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Global Warming and Electromagnetic Radiation


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Posted
  • Location: Near Newton Abbot or east Dartmoor, Devon
  • Location: Near Newton Abbot or east Dartmoor, Devon
Intended as a technical discussion. I will post a synopsis shortly . . .

It's not electromagnetism, but electromagnetic radiation :whistling: . Oh, and I think we only need one technical thread (mods, could we combine the two, and perhaps add some of the posts form the naysayers thread for context?).

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Posted
  • Location: Rochester, Kent
  • Location: Rochester, Kent

Here we go, then:

The most important concept here is temperature, and how temperature moves around. As I understand it temperature can move through three mechanisms: Radiation, Convection, and Conduction. So first, some basic definitions:

Radiation is an electromagnetic emission from a source that has heat. It travels at the speed of light and does not have a temperature even though it is sometimes referred to as thermal radiation. The more heat the source has the more radiation is emitted. The energy in radiation is converted to heat when the wave interacts with matter that can absorb it.

Convection is the movement of currents within fluids primarily due to density changes in the fluid. Warm air rises because when air is heated it expands and therefore becomes less dense, so ‘floats’ upwards.

Conduction is the transfer of heat through matter from a warm surface to a less warm surface. By definition the surfaces must be in contact. Note the term ‘warm’ is relative, here.

As the sun is hot it must emit radiation. This is important, because heat cannot move through a vacuum, but radiation can. The radiation emitted transfers in all directions throughout space, and eventually, some of it, hits the outside of the Earth’s atmosphere.

Solar radiation consists (mainly) of visible light, and near infrared in roughly 50/50 proportions. This rather shortwave (near infrared) radiation passes through our atmosphere with limited absorbtion primarily because the compounds in the atmosphere do not absorb radiation of this wavelength.

The Earth’s surface, however, is different. The sun’s radiation heats the Earth’s surface. Once the Earth’s surface has started to warm all three mechanisms for heat transfer can be employed.

Firstly, any body that has heat emits radiation by the combination of Plancks, Wien’s and Stefan-Boltzmann laws. So if the Earth is heated it must, by nature of existence, give of radiation. The primary radiation emitted by Earth is in the far infrared frequency which is longwave radiation.

Some of the components of the atmosphere can absorb this longer wave radiation; and these are rather badly termed, Greenhouse Gases.

These gases absorb the long wave radiation and heat up. Once they have heat (in this case more heat) they emit radiation in exactly the same process as the sun and the earth. Some of this radiation is absorbed into the earth’s surface, thus acting to heat the earth further, and some is radiated to space. This is the greenhouse effect. It has nothing to do with greenhouses except the notion that something below something else keeps warm. The process is entirely different.

The two other processes are what constitute our weather. The warmed Earth’s surface, by conduction heats the air in contact with it. This air, by convection, rises up through the atmosphere.

It's not electromagnetism, but electromagnetic radiation :whistling: . Oh, and I think we only need one technical thread (mods, could we combine the two, and perhaps add some of the posts form the naysayers thread for context?).
Yes good point. Can't edit the title, now, though . . .
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Posted
  • Location: Mytholmroyd, West Yorks.......
  • Weather Preferences: Hot & Sunny, Cold & Snowy
  • Location: Mytholmroyd, West Yorks.......

Whooa there trigger! We receive far more than visible and 'near infra red' from our star.As I understand it nearly the whole of the electro magnetic spectrum (and many particles) are emitted. The interaction of these 'wave energies' on our own magnetosphere may well be a way to 'trim temperatures' up or down a few degrees depending on the suns output and the impacts of 'bursts' of energy on our planet.

If we are looking here for another 'natural forcing' that could be hidden in amongst the other known AGW mediums then I'd be interested to explore all that the sun emits in the way of 'energy' and their possible interactions with our planet!

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Posted
  • Location: Brighouse, West Yorkshire
  • Location: Brighouse, West Yorkshire

X-rays, Gamma rays ,visible light, UV light, infra-red, radio waves etc. are all just terms that describe frequency ranges within the electromagnetic spectrum. All electromagnectic radiation can be described by its wavelength. The sun emits energy that covers a wide range of wavelengths.

