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Q & A: radiation and global warming

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Most recent answer: 11/29/2013
Q:
Is it possible for radiation emitted from a cooler energy source to heat a warmer energy source?
- Ian Bryant (age 57)
Paphos, Cyprus
A:

People often ask this question when they're thinking about global warming caused by greenhouse gases in the atmosphere. These gases radiate energy back to the earth, which is somewhat warmer than the atmosphere. 

The answer is that the radiation certainly warms the warmer source compared to what it would have been without that extra incoming radiation. The net flow of heat on a cold day is always from your body to your coat, but you are certainly warmer with the coat than without it. The same applies to our earth's coat of greenhouse gases. 

Mike W.


(published on 11/12/2013)

Follow-Up #1: wearing a coat

Q:
Surely the coat makes you feel warmer by preventing heat loss by (mostly) convection? Your body core temperature is not warmer than it was before you put the coat on, you just feel warmer. I cannot find any reference to cooler radiation actually being absorbed by a warmer surface *for net energy gain* as that would appear to invalidate the @nd Law of Thermodynamics. Do you have such a reference? Thanks for replying.
- Ian Bryant (age 57)
Paphos, Cyprus
A:

"Your body core temperature is not warmer than it was before you put the coat on, you just feel warmer. " Seriously?  Sure, if you're in the normal temperature range where active physiological feedback mechanisms control your metabolism to maintain a fixed core temperature, the coat or anything else won't change that core temperature much. If you're very cold, unable to maintain enough metabolism to keep proper core temperature, the coat will help. It might save your life. If you're overheating, because you can't get rid of the heat from minimal metabolism, the coat is a very bad idea indeed. It might kill you.

If, rather than having an evolved negative feedback mechanism to stabilize temperature, you had a physical positive feedback mechanism, there would be no range where the coat wouldn't heat you up. That's the situation for the Earth's surface. instead of being heated by metabolism, we're heated by solar radiation. Instead of a coat to suppress convection, we have greenhouse gases to suppress radiative cooling. Instead of a  fancy negative feedback mechanism controlled by a nervous system, we have some positive feedback mainly due to water evaporation.  So yes, thickening the coat will heat us up more.

As for "I cannot find any reference to cooler radiation actually being absorbed by a warmer surface *for net energy gain* " no, you won't find any such reference because that's not what happens. What is obvious is that the back radiation means less energy loss. In a situation where there's another energy input (metabolism, sunlight,...) that means getting warmer. Sorry if I sound frustrated, but it gets tiring to have these simple freshman physics points get obscured by a blanket of ideologically driven obfuscation.

Mike W.


(published on 11/13/2013)

Follow-Up #2: radiative heating

Q:
Gosh, Mark W, Please try to stay calm! I did not mention global warming, you did. I did not want to discuss any form of ideology, however much you may think I do. I just wanted to know if it was possible for radiation from a cooler body ever to warm a hotter body. That was it, really. I also asked for a reference if you had one. The coat analogy is inappropriate and a deflection from the original question. If you want to analogise an insulating coat to greenhouse gasses which make up less than 2% of the atmosphere, that's your business. Just out of interest, if (as you say) back-radiation cannot add energy to the warmer body, how can it reduce the energy loss? Please answer the original question. A yes or no answer, along with a reference, is fine.
- Ian Bryant (age 57)
Paphos, Cyprus
A:

OK. let's go through this slowly again.

"I just wanted to know if it was possible for radiation from a cooler body ever to warm a hotter body. " 

Yes. Radiation from B to A always warms A, regardless of their initial temperatures. Do you really need a reference for the fact that adding energy to a system raises its temperature? Try any thermal physics book in the world.

"if (as you say) back-radiation cannot add energy to the warmer body, how can it reduce the energy loss?" We said the exact opposite, Back-radiation is certainly adding radiation to the warmer body. Thus it reduces the net energy loss compared to what it would have been without that added energy. Do you want another reference for the mathematical relation that adding a small positive number to a bigger negative number reduces the absolute value of the negative number?

Mike W.


