The Planetary Greenhouse Engine Revisited

Discussion in 'Environment' started by Matthew, Jun 15, 2011.

  1. Matthew
    Online

    Matthew Blue dog all the way!

    Joined:
    Mar 16, 2010
    Messages:
    49,614
    Thanks Received:
    4,589
    Trophy Points:
    1,885
    Location:
    Portland Oregon
    Ratings:
    +15,101
    Please poke holes in this skeptical science post. Thank you:tongue:

    The Greenhouse Effect and Thermodynamics

    When we think about problems in planetary climate-- whether it be the greenhouse effect of Venus, Snowball Earth, extreme orbits, the range of habitability around others stars, or what exotic atmospheres one might encounter on other planets-- we must be prepared to think well outside the "climate box" in terms of scenarios and possibilities. Whatever alien situation we can think of however, we are necessarily constrained by the laws of physics to create a self-consistent picture that distinguishes reality from science fiction. Among these laws of physics are the many well-established rules governing the behavior of radiant energy and its interaction with air, and also the statistical behavior of gases in local thermodynamic equilibrium. Just as an incredibly trivial equation of state emerges in the thermodynamic limit from very complex molecular dynamics (which ultimately describes a relationship between fundamental variables in our atmosphere), we can make many general remarks concerning the energy balance and temperature structure of planetary atmospheres, even with exceedingly complex behavior at the interface of fluid dynamics, chemical interactions, and energy/momentum transfer.

    The nearby rocky planets (e.g. Mercury, Venus, Earth, Mars) gain and lose energy radiatively, and come into thermal equilibrium when the magnitude of the absorbed solar radiation equals the outgoing emission by the planet (which is in the far-infrared part of the electromagnetic spectrum for all planets in our solar system, but could just as well be primarily in the visible for very hot planets orbiting close to their host star). This is not always the case: on the gaseous planets, observations show that the outgoing thermal radiation exceeds the incoming solar energy by significant amounts (this excess is nearly a factor of three for Neptune). This is because the giant planets have an internal heat source. On Earth or Venus, internal heating takes the form of radioactive decay, although it is negligible for energy budget purposes, since the energy flux is many orders of magnitude smaller than the incoming solar energy flux. Radioactive decay is not responsible for the infrared excess on gas planets either; instead, the interior heat source takes the form of Kelvin-Helmholtz contraction—a way of converting potential energy into kinetic energy as the whole atmosphere contracts into the center (i.e., becoming more centrally condensed), heating the gas interiors. This is a critical component of giant gas planet evolution, and the process is also what makes young stars hot enough in the center to eventually fuse hydrogen, although Jupiter is not nearly massive enough to reach this point.

    Introducing an infrared absorbing atmosphere into the picture complicates things, since now radiation is lost to space less efficiently than with no atmosphere (for a given temperature). In essence, the surface temperature acts as a slave to the way energy flows operate between our sun , the planet, and the overlying air and eventually adjusts to maintain equilibrium at the top and bottom of the atmosphere. The critical ingredient for the greenhouse effect (aside from IR absorbers, obviously) is that the temperature structure of the atmosphere is one that declines with height. This is because in order to make the planet lose radiant heat less efficiently, you need to replace the “radiating surface” near the ground with a weaker “radiating surface” in the upper, colder atmosphere (Fig 1)
    [​IMG]
    Figure 1: Spectrum (Radiance vs. wavenumber) for a Planck Body at 300 K (purple dashed) and the OLR with an IR absorbing greenhouse gas

    Figure 1 is plotted as a somewhat “contrived” greenhouse substance that works like this: Our ground has a temperature Ts, with a colder temperature above the surface (e.g. the stratosphere). Plotted are the Planck function for the surface temperature (purple dashed) and actual outgoing radiation (OLR, curve). The Planck function gives the distribution of energy intensity vs. wavenumber (or wavelength, or frequency, depending on your favorite characterization of an electromagnetic wave) for a blackbody at some specified temperature.