The earth's surface (being solid!) is much denser than the atmosphere. Wavelengths of radiation that can pass directly through the earths atmosphere are absorbed by the surface. When an object absorbs any frequency of electromagnetic radiation it heats up.

At the kind of temperatures that you find at the surface of the earth, objects will radiate their heat energy in the infra red spectrum.

Through this mechanism, energy that might have arrived at the earth in the form of visible light (or UV light or whatever) can be emited from the earth's surface in the infra red spectrum.

The CO2 in the atmosphere is very effective at absorbing/reflecting infra red radiation. Some of the energy that would have been radiated straight into space is now reflected/radiated back down to the surface again. This results in the earth's surface/atmosphere having a higher temperature than it would have if the CO2 was not there.

/edited because I am rubbish at typing.

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Posted
  • Location: Rochester, Kent
  • Location: Rochester, Kent
The CO2 in the atmosphere is very effective at absorbing/reflecting infra red radiation. Some of energy that would have been radiated straight into space is now reflected/radiated back down to the surface again. This results the earth's surface/atmosphere having a higher temperature than it would have if the CO2 was not there.

As I understand it there is no process of 'reflection' The GHG's are heated through interactions with earth outbound radiation. By virtue of them having heat, they then emit, by themselves, further radiation which is in turn absorbed by surface creating some form of circular feedback mechanism.

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Posted
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
As I understand it there is no process of 'reflection' The GHG's are heated through interactions with earth outbound radiation. By virtue of them having heat, they then emit, by themselves, further radiation which is in turn absorbed by surface creating some form of circular feedback mechanism.

To be exact, the word "heat" should not be used in this context. Heat is what we perceive when an energetic object gives off radiation (generally infrared radiation). The word we should be using is Energy. When infrared radiation strikes a CO2 molecule, the CO2 molecule absorbs the energy, becomes more energetic, and gives off its own radiation as a result. (If the quantum, or packet, of energy it receives is big enough then the molecule becomes too energetic to sustain itself and breaks down into its constituent atoms.)

So, although the process is like a form of partial reflection of energy, in fact it is an absorption of energy rapidly followed by an emission on energy. Because a molecule (or rather all of the molecule's parts) exists only in quantum states (that is, for simplicity's sake, either a value of "1" or a value of "2", but never a value in between), the energy emitted is equal to the energy absorbed. So a CO2 molecule does not store "heat" or energy - it acts more like a wicket keeper, catching the ball and then throwing it straight back out again.

(Similarly, photons are not actually reflected by the objects they hit - what we see are re-emitted photons from the objects themselves, which have absorbed the original incoming photons.)

C-Bob

EDIT - Edited for clarity! :whistling:

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Posted
  • Location: Rochester, Kent
  • Location: Rochester, Kent

I don't agree with this. Infrared radiation is not heat. See Devonian's post elsewhere. We feel heat by thermal conduction, and radiation. Thermal conduction is when the air around us heats up and we're in contact with it, radiative heating is described above.

Perhaps I've got it wrong, can you post some links, please?

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Posted
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
I don't agree with this. Infrared radiation is not heat. See Devonian's post elsewhere. We feel heat by thermal conduction, and radiation. Thermal conduction is when the air around us heats up and we're in contact with it, radiative heating is described above.

Perhaps I've got it wrong, can you post some links, please?

The effect we perceive, at the macroscopic level, as heat is, at the microscopic level, the absorption and emission of radiation. The processes of Radiation, Convection and Conduction are the result of the cumulative effect of multiple absorptions and emissions at the microscopic level.

Conduction is the easiest to describe in this manner: imagine three atoms in a row. A single quantum of energy is sent flying towards Atom-1. Atom-1 absorbs the energy, then emits an equivalent amount of energy which flies towards Atom-2. Atom-2 absorbs this energy, then emits an equivalent amount of energy towards Atom-3. Atom-3 absorbs this energy, then emits an equivalent amount of energy back out of the system. If this quantum of energy were to then hit your hand, the energy would be absorbed by the outermost part of your skin, exciting the atoms in the process, and causing you the sensation known as heat.

