I'm afraid I'm going to have to disagree with you Greg. Neither oxygen nor nitrogen absorb IR in any significant amount in the atmosphere.
It absorbs different freq.
As you can see the horizontal scale below represents the wavelengths which correspond with freq and heat. Moving from left to right you move from hotter to colder (higher freq to lower freq. Shorter wavelengths to longer wavelengths).
As you already know, the argument is that CO2 absorbs escaping IR and re-emits it back to the earth. This isn't adequately explained either. CO2 for the most part, does not re-emit at the same freq it absorbs. Yes it excites the molecules and force collisions which in turn produce heat, but this is not the same thing.
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Is the greenhouse effect a good thing?
Well, yes, if you appreciate living.
Does the atmosphere (or any greenhouse gas) act a blanket?
At best, the reference to a blanket is a bad metaphor. Blankets act primarily to suppress convection; the atmosphere acts to enable convection. To claim that the atmosphere acts a blanket, is to admit that you don't know how either one of them operates.
Does the atmosphere trap radiation?
No, the atmosphere absorbs radiation emitted by the Earth. But, upon being absorbed, the radiation has ceased to exist by having been transformed into the kinetic and potential energy of the molecules. The atmosphere cannot be said to have succeeded in trapping something that has ceased to exist.
Does the atmosphere reradiate?
One often hears the claim that the atmosphere absorbs radiation emitted by the Earth (correct) and then reradiates it back to Earth (false). The atmosphere radiates because it has a finite temperature, not because it received radiation. When the atmosphere emits radiation, it is not the same radiation (which ceased to exist upon being absorbed) as it received. The radiation absorbed and that emitted do not even have the same spectrum and certainly are not made up of the same photons. The term reradiate is a nonsense term which should never be used to explain anything.
Sometimes diagrams are drawn which show the radiation from the Earth's surface rising into the sky and being reflected off of the atmosphere (or clouds, or greenhouse gases). This too is nonsense. The radiation was not reflected, it was absorbed and different radiation was subsequently emitted.
Does the atmosphere trap heat (in producing the greenhouse effect)?
Alas no. As rapidly as the atmosphere absorbs energy it loses it. Nothing is trapped. If energy were being trapped, i.e. retained, then the temperature would of necessity be steadily rising. Rather, on average, the temperature is constant and the energy courses through the system without being trapped within it.
Does the atmosphere behave like a greenhouse?
The name, greenhouse effect is unfortunate, for a real greenhouse does not behave as the atmosphere does. The primary mechanism keeping the air warm in a real greenhouse is the suppression of convection (the exchange of air between the inside and outside). Thus, a real greenhouse does act like a blanket to prevent bubbles of warm air from being carried away from the surface. As we have seen, this is not how the atmosphere keeps the Earth's surface warm. Indeed, the atmosphere facilitates rather than suppresses convection.
One sometimes hears the comparison between the greenhouse effect in the atmosphere (not in real greenhouses) and the interior of a parked car which has been left in the summer Sun with its windows rolled up. This comparison is as phony as is the comparison to real greenhouses. Again, keeping the windows closed merely suppresses convection.
Whether the topic is a real greenhouse or a car, one still hears the old saw that each stays warm because visible radiation (light) can pass through the windows, and infrared radiation cannot. Actually, it has been known for the better part of a century that this has very little bearing on the issue. "
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A Cautionary Note on “Re-radiation”: Scholars who should know better (I have done it myself) sometimes use the term reradiation to describe the absorption of a photon by a molecule and the subsequent emission of a photon. This usage is almost always false and misleading. It does sometimes happen that a molecule will absorb a photon of a specific wavelength and subsequently emit a photon of the identical wavelength, but this is relatively rare. The wavelengths of emitted photons are highly temperature dependent. A
molecule of water or carbon dioxide or ozone may readily absorb a short
wavelength/high energy photon of solar radiation, but is extremely unlikely to emit such a photon. Instead, the molecule will most likely emit several photons of a longer wavelength and lower energy content.
Therefore, neither the atmosphere nor the Earth’s surface absorbs and reradiates radiant energy. Both absorb radiation and both emit radiation. Over a sufficient period of time, the total energy content of the absorption will equal the total energy content of the emission if the temperatures remain constant. However, the number of photons absorbed and their wavelengths will not equal the number of photons emitted. Moreover, the set of wavelengths absorbed will not be the same as the set of wavelengths emitted."
At night though, the normal incoming heat from the sun has stopped. The heat rising from the earth could be used by CO2 but over the course of the night, the surface of the earth gets cooler and the frequency being emitted by the earth also changes. This makes it invisible to CO2. The heat from the CO2 emissions experiences the same thing. When CO2 releases heat into the cooler atmosphere (an atmosphere now cooler than that of the first absorption) frequency changes.
Another thing, if the surface of the earth is getting hotter due to CO2, then obviously that surface will eventually get so hot that it will emit photons which are too hot (or of too high a freq) for CO2 to capture. And you're right back to square one.
However, I do agree that a doubling of CO2 is not going to double heat absorption. The effect of a doubling of CO2 is logarithmic. Current observations agree with paleoclimate reconstructions show a climate sensitivity of approximately 3 to 3.5 deg. increase in GAT C for each doubling of CO2. That of course is provide that there aren't any significant other forcings in play such as the sun or increased or decreased albedo.
We're not talking about that
. The individual wanted to know about the effects of CO2.
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