    The blue curve titled “OLR” is the actual spectrum of this hypothetical planet with a hypothetical greenhouse gas in the atmosphere. The difference between that blue spectrum and the Planck (purple) spectrum for the ground temperature arises because our greenhouse gas happens to be blocking radiation from exiting directly to space at 600 cm-1 and the surrounding regions. Even toward the “wings” at 400 or 800 cm-1 it is making the atmosphere “partially opaque.” This is fairly standard qualitative behavior for a greenhouse gas, especially CO2, although there are exceptions.

    This plot is computed for a fixed temperature, so the end result of adding the greenhouse gas is to reduce the total outgoing radiation (the specific amount is whatever chunk is taken out of the Planck curve). This creates a situation where the planet temporarily takes in more energy than it loses, and as a consequence the ground temperature must rise to increase emission and restore equilibrium.

    To think about this another way, emission at wavenumbers where the atmosphere is strongly absorbing will always be closer to a "sensor" that is recording the emission than wavenumbers where the atmosphere is transparent. If the sensor is a satellite looking down from space, it will see warm, surface emission in transparent ("window") wavenumbers, but for opaque wavenumbers, emission emanates from the high atmosphere.

    Similarly, for a surface sensor looking up, emission from opaque regions is seen to come from very near the surface, whereas for transparent wavenumbers the sensor is recording the ~3 K temperature of microwave background radiation in space. In this post, we're thinking about the sensor looking down.

    Brief Technical aside: Let’s define a “mean radiating pressure" of the planet, which we’ll call pr, where the atmosphere becomes optically thin enough to lose its radiation to space directly rather than being absorbed in a higher layer. Since pressure decreases with height, the radiating pressure will decrease as the optical thickness of the atmosphere increases (i.e., more radiation is preferentially leaking out higher in the atmosphere where it is colder when you add greenhouse gases). Conversely, the radiating pressure is at the surface (pr=ps) with no greenhouse effect. It is easy to show that for an atmosphere whose temperature profile is dry adiabatic, that the radiating pressure is given by:



    where the ratio cp/R is approximately 7/2 for Earth air; the numerator in the brackets is the absorbed solar radiation, σ is the Stefan-Boltzmann constant, and Ts is the surface temperature. For Earth, the mean radiating pressure would thus be at ~650 millibars, rather than at sea level (1000 mb) with no atmosphere (in reality, it would be smaller than this, since the real lapse rate is less steep than the dry adiabat). See also Figure 2, to show how decreasing pr increases the surface temperature.
    [​IMG]
    Figure 2: Depiction of how increasing the radiating height of a planet increases the surface temperature. Equilibrium is reached when the outgoing long-wave energy curves intersect the absorbed solar radiation curve.
    Does this all violate Thermodynamics?

    The reason greenhouse warming does not violate thermodynamics is because the planet is not an energetically closed system, and receives a constant influx of energy from the sun. The reduction in outgoing energy flow by the atmosphere can therefore heat the planet toward a value slightly closer to the solar temperature. If the sun turned off, the greenhouse effect would be irrelevant (even assuming you could keep your atmosphere in the air at all without everything condensing out). Some people on the blogs have claimed that because a colder atmosphere radiates toward a warmer surface, there is some thermodynamic inconsistency with the second law. First, note that I have not said a word about back-radiation to the surface, primarily because it doesn’t give proper insight into the way energy balance is adjusted and determined. But to the point, cold objects still radiate energy and a photon doesn’t care whether it’s traveling toward a warm object. So yes, colder objects can and do radiate toward (and heat!) warmer objects. Standard measurements (from Grant Petty's Radiation book) of back-radiation should be simple proof that this occurs. Keep in mind that the net two-way energy flow is always from warm to cold.
    [​IMG]
    Let’s now compare the theoretical Fig. 1 spectrum with a real Venus spectrum (Fig 3).



    Figure 3:260 K blackbody spectrum (red) with observed Venus spectrum from The Venera 15 orbiter (blue).

