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Greenhouse effect and Earth's surface temperature

  1. Mar 19, 2015 #1
    I posed this question in the Earth section of the forums, but I think I wasn't clear enough in how I posed my question, plus I think the question is more physics than earth science.

    I have decided to try posing my question as a series of questions building to the main point. This is because I think I must have some basic misunderstanding and perhaps we can identify that early on.

    I loosely follow several blogs that are sceptical of 'global warming'. However, there is a fundamental point that I keep running into that I just don't 'get'. It is the matter of the greenhouse effect causing the earth to be warmer than it might otherwise be without an atmosphere.

    Please note I understand that climate change is not a topic for discussion here. I am not trying to question the greenhouse effect or suggest anything at all about climate change, the science, or societal views about the question. I am simply concerned with the mechanics of the greenhouse effect in terms of measured temperatures.

    My understanding is that the theoretical temperature for a body with the same albedo as earth located in an orbit at the same distance from the sun as earth and in thermal equilibrium with the incoming radiation can be calculated to be about 255K. The actual measured temperature of earth is some 287K. That warmer temperature is caused by the 'greenhouse effect' of the atmosphere.

    I think this means that an earth like body without an atmosphere would radiate at a temperature of 255K. I think it follows that the measured 'surface' temperature of that body must therefore be 255K.

    Am I correct so far?
     
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  3. Mar 19, 2015 #2

    jfizzix

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    Yes, if the Earth without an atmosphere radiated like a blackbody at a temperature of 255 K, it is reasonable to infer that its equilibrium surface temperature (without an atmosphere) is also 255 K.
     
  4. Mar 19, 2015 #3

    russ_watters

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    By the way, the ban on climate change discussion has been lifted.
     
  5. Mar 19, 2015 #4
    Thanks for the note about the change in forum policy. However, I am still only wanting to clarify understanding of some of the mechanics.

    So far it seems I have it right. The sun heats our theoretical the body which warms in response and at equilibrium its surface is (on average) 255K. That would mean that if we sampled the surface at enough points often enough, an average of that dataset would be the 255K value.

    The earth itself though is a different matter. It’s surface is rather more diverse (ie desert, snow, ocean, grass, jungle) and the atmosphere is an attenuating layer.

    In the case of the earth, I understand that the atmosphere is largely transparent to incoming solar radiation. This radiation heats the earth’s surface which then emits long wave infrared radiation. That radiation is absorbed by greenhouse gasses and re-emitted in all directions, including back down.

    The end result is that the surface temperature is on average 287K. Hence the ‘greenhouse effect’. But… what is the ‘surface’ temperature that is measured at 287K?

    I had understood this to be an average of the various temperature indices, which are measurements of the lowest layer of the atmosphere via thermometers. However, I also understand that satellites can measure the actual surface itself.

    Is the 287K measured average derived from thermometer data (atmosphere) or satellite measurements (actual surface)?
     
  6. Mar 19, 2015 #5

    russ_watters

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  7. Mar 20, 2015 #6
    I have done some digging but generally couldn't find the direct answer. You say temps from just above ground must be similar on average but I've never seen that said anywhere. As far as I know, the major indices like HADCRUT or GISS etc are derived from thermometer data. As your link shows. RSS/UAH are sampling radiance at various altitudes and according to your wiki link, inferring temperature from these. So 'surface' temperature, at least as far as these data go, are actually atmospheric temps.

    My question really is whether or not sampling air temperature is the same as directly sampling surface temperature. In the case of the theoretical body, the temperature is derived from a calculation of radiance at equilibrium directly from the surface which should be the same as directly sampling the surface temperature.

    With the earth, air temps can differ markedly from actual ground temperature, depending on surface. For example, sea surface temps rarely exceed about 30C as far as I know, but air temps at say 2 metres could be substantially different. Similarly on land, the ground can be quite cool to touch while a warm wind is blowing. A cool change can cause an air temp drop of many degrees, but I doubt the land responds as quickly. Equally at say the poles, what is the radiating temperature of the ice itself? The air temps can get down to what, -30 or less C.

    But really, I don't know. It just seemed to me that saying you are measuring surface temperature when you are really measuring air temperatures is not an apples and apples thing.

