onsdag 6 juli 2011

Cloud or No Cloud

Much confusion surrounding the debate concerning the fundamentals of the greehouse theory, in my opinion can often be traced back to a careless hopping between concepts from equilibrium- and non equilibrium thermodynamics respectively. Here I will present a simple thought experiment that might highlight this issue. Consider a body which by regulated inner chemical reactions maintains a temperature of 37 degrees Celsius. Now consider two situations:

1. The body is placed in vacuum (outer space)

2. The body is surrounded by a nitrogen cloud which has a temperature of 20 degrees Celsius.

In case 1 it is obvious that the body will radiate energy to space at a rate which depends on its temperature. The exact law governing the magnitude of this radiation is not really important here, but let's suppose that it follows a T^4 law. But what about case 2? It seems obvious that the body will transport heat to the gas due to the temperature difference but how much will it radiate? Greenhouse theory given an unequivocal answer to that: It will still radiate as much as it did in vacuum, and since there are no greenhouse gases around there will be no back-radiation. In case 2 we are thus left with the conclusion that the body loses more heat to the surrounding than it did in case 1. But does this really make sense? I can think of several ways to argue that the body will in fact not radiate nearly as much as it did in vacuum, and I will for the moment leave it to the reader to think about this. If, however, you believe that the surrounding gas will reduce the amount of radiation it does have implications for the greenhouse effect, it means that the application of an equilibrium radiation law, like the Stefan-Boltzmann law, to a surface which is not equilibrated to the surrounding gas (with or without greenhouse gases) is invalid.

If you are still hesitant I might just through out the question quite bluntly:

Which would you like to try, cloud or no cloud?

4 kommentarer:

  1. Hi Anders,

    I'm afraid you're not clearing up confusion, but demonstrating it.

    You write that 'In case 1 it is obvious that the body will radiate energy to space at a rate which depends on its temperature.'

    But of course it is also 'obvious' that the body will also be warmed if there are other sources of electromagnetic radiation around it. Like the sun, for example. In sunlight, NASA's space-walking astronauts experience temperatures on their suit-surface around 400K.

    Yet you assume only a cooling effect due to radiative loss - because you have chosen arbitrary (purely imaginary) starting conditions with no outside EM sources.

    Whereas, in your case 2., the gaseous N is warmed to 20C. Of course, if it were otherwise unheated, and surrounded by the vacuum of space, as for example with the atmosphere of a planet, it would also cool very quickly to near 0K. Unless it was warmed to an equilibrium point of 20C by something like the sun.

    So in case 2. you assume the sun, or something like it. But in case 1. you assume its absence.

    What you present as a 'simple thought experiment' is in fact a simple example of drawing a mistaken conclusion from mistaken assumptions.

  2. Hi Alien Technician,

    Thanks for the comment. The main question I ask, though maybe I didn't communicate it clear enough, is whether the non-greenhouse gas surrounding for example a human beeing momentarily reduces the rate of radiative heat loss. The "imaginary" starting conditions I think could be made more concrete if you imagine a summers night with an outside air temperature of 20 degrees C. Let's suppose that there were no greenhouse gases in the atmosphere and that it would somehow disappear for 30 seconds and then come back. Would you perceive these 30 seconds as colder or warmer (equipped with oxygen mask and not suffering too much from the reduction in air pressure)? If you say colder, then the only way I can make the math go round is that the conclusion must be that the surrounding gas somehow reduces the rate of radiative heat loss compared with the vacuum case. Do you agree?

  3. Hi Anders,

    Good to see you clarify the question that is troubling you.

    I think there is one more note to make that will help still further.

    You make a distinction between 'greenhouse' and 'non-greenhouse' gases. But you also imply that the difference is that 'greenhouse' gases emit 'back-radiation'.

    A more standard way to think about this is that _all_ gases, indeed all matter at a temperature above 0K, will emit electromagnetic radiation at wavelengths that vary with temperature. there is _no categorical difference_ between 'greenhouse' and 'non-greenhouse' gases in this respect.

    Those gases considered as major 'greenhouse' gases are notable rather because they strongly _absorb_ radiation in the far infrared spectrum. In other words, they are subject to radiative heating by the Earth as a black body source.

    With me so far? Good.

    Now, the bad news is that your new example illustrates nothing about radiative heat loss or IR absorbtion, but the ways in which it is wrong might help you to think through the question again.

    So, in your new example, on a summer night, the subject is initially surrounded by air at an ambient temperature of 20C. Then you take away this atmosphere, and ask whether the subject would begin to cool (or subjectively experience cooling, but let's be empirical here).

    Let's assume that the situation is, indeed, a summer night on Earth. What could we expect the temperature to be without an atmosphere? For a working approximation, let's use the moon's nighttime temperature - around 120K.

    From this rough assumption, we see a differential of -170K. So we can guess it will be a little colder ;-)

    Of course, this temperature drop wouldn't happen all at once, as the planet's surface would cool gradually. So the subject might still be warmed by radiative heating from the Earth itself, plus some conductive heating. But there is no doubt that the subject's temperature would begin to drop immediately.

    However, I'm not sure why anyone would find this surprising.

    You suggest that 'the conclusion must be that the surrounding gas somehow reduces the rate of radiative heat loss compared with the vacuum case.'

    But that's irrelevant in this example. If you are surrounded by an enormous bath of hot gas, then somebody magically takes that away and replaces it with a -170K temperature gradient, you'll get colder. This would seem to be the intuitive answer, as well as the correct one.

    Meanwhile, the subject's rate of radiative heat loss remains entirely unaffected by the presence or absence of the surrounding gas. (in fact it will indeed fall as the subject's temperature decreases, but as you know that's for different reasons).

    Hope that helps!

  4. & I realise I assumed above that you know that for a substance to be IR-absorbing means it's also IR-emitting. ie it's the fact 'greenhouse' gases absorb and emit more strongly in the _far infra-red_ region while being more transparent to other wavelengths that drives Earth's greenhouse effect. while this isn't relevant to the discussion above, it is important to an understanding of the climate.