    Here, the red curve is a 260 K blackbody Planck spectrum and the blue is a typical Venus spectrum I plotted which was obtained from the Soviet Venera 15 orbiter. Keep in mind that the Venusian surface radiates at ~735 K, so the fact that the whole spectrum is seen to radiate at Earth or Mars like temperatures is a good indication that the atmosphere is highly opaque in the infrared spectrum. Most of this is CO2, but other constituents like water vapor, SO2, and sulfur-water clouds are very important too, along with some other minor species.
    Some Remarks about Pressure

    It has been argued on some blogs that high pressures can cause high temperatures, and the argument has taken a variety of forms. One is that p= ρRT (the ideal gas law) implies that a high p means a high T. Of course, the pressure is 90x higher on Venus but the temperature is only 2-3 times higher than Earth, so such a straightforward proportion obviously doesn’t work. The temperature must satisfy energy balance considerations, so a better way to think about the problem is to fix T (with other information, namely radiation) and solve for the density, which is of course much higher on Venus. You can't get all the information from the equation of state alone. The other argument is that some “insulative” property of gases could keep Venus hot at high pressure, even if the whole atmosphere were transparent to outgoing light. One way to heat Venus would be to compress its atmosphere, but this would be temporary and eventually the temperature must relax back to its equilibrium value determined by energy conservation considerations. The way things work is that heat is sluggishly migrated upward by radiation or convection until it finally reaches a point where the air is optically thin enough to let radiation leak out to space. This doesn’t happen in a transparent atmosphere.

    So does pressure matter for the greenhouse effect? The answer is yes, and the prime reason it matters is that collisions between molecules act to “smooth out” absorption and fill in the window regions where air is transparent. Unlike the quantum nature of absorption and emission, the kinetic energy of moving molecules is not quantized, so it is possible for colliding molecules to impart kinetic energy on the absorber and make up the energy deficit required to make a quantum leap from one energy level to another. There are some other broadening mechanisms too, but this is by far most important in the lower atmosphere.

    Aside from the fact that a 90 bar atmosphere can hold much more greenhouse gas, pressure broadening is huge on Venus, but you can only smooth things out and fill in the windows so much. Where pressure broadening would really make a difference is to put in a 1 bar atmosphere (even N2) on a very low dense atmosphere like Mars. The reason why Mars does not currently generate a strong greenhouse effect, even at over 90% CO2, is that the spectral lines are too narrow to have a sizable effect. Even with almost two orders of magnitude more CO2 per square meter than Earth, the equivalent width is less on Mars. The equivalent width is a measure of the area of absorption taken out by a molecule (see the wiki article for further explanation on its definition). The following diagrams illustrate the OLR change in a 250 ppm CO2 atmosphere at Earthlike pressure (Fig. 4a) and 100x Earth pressure (Fig. 4b) (note that the same mixing ratio in the 100 bar atmosphere implies more greenhouse gas overall).

    [​IMG]

    [​IMG]

    Figure 5: 250 ppm CO2 mixing ratio for an atmosphere at a) Earthlike pressure and b) 100x Earth pressure

    Note that at very high CO2 concentrations, a lot of new absorption features come into play that are irrelevant on modern Earth. The water vapor and sulfur-bearing compounds on Venus also help to fill in some window regions considerably. Also unlike Earth, Venus has a non-negligible scattering greenhouse component too (by inhibiting cooling through IR scattering rather than absorption and emission). These make direct planetary comparisons useless, except that Venus is a case in point of how much a greenhouse effect can matter in planetary climate discussions.

    Note also that very dense atmospheres also raise the albedo through Rayleigh scattering; this is the same process that make our skies blue. A pure Venusian CO2atmosphere raises the albedo to a moderately high ~40%, somewhat short of its current albedo (~77%, because of clouds), but still higher than Earth. This remark is primarily true for planets orbiting sun-like stars, but for lower temperature stars (like M-dwarfs) the Rayleigh scattering is much less important, since the spectrum of the starlight itself is red-shifted, and Rayleigh scattering favors shorter (bluer) wavelengths.
    Could a purely diatomic molecule atmosphere generate a greenhouse effect?