    I would be interested to see if there is any example of data plotted for actual surface temps vs air temps. I'm just having trouble believing at face value that air temps would on average be directly similar to actual surface temps.
     
  8. Mar 20, 2015 #7

    russ_watters

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    You can see it in the graph, but think about it logically: if the average surface and air just above the surface temperatures were didfferent, heat would flow from one to the other because they are touching each other.
    Where exactly are you seeing that? There was a click-through to more specific info on the satellites that says this:
    And you can click through a page deeper to get the precise details about how AVHRR works/what it does.

    But again, as you can see from the graph, the plots lie pretty much right on top of each other, so I don't see a hair here to split.
    The same how? It is done by a different method, but returns nearly the same results.
    Is there something wrong with the graph I posted?
     
  9. Mar 20, 2015 #8

    CWatters

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  10. Mar 20, 2015 #9
    CWatters, that's a good find. Would it be the same for say the ocean? Polar ice? I will read your links when I get a chance, bedtime calls.

    russ_watters, ditto re reading deeper at your links, I'll tackle that tomorrow. Thanks for the tip. Logically, heat may indeed flow, but surely for both surface and air to be close in temperature we'd need a fairly static situation? Air and ocean currents, weather systems, local effects must all have some effect? Again, I have no idea so you may well be right but I am not too keen on just taking your word for it. The air temp in the desert can get to something like 55-60C maximum. Yet the moon's surface can get to 123C. How hot could desert sand be? I have read it can be 5-10C hotter than air temps.

    I am also not convinced by your suggestion that satellite measurements in your graph are taken directly from the surface. Satellites may be able to do that, but I don't think UAH and RSS represent that. Wikipedia says:

    Satellites do not measure temperature directly. They measure radiances in various wavelength bands, from which temperature may be inferred.[1][2] The resulting temperature profiles depend on details of the methods that are used to obtain temperatures from radiances. As a result, different groups that have analyzed the satellite data have obtained different temperature data.

    And

    UAH provide data on three broad levels of the atmosphere.
    • The Lower troposphere - TLT (originally called T2LT).
    • The mid troposphere - TMT
    • The lower stratosphere - TLS[3]
    http://en.wikipedia.org/wiki/UAH_satellite_temperature_dataset

    Do you have a source that certifies ground temperatures to be on average very similar to UAH/RSS?
     
  11. Mar 20, 2015 #10

    CWatters

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  12. Mar 20, 2015 #11
    My question is simple, and as you and russ have noted it may be completely beside the point. Without any deep understanding of the physics, I wondered whether the surface temperature is the same as the surface air temperature. If the surface warms the atmosphere, which it does as I understand it, and the air is always losing heat up to space, it seems to me that the average surface air temperature if it's taken at a point several metres above the ground may not be the same as the ground. For all the reasons I mentioned.

    For example, the average sea surface temperature is 17C. Right there is a substantial planetary surface warmer by 3C than the theoretical average surface air temperature. And oceans cannot warm over about 30 or cool below 0.

    The argument is that the surface is 33C warmer than it should be due to the atmosphere, but strictly speaking the claim is that the near surface air temperature is 33C warmer than the surface temperature of a theoretical blackbody. To me that's a different argument.

    So, is the earth's actual average surface temperature the same as its average near surface air temperature. I have never seen this stated or discussed anywhere.

    But then, I've hardly done an exhaustive search...
     
  13. Mar 20, 2015 #12

    Bystander

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    Variety of definitions from 10 cm depth, to frostline, to "isothermal."
    Your original question.
    Plus these other somewhat nebulous definitions
    have by now given you a "feel" for what is and is not defined?
     
  14. Mar 20, 2015 #13
    Bystander, what???

    It's a simple question tho perhaps poorly explained. And maybe it's an entirely meaningless question. The average temperature of an earth, otherwise exactly the same as our earth, but minus atmosphere, would be whatever its radiating surface could be measured as. According to explanations of the greenhouse effect, that is calculated to be -18C. The average of the real earth's near surface atmospheric temperatures is +14C. OK, but what is the average of the real earth's actual surface radiating temperature?