It all comes down to quanta of energy flitting around and stimulating any particles with which they come into contact.

:whistling:

C-Bob

EDIT - You're right: infrared radiation is not heat, but infrared radiation's interaction with other particles is what we call "Heat".

EDIT #2: Here's a link to wikipedia's entry for "Heat":

http://en.wikipedia.org/wiki/Heat

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Posted
  • Location: New York City
  • Location: New York City
To be exact, the word "heat" should not be used in this context. Heat is what we perceive when an energetic object gives off radiation (generally infrared radiation). The word we should be using is Energy. When infrared radiation strikes a CO2 molecule, the CO2 molecule absorbs the energy, becomes more energetic, and gives off its own radiation as a result. (If the quantum, or packet, of energy it receives is big enough then the molecule becomes too energetic to sustain itself and breaks down into its constituent atoms.)

So, although the process is like a form of partial reflection of energy, in fact it is an absorption of energy rapidly followed by an emission on energy. Because a molecule (or rather all of the molecule's parts) exists only in quantum states (that is, for simplicity's sake, either a value of "1" or a value of "2", but never a value in between), the energy emitted is equal to the energy absorbed. So a CO2 molecule does not store "heat" or energy - it acts more like a wicket keeper, catching the ball and then throwing it straight back out again.

(Similarly, photons are not actually reflected by the objects they hit - what we see are re-emitted photons from the objects themselves, which have absorbed the original incoming photons.)

C-Bob

EDIT - Edited for clarity! :whistling:

This is a good post, that says a lot of what I was gonna say. I highlighted the bits in bold which is partially correct. The first bit is correct, but they need not be the same photon. A molecule can go thru various energy levels, electronic, rotational and vibrational, it can be its this process that causes the emitted radiation from the earth to be a different wavelength from the incident. This process occurs in the atmosphere with the carbon dioxide which causes the atmosphere to heat up a bit, i believe, but I'm not an expert on that field.

Vibrational energy levels concern infrared radiation, "heat"

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Posted
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
This is a good post, that says a lot of what I was gonna say. I highlighted the bits in bold which is partially correct.

Thanks Hiya! Re-reading that post it does suggest that it's the exact same quantum or photon getting thrown back out again, doesn't it? I didn't intend for it to read that way, honest! :whistling:

Now that we've cleared up the issue of what "Heat" is, where are we going with this...?

:blink:

C-Bob

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Posted
  • Location: Rochester, Kent
  • Location: Rochester, Kent

To paraphrase, for my simple, mind; energy races around in the form of radiation. This has no apparent temperature is really only energy. When this radiation contacts something it interacts with it. In the instance of infrared wavelength we perceive that particular wavelength of radiation to be heat. Conduction is the process of getting molecules to excite in such a way to get it to give off more ir radiation, and encouraging neighbouring molecules to do the same, hence conduction.

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Posted
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
To paraphrase, for my simple, mind; energy races around in the form of radiation. This has no apparent temperature is really only energy. When this radiation contacts something it interacts with it. In the instance of infrared wavelength we perceive that particular wavelength of radiation to be heat. Conduction is the process of getting molecules to excite in such a way to get it to give off more ir radiation, and encouraging neighbouring molecules to do the same, hence conduction.

Yup! That's pretty much it in a nutshell, I think! :good:

So I guess next we want to work out how much incoming energy the CO2 molecules receive, how many molecules receive it, and how much those molecules re-emit back towards the ground...?

:doh:

C-Bob

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Posted
  • Location: Rochester, Kent
  • Location: Rochester, Kent
Yup! That's pretty much it in a nutshell, I think! ;)

So I guess next we want to work out how much incoming energy the CO2 molecules receive, how many molecules receive it, and how much those molecules re-emit back towards the ground...?