(published on 11/15/2013)

Follow-Up #3: radiative energy exchange

Q:
I'm confused, when you have two bodies radiating towards each other, doesn't the radiation from the cooler body simply reduce the rate of energy lost by the warmer body? If the radiation from a colder body could add energy to a warmer body, so should radiation from a body the same temperature, right? Wouldn't that mean you could put two objects the same temperature side by side and have them mutually heat up to arbitrarily high temperatures?
- Max (age Thy)
Memphis, TN, USA
A:

Right, the radiation from colder (B) to hotter adds some energy to the hotter body (A), but not as much as (A) was losing by radiating. So it reduces the rate at which the hotter body (A) cools off. If something else (say radiation from an even hotter source like the sun) was already heating (A) enough  to balance A's outgoing radiation, the extra radiation from (B) can cause (A) to heat up.

It's really not complicated. Just write down each energy flow (A->B, etc.) Each one is positive, and the 2nd law says that the flow from hotter to colder is always bigger than the reverse. The basic point for the most important application is that inserting a cool absorbing atmosphere between the earth and cold outer space gives a little back-flow of radiation toward earth, raising its temperature above what it otherwise would have been.

Mike W.


(published on 11/26/2013)

Follow-Up #4: radiation carries positive energy

Q:
Aside from the sarcasm, which is entirely unnecessary, thank you for your explicit statement: ["Yes. Radiation from B to A always warms A, regardless of their initial temperatures"]. Now, as for this part: ["Do you really need a reference for the fact that adding energy to a system raises its temperature? Try any thermal physics book in the world."] Yes, I really would like a reference which supports your statement with reference to radiation and my original question. Please, any thermal physics text which states that radiative energy emitted by a 'cooler' body will ADD to the energy state of a warmer body. That is exactly what I wish to see referenced. Your sarcastic reference to addition of positive and negative numbers will depend entirely on whether or not the positive number can legally be added to the negative number. Hence the original question. If the radiation from the cooler source passes straight through - or is deflected by - the warmer body (or is in any way not absorbed for energy gain) then the cooler radiation cannot be numerically added to the system. Reference please. "Yes. Radiation from B to A always warms A, regardless of their initial temperatures Do you really need a reference for the fact that adding energy to a system raises its temperature? Try any thermal physics book in the world." I would like a reference to support your assertion that any radiation from a cooler body will ADD to the energy level of the warmer body. How do you know that the radiation from B is being added to A to make it warmer? Reference please.
- Ian Bryant (age 57)
Paphos, Cyprus
A:

I've combined your two questions for efficiency.

First, the simple part, a reference that all electromagnetic radiation transports positive energy. Try any standard old freshman text, e.g. Hugh Young, University Physics, Eight Edition, pp 928-931. 

Your other question is a new one, a big switch from your previous line of questions. You want to know what happens if the hot object has no ability to absorb radiation, i.e. is a perfect reflector or totally transparent. There is an object- a vacuum, maybe with some neutrinos- that's perfectly transparent, at least enough for practical purposes. Why would we be discussing it? There are no objects that are perfectly reflecting. 

There's some point you're trying to make but it is obscure. Wouldn't it be simpler to just describe an actual physical situation instead of playing around with impossible hypotheticals? What is it that you're actually wondering about? I promise not to be sarcastic if we get some sort of straightforward question rather than word games.

Mike W.


(published on 11/21/2013)