    The answer, again, is yes. This may be surprising because something like H2 or N2doesn’t have the molecular symmetry (to make a dipole moment) that we commonly attribute as a defining characteristic of greenhouse gases. Similarly, Pressure broadening doesn’t broaden anything that isn’t there to begin with. But for very dense atmosphere, frequent enough collisions between diatomic molecules can temporarily make a ”four-atom” molecule that behaves like a greenhouse gas. This effect is much more pronounced at colder temperatures, since the time of collision is longer at low velocities. Collision induced (as opposed to broadened) absorption has been best studied on Titan, but it’s important on the gaseous planets, as well as some theoretical atmosphere with several tens of bars of H2 or He that are relatively dense and cold. It’s unimportant on Earth, since the temperatures are high enough and density low enough.
    Lapse Rates and Tropopause Height

    Several other bloggers have been under the impression that the lapse rate “causes” high surface temperatures on a place like Venus, the idea being that the tropopause is very high and so one can extrapolate down the adiabat very far to reach a high temperature. As should be obvious from the preceding section, the entire reason why you’re allowed to extrapolate such a far distance is because of the greenhouse effect, which increases the altitude where emission in the opaque regions of the spectrum take place. In fact, on Venus the high tropopause is a a consequence of the high optical thickness.

    In radiative-convective equilibrium, the atmosphere transports sufficient heat vertically (by convection) to prevent the lapse rate from exceeding some critical value, so that a stratosphere can exist in radiative equilibrium (with a thermal balance between ozone heating and CO2 cooling) atop a troposphere where both radiative and dynamical fluxes are important. The lapse rate just describes the manner in which temperature changes vertically; it isn’t some supply of energy and you need to specify the temperature at the surface by some other means. The reason an adiabatic lapse rate might develop and the height to which it extends is most certainly not independent of radiation, which provides a basis for global energy flows.

    An adiabatic lapse rate only needs to develop by convection where air parcels at the surface become buoyant with respect to the air above it. In an infrared transparent atmosphere with no sources and sinks of energy, convection would eventually give out and the tropopause would migrate to the surface, developing a deep isothermal region.

    In conclusion, the "greenhouse effect" is a very real physical phenomenon and has no inconsistencies with thermodynamics or any other field of inquiry (and in fact,emerges from these disciplines). It can be just as important in determining the global temperature as the distance to the sun, and is especially important on Venus.

    The Planetary Greenhouse Engine Revisited
    Posted on 15 June 2011 by Chris Colose
     
    • Thank You! Thank You! x 1
    Last edited: Jun 15, 2011
  2. CrusaderFrank
    Offline

    CrusaderFrank Diamond Member

    Joined:
    May 20, 2009
    Messages:
    81,150
    Thanks Received:
    14,897
    Trophy Points:
    2,210
    Ratings:
    +36,860
    About half way through when it dawned on me that according to the Warmers we could terraform Mars and make it hospitable to human habitation just by dropping a few blocks of dry ice (Frozen CO2) on the surface
     
    • Funny Funny x 1
  3. Matthew
    Online

    Matthew Blue dog all the way!

    Joined:
    Mar 16, 2010
    Messages:
    49,614
    Thanks Received:
    4,589
    Trophy Points:
    1,885
    Location:
    Portland Oregon
    Ratings:
    +15,101
    The atmosphere of mars doesn't have enough density for it is to small to hold onto a thick enough atmosphere to rise the surface pressure enough for the heat energy molecules to bump into each other to create warmer temperatures. So it wouldn't work.
     
  4. CrusaderFrank
    Offline

    CrusaderFrank Diamond Member

    Joined:
    May 20, 2009
    Messages:
    81,150
    Thanks Received:
    14,897
    Trophy Points:
    2,210
    Ratings:
    +36,860
    Yeah, I just caught up with that.

    I had to print it out to read later. It's not light reading like say anything OR ever posted
     
  5. konradv
    Offline

    konradv Gold Member

    Joined:
    Mar 23, 2010
    Messages:
    22,541
    Thanks Received:
    2,554
    Trophy Points:
    280
    Location:
    Baltimore
    Ratings:
    +5,660
    Got a cite for that? PUT UP OR SHUT UP.
     
  6. waltky
    Offline

    waltky Wise ol' monkey Supporting Member

    Joined:
    Feb 6, 2011
    Messages:
    20,776
    Thanks Received:
    1,788
    Trophy Points:
    215
    Location:
    Okolona, KY
    Ratings:
    +3,858
    Greenhouse Gas Levels Set Record...

    Atmospheric Greenhouse Gas Levels Hit Record, Report Says
    NOV. 10, 2015 - Global concentrations of carbon dioxide in Earth’s atmosphere surpassed 400 parts per million this past spring for the first time, breaching a symbolic barrier set by climate scientists and policy makers, according to a report released Monday.
     