:)

C-Bob

There's that 're-emit' again :(

Given that a molecule of a gas, in this case CO2, can absorb radiation from any source, terrestial, or otherwise, if, indeed, that molecule is an absorber of said spectrum of radiation, then the emmision must therefore be a function (linear or otherwise) of the sum of all absorbed radiation (given that radiation causes the molecule to excite which causes the molecule to heat, and emit radiation) whether it came from the Earth, Space, or alien radar. It is therefore not re-emmitting the Earths radiation -which implies reflection- but merely emitting based on the sum of all the radiation it has absorbed regardless of the source.

Are there, for instance, any extra-terrestial sources of LW radiation? (I ask because I am assuming that CO2 is a good absorber of LW radiation) such as LW emmissions from planets (if there is a vacuum between us and them, I presume that, like light, it doesn't 'fade' and therefore is perfectly capable of reaching us - and there's an awful lot more LW producing surface area in our solar system than just Earth!) and if there is, is this significant, and even more so, if it's significant is it accounted for in climate models?

Another question: is abosrbtion (of whatever wavelength) a function of pressure? Would we expect absorbtion to drop with altitude, for instance?

:huh:

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Posted
  • Location: Rochester, Kent
  • Location: Rochester, Kent

Found this which looks reasonably good; which, I must add, mentions the process of radiative reflectivity; not that it mentions CO2 LW reflectivity (if it's absorbing can it also be reflecting? Does it work like a sponge - once the maximum absorbtivity has been reached does the CO2 acts like a reflector?)

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Posted
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
There's that 're-emit' again...It is therefore not re-emmitting the Earths radiation -which implies reflection- but merely emitting based on the sum of all the radiation it has absorbed regardless of the source.

That's a good point - I used the word "re-emit" wrongly, though I intended to use it for the right reasons! "Re-emit" was supposed to suggest that the emission of radiation was as a result of an equivalent amount of radiation being absorbed, not that it was the same radiation that had been absorbed.

I've heard before that a given amount of CO2 will only "reflect" (by the process above, not through actual reflection, as far as my understanding goes) a certain amount of "heat" (or, more accurately, energy) because it becomes saturated. This does give the impression that CO2 molecules somehow hold heat (energy) in (acting like capacitors, as you mentioned in an earlier post), but my understanding is that this is rubbish. The reason only a certain proportion of energy will be "reflected" back to Earth is surely because the CO2 is so diffuse in the atmosphere (so most of the radiation simply "misses" the CO2 molecules)? Or am I wrong? Anyone want to clarify this?

I'll check out your link, VP, and get back to you a bit later on.

:huh:

C-Bob

EDIT - Presumably absorption would be a function of pressure - the higher the pressure is, the more dense the gas is, which implies that at higher pressures the CO2 molecules would be closer together and so less radiative energy would "slip through the cracks" between molecules. Note that the rate of absorption would not increase at the molecular level, but rather it would increase as the result of there simply being more molecules to absorb the radiation in any given area.

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Posted
  • Location: New York City
  • Location: New York City
There's that 're-emit' again :(

Given that a molecule of a gas, in this case CO2, can absorb radiation from any source, terrestial, or otherwise, if, indeed, that molecule is an absorber of said spectrum of radiation, then the emmision must therefore be a function (linear or otherwise) of the sum of all absorbed radiation (given that radiation causes the molecule to excite which causes the molecule to heat, and emit radiation) whether it came from the Earth, Space, or alien radar. It is therefore not re-emmitting the Earths radiation -which implies reflection- but merely emitting based on the sum of all the radiation it has absorbed regardless of the source.

Are there, for instance, any extra-terrestial sources of LW radiation? (I ask because I am assuming that CO2 is a good absorber of LW radiation) such as LW emmissions from planets (if there is a vacuum between us and them, I presume that, like light, it doesn't 'fade' and therefore is perfectly capable of reaching us - and there's an awful lot more LW producing surface area in our solar system than just Earth!) and if there is, is this significant, and even more so, if it's significant is it accounted for in climate models?

Another question: is abosrbtion (of whatever wavelength) a function of pressure? Would we expect absorbtion to drop with altitude, for instance?

:huh:

- Not quite sure what you mean by this but when a molecule absorbs a photon it can either emit a photon of the same energy to return to its ground state or it can lose multiple photons to return to its ground state.