    Is it the same as, more than, or less than, the near surface air temperature? Is there any data anywhere to illustrate the answer? I am after actual data, not people's opinions. russ_waters graphs above I *think* are of tropospheric temps, NOT actual surface.
     
  15. Mar 20, 2015 #14

    Bystander

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    Clear day, summer, "more than" resulting in thermal convection. Clear night, summer, "less than" resulting in "temperature inversions" (actually a normal density gradient), or cool air piling up against colder ground. Average/mean? Ground temperature is increasing from vernal equinox to solstice along the front range. Daytime onshore breezes on tropical and temperate zone coastlines are another example of land surfaces warmer than air temperatures driving daytime convection; none of the coasts I've visited ever exhibited the complementary nighttime offshore breeze (sea surface temperature driving convection the opposite direction).

    Heat capacities, thermal conductivities, thermal expansion coefficients, and phase behaviors of air, ground, and water affect heat transfer processes more than is suggested or implied by "average temperature" data. Definitions of terms with which to ask and answer questions still leave a lot to be desired.
     
  16. Mar 20, 2015 #15
    Sure, but that is what science is about.
    Examining stuff and trying to figure out/imagine what is going on.
    as best as possible, (and without having prior conclusions)
     
    Last edited: Mar 20, 2015
  17. Mar 21, 2015 #16

    CWatters

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    I don't know but the data appears to exist because the surface temperature is used as a proxy for the air temperature...

    http://data.giss.nasa.gov/gistemp/FAQ.html

     
  18. Mar 21, 2015 #17
    OK... but using anomalies to evaluate trends in temperatures is rather a different thing is it not? By striking an average value and then looking at anomalies from that average over time, we can see a trend in a temperature series. That FAQ seems to suggest that sea surface and air temperatures share similar anomaly values when tracked over time (though I think there is a logical inconsistency in what they write there), but that tells us little about the actual values. In other words, SST and SATs may tend to rise and fall in a synchronous pattern, but that does not inform us whether one is typically more than, less than or equal to the other.

    They also note that they can "use SST anomalies as proxies for SAT anomalies in regions without sea ice", but I think they mean only insofar as using anomalies to track the series trend.

    My interest is the extent to which an average surface temperature is higher or lower than an average near surface air temperature. Your quote above observes that "SATs and SSTs may be very different " and "This is not true in the presence of sea ice, since in that case water temperature will stay at the freezing level". Which sort of supports my idea that an average air temp may be different from an average surface temperature.
     
  19. Mar 22, 2015 #18
    Most global temperature collections no longer use the term "surface temperature". Instead, they prefer "Near-surface temperature". This is because these readings are taken from ordinary national meteorological stations and from ships at sea. A standard instrument shelter on the ground will have its thermometers located about 1.5 meters from the actual ground surface. Some shelters are on the roofs of various federal buildings. On ships at sea, the measurements may be taken tens of meters above the sea surface and from a moving vessel. For all of these reasons, the term "near-surface" is preferred and is more descriptive.

    It is worth noting that countries that report "ground frost" as well as shelter frost show the former to be far more prevalent. Similarly, desert surface temperatures can be tens of degrees Kelvin above shelter temperatures. In any case, most of the outgoing terrestrial radiation comes from the atmosphere. Of the 375 Wm2 of outgoing terrestrial radiation, only about 87 Wm2 comes directly from the surface.
     
  20. Mar 22, 2015 #19
    Why is the hypothetical emissivity of the atmosphere-free Earth significant in any way? How can this value be used to solve any significant real-world problems. The Earth's estimated surface emissivity is usually given as 375 Wm2. This is the emissivity that would be produced by a surface at a mean temperature of 285 Kelvin with a coefficient of emissivity of 0.95. This is very close to the 287K you mention as the estimated mean annual near-surface temperature. This is a good starting point for heat budget studies. The imaginary 255K temperature has no predictive value and is useless in global heat budget studies. It just gets in the way.
     