    • Funny Funny x 1
  7. Billy_Bob
    Offline

    Billy_Bob Platinum Member

    Joined:
    Sep 4, 2014
    Messages:
    7,086
    Thanks Received:
    1,512
    Trophy Points:
    975
    Location:
    Top Of The Great Divide
    Ratings:
    +4,453
    One very simple premise they failed to even give notice is molecular interaction and how it has both positive and negative effects on differing concentrations of gases, how those change with concentration, not to mention how water vapor reacts.

    The basic failed premise of the SKS boys is, they assume water vapor is always net positive in its reaction when empirical evidence shows that false. The convection cycle increases in rate. This allows increased IR loss at higher altitude which combats the increased thermal lapse rate at low altitude, ending in a negative balance. Water vapor is not slowed by CO2 as O2 and N2 thinning increases water vapor rise speed. (Solar wind pressure on our atmosphere has a major effect as well)

    We've been cooling now for 14 years and 4 months from precisely this effect.

    This should sink the SKS-TITANTIC... That gaping hole should put it on the bottom of the science ocean...
     
    Last edited: Nov 11, 2015
  8. flacaltenn
    Online

    flacaltenn USMB Mod Staff Member Gold Supporting Member

    Joined:
    Jun 9, 2011
    Messages:
    30,016
    Thanks Received:
    4,652
    Trophy Points:
    1,130
    Location:
    Hillbilly Hollywood, Tenn
    Ratings:
    +13,339

    It's really a rambling esoteric cruise thru the various mechanisms of the GHouse effect as it applies on other planets. It's "interesting" --- but probably not what you THINK it is..

    For instance, on the Earth, the GHouse effect is actually a very sensitive fine-tuned set of happy coincidences. Some of those are --- the actual radiating BBody frequency of the Earth MATCHES the absorption spectrum of gases that are GH gases.. Where on Venus, because the surface is "tuned" slightly differently for updwelling long wave -- Wouldn't much matter what GH gases you had.

    Some of the other "frailties" of the GH on Earth are the fact that the majority of the Sun's spectral output can pass unhindered due to the lack of absorbing or shielding influences.. As stated....

    Except that "scattering mechanism" would also affect wider bands for incoming solar radiation as well..

    Sorry to disappoint Matthew. Ain't no smokin' gun here. Just a few smokin' hot planets and bunch of atmos. physics..
     
    • Agree Agree x 1
  9. RollingThunder
    Offline

    RollingThunder VIP Member

    Joined:
    Mar 22, 2010
    Messages:
    4,398
    Thanks Received:
    415
    Trophy Points:
    85
    Ratings:
    +886
    The rest of your post is just Dunning-Kruger Effect inspired silly nonsense.....but this part here is out-and-out reality-denying insanity!

    These studies were done in 2014, before they were totally sure that it would wind up being the hottest year on record...which it did...and way before anyone realized that 2015 would end up being even hotter. But even then in 2014 they knew that....

    14 of the 15 hottest years on record have occurred since 2000, UN says

    It's even hotter this year and it may be even hotter than that next year, scientists are saying. At the end of this December, the experts will announce that three of hottest years on record since at least 1880 have now occured in just the last five years.
     
    • Funny Funny x 1
    • Winner Winner x 1
    Last edited: Nov 12, 2015
  10. Billy_Bob
    Offline

    Billy_Bob Platinum Member

    Joined:
    Sep 4, 2014
    Messages:
    7,086
    Thanks Received:
    1,512
    Trophy Points:
    975
    Location:
    Top Of The Great Divide
    Ratings:
    +4,453
    Oh you silly person.... THE UN SAYS is all that needs to be stated to show that it is purely politically motivated bull shit.

    Then we have facts in the UAH and RSS satellite data sets which show NO WARMING FOR OVER 18 YEARS 8 MONTHS and cooling now for over 14 years.

    Tell me how you defend the divergence from these data sets by the HCN, which has been adjusted some 26 times in the last 9 years alone and always upward, because they wanted to hide the cooling trend that has formed.
     

Share This Page