Radiation and Global Warming
Most recent answer: 11/12/2013
- Ian Bryant (age 57)
Paphos, Cyprus
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
- Ian Bryant (age 57)
Paphos, Cyprus
"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
- Ian Bryant (age 57)
Paphos, Cyprus
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: radiation carries positive energy
- Ian Bryant (age 57)
Paphos, Cyprus
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 #4: radiative energy exchange
- Max (age Thy)
Memphis, TN, USA
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 #5: global warming
- Ian Bryant (age 57)
Paphos, Cyprus
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
- Max (age 33)
Memphis, TN, USA
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)