earth have long adapted to the nature of this sunlight. Blue-green light penetrates most deeply into the sea, so visual acuity of deep-living fishes is greatest in the blue-green part of the spectrum. Our own eyes are specialized for yellow, green and red (the colors of traffic lights). Plants use mostly red light to grow and reflect the rest, making them appear green.
The nuclear fusion generator that powers the Sun is deep in the Sun’s center (called the “core” of the Sun), hidden by a thick layer of hot hydrogen and helium. This is fortunate for us, because no one could look at the Sun's power plant and survive the experience: the temperature is near 15 million degrees of Kelvin. The reason the power plant does not blow the sun apart is that the enormous pressure of the solar matter surrounding it prevents it from doing so. Conversely, the Sun does not collapse because of the counter-pressure generated in the core because in the Sun the gravitational and radiative pressures are in balance. It takes about a million years for the energy to made in the core to reach the surface of the Sun. From there it takes less than 10 minutes at the speed of light to reach Earth.
The energy the core generates comes from the fusion of hydrogen nuclei to make helium nuclei. Through this process some of the mass is “lost” and reappears as energy (according to the famous equation of Albert Einstein E=mc2), resulting in the loss of 4.5 million tons of mass from the Sun each second. Not to worry however: there is still plenty of hydrogen to burn —about two-thirds of the mass of the sun consists of hydrogen — and the process has been going on for some 5 billion years and will do so in the future for about as long (Also see the Glossary entry on Solar constant.).
The Earth’s Radiation
Just as the temperature of the Sun's surface determines the kind of electromagnetic radiation it delivers, so the temperature of the Earth determines what kind of radiation it puts out to space, which turns out to be infrared, or “heat” radiation. As mentioned, the amount of heat Earth has to get rid of is entirely determined by the amount it receives from the Sun in the first place minus the portion it immediately reflects back into space. (The reflected portion cannot be included in the portion that is re-radiated because it does not actually heat the Earth. However, this reflected portion is visible from a spacecraft or when standing on the Moon: the Earth is reasonably bright as planets go, mainly because of its clouds and its ice caps and reflects 30% of the light it receives back to space. This proportion, called the “albedo” of Earth, is less than that reflected by Venus, but more than that of Mars - see the Glossary for more on albedo.).
The kind of infrared radiation given off by the various areas of Earth's surface depends on their temperature, which in turn depends on a number of factors such as the amount of sunlight absorbed and the heat spent in evaporating water. In the desert, after sundown, one can readily sense the high-energy infrared given off by rocks recently warmed by the Sun's rays, but all surfaces radiate heat, whether recently warmed by the Sun or not. Typically, temperatures on the surface of Earth vary somewhere between freezing and 90°F, which roughly defines the broad "spectrum" of infrared radiation emitted upward into the atmosphere.
Graph showing the mostly visible radiation spectrum emitted from the Sun as seen from the surface of the Earth (left curve) and the IR radiation emitted Earth as seen from space (right curve). Compared it to Figure 1 this graph shows the wavelengths that are absorbed by atmospheric gases, causing gaps in the spectrum. For the left curve, it is obvious how ozone acts to absorb UV light
before it reaches the Earth’s surface with some visible light also being absorbed by water vapor. For the right curve, the impact that the major greenhouse gases
have on the amount of IR that Earth emits to space is clear: carbon dioxide, ozone
, and water vapor act together to absorb and re-emit radiation that stays trapped in the lower atmosphere (that is, the outgoing radiation from Earth is much less intense than expected from the temperature one would measure in space due to the effect of greenhouse gases). Carbon dioxide absorbs at wavelengths centered on 15 microns, ozone at wavelengths of 10 microns, and water vapor over broad ranges of wavelengths.
The Absorption of IR by Greenhouse Gases
Now that we have explained the relationship between the radiation emitted by an object and its temperature, we can explain how greenhouse gases warm the Earth. Certain “lines” within the electromagnetic spectrum, specifically certain wavelengths of infrared radiation, have precisely the right energy to interact with certain molecules present within Earth’s atmosphere. When such a special packet of light (called a “photon”) interacts with the appropriate molecule, the molecule absorbs the energy, and increases its temperature accordingly. It then re-radiates heat to its surroundings. When measured with an instrument, this absorbed heat forms absorption lines or even “absorption bands” that are broader than lines and may include several lines.
The absorption bands of different greenhouse gases may or may not overlap with each other. When a greenhouse gas is very abundant the absorption lines for which it is active are said to “become saturated,” that is, most of the available IR will have been absorbed by the molecules of that gas. Adding more of that gas will not absorb more IR in the proportion of the addition. For example, many of carbon dioxide’s absorption lines are fairly well saturated. This is the fundamental reason that the 30 percent increase in carbon dioxide since the industrial revolution has not increased the background greenhouse effect by 30 percent. Only a doubling of CO2 will have a substantial effect, through the amplification caused by water vapor (resulting in a 4 to 6°F increase, according to the best estimates). Another doubling on top of this presumably will have a similar effect, in part through a broadening of the absorption lines affected.
Other greenhouse gases that were once rare or even nonexistent are now being introduced vigorously by human activities, and they can take up new absorption lines that have not been previously occupied. Thus, their effect on interception of IR and the associated greenhouse effect is accordingly enhanced. Methane is such a gas, as are the chlorofluorocarbon compounds (abbreviated CFC’s) formerly used in refrigeration.