2/7/00
RADIATION
Electromagnetic radiation can be thought of as energy traveling in the form of waves at the speed of light - 186,000 miles per second or 300,000 kilometers per second. It can also be described as a stream of massless, energetic particles knowns as photons.
E/M radiation is most often characterized according to its wavelength; each photon has a certain wavelength and the amount of energy carried by that photon depends on its wavelength. Photons with longer wavelengths carry lower energy, while photons with shorter wavelengths carry higher energy. Typical wavelengths are very small - about one millionth of a meter; this unit is called the micrometer or micron. Different types of radiation are classified according to the wavelength. Figure 2.6 shows these classifications, and the amount of energy per photon associated with each class. Together, all these types of radiation over the entire range of wavelengths, from a fraction of a micrometer to hundreds of meters, is referred to as the electromagnetic spectrum
For the earth and atmosphere, we are most interested in the following wavelength regions of the electromagnetic spectrum:
- Ultraviolet (UV): wavelengths from about 0.1 to 0.4 micrometers
- Visible: wavelengths from 0.4 (violet) to 0.7 (red) micrometers
- Infrared (IR) : wavelengths from about 0.7 to 10 micrometers
All objects emit electromagnetic radiation continuously. The radiation emitted by a solid object (or a liquid) has certain fundamental characteristics which are described using the term ``blackbody radiation''. This term has nothing to do with an object's color, but instead it refers to how it absorbs and emits radiation. A blackbody is considered to be perfect absorber and emitter of energy (everything it absorbs it will emit back, with no energy lost). For this course we will assume the earth's surface and liquid water in the atmosphere emit blackbody radiation with the following characteristics:
- they emit radiation over a wide range of wavelengths; the amount
of energy emitted at each wavelength is not the same however. If you plot
the amount of energy emitted at each wavelength, you get a curve like that
shown in Figure 2.7. Note that the wavelength where most of the sun's energy
comes from (or "wavelength of peak emission") is in the visible portion of
the spectrum (it occurs exactly at 0.5 micrometers, which is the wavelength
of green light. Why then isn't the sun green?)
- the wavelengths of radiation an object emits depends on the object's
temperature; a hotter object will radiate more energy at shorter
wavelengths, a cooler object radiates most of its energy at longer
wavelengths. The wavelength of peak emission (see Figure 2.7) is determined
by Wien's Law. (You should know the mathematical formula for Wien's
Law.) Bacause the sun has an average temperature of about 5800 K, it emits
most of its energy at 0.5 micrometers, according to Wien's Law. The earth
has an average temperature of about 288 K; it emits most of its energy at
about 10 micrometers.
- The total amount of energy an object emits is also related to its temperature. Objects with a higher temperature will of course emit more energy than cooler objects. The mathematical formula describing this is the Stefan-Boltzmann law. (You should know this formula too.)
2/9/00
RADIATION (cont'd)
Gases: Selective absorbers and emitters
Unlike solid objects or liquids, gases in our atmosphere do not behave as blackbody emitters/absorbers of radiation. Instead, they are referred to as selective absorbers and emitters. Gases only absorb and emit radiation at specific wavelengths. This is different from solids or liquids which emit and absorb over a wide range of wavelengths.
The specific wavelengths at which a gas aborbs radiation is determined by its molecular structure, and so different types of gases absorb at different wavelengths. Figure 2.9 shows the abosrbing wavelengths for some selected gases in our atmosphere as well as for the entire atmosphere (bottom panel). Note that the gases in our atmosphere absorb very little in the wavelength intervals between 0.3 - 0.7 micrometers and 8 - 11 micrometers. These intervals of little or no absorption are called window regions because radiation at these wavelengths is free to travel through the atmosphere to the surface or out to space with no losses.
The window regions are important for determining the energy balance at the earth's surface. Most of the sun's energy is in the visible portion of the spectrum (Wien's Law). This passes through the 0.3 - 0.7 micrometer "window" and is absorbed at the earth's surface. The earth emits in the infrared region near 10 micrometers, which passes through the 8 - 11 micrometer window and out to space. during the day, the sirface absorbs more energy than it emits, and heats up. At night, the surface emits more than it absorbs, and cools off. [This is called radiative cooling].
Clouds, remember, are not selective absorbers; instead they absorb infrared
(IR) radiation over a wide range of wavelengths. As a result, the presence of
clouds in the atmosphere will in effect "close" the atmospheric window. At
night, the infrared radiation emitted by the earth's surface would normally
pass through the 8 - 11 mocrometer window, but in the presence of clouds this
radiation is absorbed and re-emitted back to the surface. The net effect is
that, at night, clouds can keep the surface warmer than it would be
otherwise.
2/11/00
GREENHOUSE EFFECT
Because gases such as water vapor and carbon dioxide are selective absorbers of infrared radiation, they keep the sirface of the earth warmer than it would be otherwise through the so-called greenhouse effect, which operates as follows:
- Solar radiation at visible wavelengths passes through the atmosphere to
the surface without being absorbed
- The earth's surface absorbs this radiation, heats up, and emits infrared
radiation upward
- Greenhouse gases (water vapor, carbon dixoide, methane, ozone, CFC's)
absorb some of this upward radiation; some of this energy is radiated
back down to the surface by the gases (note: energy is radiated, not
reflected!).
- Net effect: surface is warmer than it would be if no greenhouse gases are present (about 60 degrees F warmer)
Positive and negative feedback
What happens if we keep putting more carbon dioxide into the atmosphere? If we keep going at the present rate, we will double the amount of CO2 in our atmosphere (500 ppm by 2100 AD) compared to preindustrial times (250 ppm). The effect of increasing CO2 alone is believed to warm the surface by about 1 degree Celsius on average. Whether the surface continues to warm depends on the presence of feedback in the earth/atmosphere system, a process by which an initial change can either be reinforced (positive) or weakened (negative).
Positive feedback: This is based on the fact that water vapor is the most important greenhouse gase, mainly because there is a lot of it in our atmosphere. This is also based on the fact that "warmer air holds more water vapor"; the exact meaning of this will be discussed in Chapter 4, so for now we will just take it on faith. Positive feedback scenarios generally work as follows:
- More CO2 causes surface to warm (about 1 degree C under double-CO2 conditions)
- Warmer surface means warmer air near surface; this warmer air will drive additional evaporation from the ocean, and so there will be more water vapor in the air
- More water vapor means more greenhouse gases in the atmosphere, so surface continues to warm
- Return to step 2.
Negative feedback: This is based on the role that clouds play in the radiative balance of the earth's surface. As stated above, clouds at night can warm the surface. During the day, clouds will of course cool the surface by shading it from the sun's rays. Overall, the net effect of clouds is believed to be a cooling effect, meaning on average, the earth's surface would be warmer if there were no clouds in our atmosphere. Negative feedback involving clouds works as follows:
- More CO2 causes surface to warm
- Warmer surface means warmer air near surface, and thus more water vapor in the atmosphere
- More water vapor leads to more clouds; more clouds means, on average, a cooler surface
- Initial warming (step 1) is weakened.
Clearly, whether positive or negative feedback takes place depends on the role of clouds. Clouds are still not very well understood, in terms of their radiative effects on climate, which is why scientists currently can't predict with absolute certainty what will happen in the future. What is clear is that we are putting more CO2 into the atmosphere, and it will change the radiative balance of the earth, one way or another.