There is a big urban legend out there: the Bad Greenhouse
Effect. It is also frequently mixed up with global warming.
To give an example, I quote Zumdahls' book Chemistry: "The atmosphere, like window glass, is transparent to visible light but does not allow all the infrared radiation pass back into space." The atmosphere does not operate like a window glass or a real greenhouse. This is a false
analogy. A window suppresses convection, the atmosphere facilitates it.
The name "greenhouse effect" is a confusing misnomer.
The correct statement is:
The surface of Earth is warmer with an atmosphere, because it
receives radiation from two distinct sources: the Sun and the atmosphere.
To visualize: Look at a random point in the sky on Earth. Most probably you see the atmosphere: scattered blue light, clouds, or only the twinkle of stars at night. The same on the Moon - most probably you see the unfluctuating black of the interstellar void. Which would radiate more heat?
A given point on Earth receives radiation not only from the Sun, but from the whole sky, even at night when the Sun isn't there. This "greenhouse
effect" is essential to life, as it stabilises the temperature(1 30 K above the no-atmosphere equilibrium. The temperature difference between night and day is only 10-50 K. To compare, Moon has no atmosphere and there the difference can be as much as 300 K. The atmosphere can be compared to a dark coating on the surface of the Earth. We're in it, and receive the warmth it absorbs from the sun from all directions, not directly from the sun itself.
See? We didn't need blankets, wavelengths, reradiation or anything
complicated. The atmosphere is no different from a rock taken from a camp fire, the Sun or any hot object, so it does radiate because it has a finite temperature. This radiation is black-body radiation(2, which has no preferred direction. Thus the phrases "reradiate" and "radiate back" are confusing and should never be used, because they implicitly state that the radiation would have a preferred direction.
Let's look at some common misconceptions.
- Atmosphere does not "trap" heat. If it did, it would heat up. The Earth is
isothermal(3, so it must absorb the same amount of heat it emits.
- Parked cars, greenhouses and blankets operate by suppressing convection.
The air in them would rise up without the blocking of air flow. This mechanism
is completely different from the effect keeping the surface of Earth warm, as
it is relevant only in an atmosphere. Space is vacuum.
- Atmosphere does not "reflect heat back and forth", because it has no reflective surfaces. It merely absorbs the energy as heat and eventually radiates this energy to space, along with the rest of the black-body radiation. No specific, individual sunlight rays are involved, that is, there is only energy in and energy out.
- Atmosphere does have an absorption spectrum, but its effect is very small
compared to the greenhouse effect (as defined above). So, atmosphere can be said to "let visible light pass but absorb infrared" to an extent, but this has no significance in the context of the "greenhouse" effect. What is important that the atmosphere is an overall absorber rather than a frequency-specific absorber. Or in more scientific terms, integrate the energy absorption over the entire spectrum and consider the temperature that gives the corresponding emission of energy.
A note on the "reflective surface" point: There are clouds in the lower atmosphere. When they reflect radiation, it reflects the incoming visible light back into space, so the effect is cooling, not warming. This happens every once in a while when a large volcano erupts: the following summers are cool and rainy. The last one was Mount Pinatubo.
The absorption spectrum does have a significance in the context of global warming. Atmosphere receives heat from the Sun. Consequently, its temperature
depends on the efficiency of absorption of Sun's radiation. Water vapor, carbon dioxide and methane absorb and retain(4 thermal energy more efficiently than dry air. Adding them to the atmosphere causes an elevation of the equilibrium temperature. This effect is called "global warming".
An analogy to global warming is black asphalt vs. white paint surface. Asphalt absorbs heat with a larger power (joules/second) than white paint, so it has to have an equal addition to the power of thermal radiation - and thus, have a higher equilibrium temperature. Both surfaces do emit heat as much as they absorb, so there's no "trapping" heat. Carbon dioxide and methane are "black" compared to dry air.
The reason for the "IR blackness" of carbon dioxide and methane is simple molecular geometry: air is mostly composed of the diatomic gases nitrogen and oxygen. Diatomic molecules have only one bond, and so only one mode of vibration: stretching and contracting along the one bond. This corresponds to a single frequency; a single "IR color" only, so to speak. Carbon dioxide has two bonds, so there are more degrees of freedom for modes: stretching both; stretching one, contracting other; contracting both; and bending the molecule while doing the previous. This corresponds to many more "IR colors" and larger overall absorption of sunlight. Methane with four bonds has even more degrees of freedom for vibration, and consequently it is a 30 times stronger greenhouse gas than carbon dioxide.
It is a different question if the global warming is in progress, and if it
is, is it natural or caused by fossil fuel emissions.
Read Dr. Kevin Lehmanns explanation:
http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html
(1 Average temperature at surface is 22 C or 295 K.
(2 That is, atmosphere obeys the Stefan-Boltzmann Law. I am not implying atmosphere would be a perfect black body. It isn't.
(3 Here, "isothermal" means that the average surface temperature is constant in the long run.
(4 In this context, "to absorb more efficiently" means that the absorption spectrum of the gas matches better with the emission spectrum of the Sun. "To retain" refers to the higher isobaric heat capacity of vapor than that of air, not "trapping of heat".
The quote was from Chemistry, 5th Ed., p. 270, by Steven
Zumdahl and the Houghton Mifflin Company. Note that the book is for university,
that is, for educating future scientists!
Bad Spelling corrected by C-Dawg :)