Follow-Up #5: global warming

Q:
Mike W, Please do not change my question. It was specific and very clear: ["Is it possible for radiation emitted from a cooler energy source to heat a warmer energy source?"]. Ok? I do not have a copy of the Hugh Young book to hand (although I will try to find one), but if your reference relates to "all electromagnetic radiation transporting positive energy", then it is not valid. I know electromagnetic radiation transports energy. That was not the question. The question was whether the radiation from a cooler body can warm a warmer body. Every object above absolute zero radiates. The point is, can that radiation add thermal energy to a body which is already at a higher state of thermal energy? Your assertion : ["Yes. Radiation from B to A always warms A, regardless of their initial temperatures."] is an answer - fair enough - but it does not make sense, so I therefore have requested a reference from you to support that specific assertion. Why does it not make sense? Because if radiation from B always warms A even if B was cooler than A, then if you place two objects (A and B, where A is warmer than B) near each other, then - according to your assertion - A will warm even further. If so, it will then radiate more energy to B, which will absorb that energy for energy gain and then re-radiate to A. If A absorbs this new, slightly-wamer-but-still-cooler-than-A radiation for energy gain, then you will end up with continuously warming objects (A and B). This does not make sense to me but, as I am not a scientist, I should accept your word. However, I would prefer if you could provide me with an explicit reference which supports your assertion. There was no other question on another subject. I just asked "How do you know the radiation is added to make A warmer?" I was not asking about a non absorbing or transparent body. If you want to read a point into my question, that's fine. The point is the original question. I don't want to 'just describe an actual physical situation' because that would just degenerate into why we disagree on the veracity of the back-radiation=AGW argument and I do not want that. I just want a reference which supports your assertion/answer to my question. Trying to analogise the answer just gets complicated. My understanding is that an object warms when its energy level is increased by absorption of energy from a higher frequency source, not from a lower frequency source. I.e, how do photons of longer wavelength (from body B) raise the (thermal) energy body A when A's own energy state is already higher? Does that help? I suspect that you cannot just ADD radiative energy irrespective of wavelength. Thank you for refraining from the use of sarcasm. I have no agenda other than a referenced answer to my original question. I am trying to avoid word games and have not introduced other factors - although you keep trying to do so.
- Ian Bryant (age 57)
Paphos, Cyprus
A:

I'll answer the new parts.

"My understanding is that an object warms when its energy level is increased by absorption of energy from a higher frequency source, not from a lower frequency source." Your understanding is wrong. Absorbing energy from any source warms things up.

"I suspect that you cannot just ADD radiative energy irrespective of wavelength." Your suspicion is wrong. So long as there's any finite absorption crossection, regardless of temperature, you can raise the temperature by absorbing more energy from any source.

Finally, before you wrote: "If the radiation from the cooler source passes straight through - or is deflected by - the warmer body (or is in any way not absorbed for energy gain) then the cooler radiation cannot be numerically added to the system. ". Now you write "I was not asking about a non absorbing or transparent body. ".

We have many thousands of unanswered honest questions. It's  time to get back to them.

Mike W.


(published on 11/28/2013)

Follow-Up #6: thermal radiation balance

Q:
I thought electromagnetic radiation being transferred between two bodies involved vectors. Is that incorrect, wouldn't you end up with the remainder of the greater out of the two opposing vectors? If so, wouldn't that make the transfer equation: Q = σ(T_h^4 T_c^4) ...rather than treating each quantity alone?
- Max (age 33)
Memphis, TN, USA
A:

The thermal radiation balance for the exchange between two perfectly absorbing objects ("black bodies") takes the form you gave, c(Th4-Tc4), i.e. the Stefan-Boltzmann law. Real objects (e.g. the Earth) are only partially absorbing, so the balance gets modified. The modification isn't just a change in the constant c, because the absorption/reflection ratio is frequency dependent. Since the radiation frequency spectrum depends on T, the partial absorption can change that balance, although it never reverse the direction of the net energy flow.

The particular situation being discussed here is what happens when an infrared absorbing blanket around the earth is thickened. That interferes more with the outflow of heat  via infrared radiation than it does with the inflow of heat from the Sun, which includes a lot of visible radiation. Our infrared-absorbing atmosphere causes the Earth to be significantly hotter than it would be if it were just a black body spinning at the same distance from the Sun. The Earth's reflectivity would decrease the steady-state temperature by ~23°C below the ideal black-body T, but the greenhouse effect increases the steady-state T even more, leaving the net T about 10°C hotter than it would be for an ideal black body.  Adding more infrared absorbing gases (mainly CO2 and methane) increases the warming effect.

Mike W.


(published on 11/29/2013)

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