- IR radiation is redshifted from other planets, however shorter wavelengths are redshifted to IR when they reach us but due to the laws of intensity it makes no difference to us, infact we need a powerful telescope to detect it.

- The sun

- It is light, just a different wavelength from the light we see

-

Found this which looks reasonably good; which, I must add, mentions the process of radiative reflectivity; not that it mentions CO2 LW reflectivity (if it's absorbing can it also be reflecting? Does it work like a sponge - once the maximum absorbtivity has been reached does the CO2 acts like a reflector?)

- only one photon of the intrested radiation band will be absorbed per molecule of carbon dioxide ~4.2e-6m afterwhich it begins the process of returning to its ground state thru emission, quenching etc. A shorter photon will cause a different effect on the molecule, the one we're intrested in is the asymmetrical stretch, if two photons come along in quick sucession the second will more than likely pass straight thru.

That's a good point - I used the word "re-emit" wrongly, though I intended to use it for the right reasons! "Re-emit" was supposed to suggest that the emission of radiation was as a result of an equivalent amount of radiation being absorbed, not that it was the same radiation that had been absorbed.

I've heard before that a given amount of CO2 will only "reflect" (by the process above, not through actual reflection, as far as my understanding goes) a certain amount of "heat" (or, more accurately, energy) because it becomes saturated. This does give the impression that CO2 molecules somehow hold heat (energy) in (acting like capacitors, as you mentioned in an earlier post), but my understanding is that this is rubbish. The reason only a certain proportion of energy will be "reflected" back to Earth is surely because the CO2 is so diffuse in the atmosphere (so most of the radiation simply "misses" the CO2 molecules)? Or am I wrong? Anyone want to clarify this?

I'll check out your link, VP, and get back to you a bit later on.

;)

C-Bob

EDIT - Presumably absorption would be a function of pressure - the higher the pressure is, the more dense the gas is, which implies that at higher pressures the CO2 molecules would be closer together and so less radiative energy would "slip through the cracks" between molecules. Note that the rate of absorption would not increase at the molecular level, but rather it would increase as the result of there simply being more molecules to absorb the radiation in any given area.

- as above

- no its not, the half life of molecular vibrations mean that a capacitor analogy is viable.

- Yeah thats the basis of CO2 "greenhouse effect" increasing concentration of carbon dioxide, more IR reflected back to earth. Conversely more of this same wavelength reflected back into space from the sun :)

- absolutely correct, its called the Beer-Lambert law. It even has a special form that is used for atmospheric purposes.

I think villageplank would find this http://en.wikipedia.org/wiki/Bohr_model quite educational, which is with regards to electronic transitions, but the same principle applies to the other three types too.

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Posted
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
  • Location: A small planet somewhere in the vicinity of Guildford, Surrey
...the half life of molecular vibrations mean that a capacitor analogy is viable...

Great post, Hiya. That cleared some things up for me, too. My area of interest is atomic and sub-atomic physics (then I skip several orders of magnitude and get interested in things like stars and galaxies and, eventually, the whole universe!), but I'm a bit fuzzy on molecular actions and interactions!

Thanks for that :huh:

C-Bob

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Posted
  • Location: Rochester, Kent
  • Location: Rochester, Kent

Thanks for the info, chaps.

With this amount of knowledge flying around is it possible, then, for us to create a model of CO2 absorbtivity, and emissivity as a function of incoming radiation and pressure?

Has someone already done this? I've googled, but google doesn't want to be my friend and returned nothing of interest.

Anyone?

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Posted
  • Location: New York City
  • Location: New York City
Thanks for the info, chaps.

With this amount of knowledge flying around is it possible, then, for us to create a model of CO2 absorbtivity, and emissivity as a function of incoming radiation and pressure?

Has someone already done this? I've googled, but google doesn't want to be my friend and returned nothing of interest.

Anyone?

I can't do that, although I can help. If capt bob knows what he's doing I'll keep him right on the molecular level.

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Posted
  • Location: New York City
  • Location: New York City
I can compile the model, I just need to relationships . . .

The more I think about it the more things you need to include in this model.

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