  21. Mar 22, 2015 #20
    The kinetic energy, KE, of particles in a small volume is an averages of the KE of their individual ke. These atom or molecules collide and exchange energy with each other. Typically a very fast moving one (higher than the average energy) will leave the collision with less energy than it had just before the collisons. Very rapidly a velocity distribution will be established that can be described with a single free parameter or variable, T, which is the temperature of that small volume of many thousand of particles. We then say that Local Thermodynamic Equilibrium, LTE, exists. However, photons also carry or posses energy and exchange energy with atoms or molecules, both by scattering and by absorption or emission.

    Inside a closed box with all the wall materials at same temperature T, the distribution of photons (how many of each wavelength) is also given by a math expression with a single parameter, Planck's equation for "black body radiation." The value of that parameter is the same T that describes the temperature of the wall, both on the Kelvin scale. If there is a tiny, compared to the box's internal surface area, hole in the box, black body radiation will be coming out of that hole.

    The atmosphere surrounding the earth is not confined inside an isothermal box, so one cannot expect the radiation leaving earth's surface to be black body radiation, not even very locally, say from a square cm which is all at same temperature T; for two reasons.
    (1) LTE does not exist as there is a net loss of energy and (2) Black body radiation is called that as it comes only from a surface that is perfectly black. I. e. absorbs all radiation that falls on it. Earth is not "perfectly black."

    Many will tell you that a good absorber is a good emitter too, but forget to add "at the same wavelength." Thus a square cm of earth's surface that has nearly an absorption coefficient, a, of 1, say 0.9, at one wave length may have an emissivity "constant" ,e, at some other wavelength of only 0.4 but for the wave length at which a = 0.9, e = 0.9 also. I won't go into details but just note that if this were not always true, you could violate conservation of energy. - Made a device that produces net energy.

    So now to come more to your question: The surface of the earth is at many different temperatures and has differently varying (with wavelength) coefficients, a & e. As if that were not enough complexity, you need to understand the concept of "optical depth" to understand what satellites looking down at the earth see as outbound IR radiation.

    Although symmetric molecules like O2 & N2, don't change their "dipole moment" when they vibrate or rotate molecules like CO2 can. CO2 is actually linear molecule: 0---C---0 with the Os having "stolen" some negative charge, on average, from the C. Its dipole moment is zero when in it ground vibrational state but not when excited in the "a-symmetric" vibrational mode.
    I.e. with this configuration O--C----0 and then half a cycle later is O----C--O. When this vibrational state return to the ground state, its loss of energy is given to the IR photon emitted. I'll call the wave length of that photon "L."

    When a photon of wave length L is emitted by the earth's surface and there are no clouds in the way, it still has only about 1/3 probability of escape to space now that the concentration of CO2 is about 400ppm. And to even get that ~33% chance, the photon that does leave earth is not the same as the one that left the surface originally. That original wave length L photon, was absorbed and its energy re-radiated by some other CO2 molecules at a different altitude. I.e.. the escape to space of wave length L photons is a random walk process. Most that do escape, have come from a high altitude, where the temperature is much lower than that of the earth's surface below them.

    They come crudely speaking, from the top "optical depth" of the atmosphere, which is a smaller layer than for most photons not of wave length L. That high layer's colder temperature limits the intensity of this L wave length IR as no surface or volume can radiate more intensely than a black body at the same temperature can. Thus man is lucky that CO2 effect on Global Warming is limited. The CO2 concentration could increase five fold and the IR loss via wave length L radiation would not even double. We say the CO2 absorption bands are nearly "saturated."

    Unfortunately that is not true of CH4, which is a 3-D molecule, with more and stronger absorption bands. During the first year after a Kg of CH4 is released it does 120 times more global warming than a Kg of CO2 does, but as its half line in the air is only 12.6 years* during the first decade after puff was released it does only about 84 times more global warming as same mas of CO2 does as CO2 half life is about 1000 years.

    I'll stop here as getting too far from your question, but at least you should now understand it was a quite complex question.

    * I need to note that CH4's half life is now increasing by about 0.3 year each year, because it is mainly destroyed by the OH- radical but now, unlike the last 800,000 years, the harsh solar UV that make the OH- is not able to do so at the rate CH4 is being released so OH- concentrations are falling and CH4 is both increasing and remaining in the air more years.
     
    Last edited: Mar 22, 2015
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