The Energy Balance and the Greenhouse Effect: 2 of 3


As I discussed in Part 1, the Earth’s surface warms the atmosphere.  Without that heat transfer, the Earth would be a much colder place.  The normally accepted value (that I agree with) is 33 °C.  There are differing ideas as to the precise contributions that result in the Earth being warmer than it would be otherwise.  I will discuss the common one and then present my own ideas about it that are based on the Energy Balance I recently presented.

The most common idea is that the Greenhouse Effect  (GHE) is actually about 60 °C instead of 33 °C.  Then there are cooling effects that drop the overall effect down to 33 °C.  This is primarily based on the idea that the radiative flux would induce significant warming, but then the evaporation from the oceans and convective transport have a cooling effect.  My problem with this idea is that both convection and evaporation are other forms of energy transfer.  They cool the surface, but warm the atmosphere.  Large temperature gradients just don’t naturally exist for any period of time.  In addition this idea doesn’t focus on NET energy transfer.

Here is where I differ in my views.  Energy (heat) transfer causes warming of the atmosphere.  In the case where there is no GHE the Earth would be similar to the Moon.  Very hot for the surface in the sun, very cold for the surface not in the sun.  On average it would be much colder.  The accepted value for the Earth is 255K.  The differences between the situation with no GHE and the actual GHE are interesting.

When 240 W/m2 are both absorbed and radiated away from the Earth, it is in a steady state situation.  That means even though energy is always flowing, there is no real change.  In that situation the temperature is only influenced by the energy input from the sun.  The day of the closest approach to the sun (currently Jan 5th) would be the hottest day of the year and the farthest would be the coldest it happens to be July 4th right now.  That day would always be colder.  This behavior is opposite of what is currently observed.  More on that topic here.

The Moon, courtesy of NASA

Now lets consider the Moon.  It experiences the same orbit as the Earth in relationship to the sun.  Lets imagine that it had a pure nitrogen atmosphere.  What would happen there.  The very first thing would be wind.  When the sun was warming the day side up, convection would happen at the surface and that would cause wind to blow.  This warm nitrogen would balance out the temperature gradients that exist on the moon.  Cold places in the shade would warm up and and the hot places always in the sun would cool down as the energy was transferred by wind.  Convection would be very important to the Moon due to the very long days (sunlight for 13.6  Earth days straight).  Since the high temperatures would be tempered, there would likely be an overall decrease in RHT away from the Moon.  This would also help reduce the heat loss and keep the Moon warmer.  There would be a GHE with nitrogen only.

If water vapor and CO2 were added to the Moon it would warm up even further.  Those greenhouse gases would absorb the energy from the surface and cause additional heating of the atmosphere.  Instead of the surface radiating energy away, the atmosphere would start to be a source of the heat loss for the moon.  The more energy the atmosphere can hold, the warmer the Moon would be.

Finally lets add some surface water to the Moon.  This really warms the Moon up.  The water stores excess energy when the sun is up (lots of it due to the long day).  This increases the water vapor in the atmosphere which also increases the temperature more.  Primarily this would help the side of the Moon that happened to be dark from cooling down as much.  As the air cooled, the surface water would help keep the air warm by transferring energy to the atmosphere.  The Moon would not get as warm as the Earth (albedo is too low for the Moon), but it would be much warmer than it is now.

Each additional effect would increase the average temperature of the Moon.  On the Earth, all those conditions are in place and that is why there are less extreme temperatures on the Earth.  At any given time the temperature range on the Earth has a temperature range from -50 to 45 °C with the highest measured extremes being about 150 °C apart.  The Moons known temperature extremes are more than double that.  The same orbit and position from the sun and it can have temperatures that are 320 °C apart at the same time.  Those temperature extremes can even be physically close together.

An atmosphere would change all of that, much like it has made the Earth the place that it is today.

The Inconvenient Skeptic

Terraformed Moon

Posted in Energy Balance by inconvenientskeptic on December 3rd, 2010 at 2:15 am.

20 comments

This post has 20 comments

  1. Malaga View Dec 3rd 2010

    There would be a GHE with nitrogen only.

    Now things are beginning to fall into place…..

  2. inconvenientskeptic Dec 3rd 2010

    It is interesting to picture a world with nitrogen only being warmer, but it would. If nitrogen only is enough to cause an atmosphere to warm without any RHT, then convective heat transfer is part of the total GHE and not a reduction in the GHE.

  3. Richard111 Dec 4th 2010

    It would appear all planets with an atmosphere exhibit a “greenhouse effect” which is purely a function of solar energy and gravity. There are many lapse rate graphs showing this on the internet. If the earth’s atmosphere was pure CO2 it would be 2 degrees cooler. Convective heat transfere through CO2 exceeds that through N2.

  4. inconvenientskeptic Dec 4th 2010

    Richard,

    Pure CO2 with the density of the Earth’s atmosphere would be more complex. Some of the absorption bands that can be safely ignored would start to play a significant role.

    So while the effect of convection might be less, the radiative effect would be much greater. Some of the effect associated with water vapor would be picked up by CO2 as well.

    If the water vapor was replaced by CO2 and the latent heat transfer was removed from the balance, then I agree that the Earth would be cooler by at least 10 C.

  5. SoundOff Mar 11th 2011

    TCS, you said: “The Moon would not get as warm as the Earth (albedo is too low for the Moon) …”

    Low albedo (less reflection & more absorption) is an advantage to warming that the Moon has. It explains why the black-body temperature of the Moon is 270.7 K (-2°C) while the same for the Earth is 254.3 K (-18°C). The Earth is effectively starting from a 16°C disadvantage to get to its +15°C average temperature. Add equivalent atmosphere and oceans to the Moon, and things would be downright toasty (a least until clouds and glaciation take hold and increase the Moon’s albedo).

  6. inconvenientskeptic Mar 11th 2011

    Sound,

    Yes, lower albedo higher blackbody temperature. Thank you for correcting that. 🙂

    With a nitrogen only atmosphere that was as dense as the Earth’s it would be much warmer though without any radiative forcing changes. That might be enough to catch the Earth.

    I will have to look at why the moon is colder than the blackbody.. That is an interesting fact.

    Mars matches perfectly the blackbody and actual.

  7. SoundOff Mar 11th 2011

    TCS, some of your conjecture about an all nitrogen atmosphere on the Moon might need rethinking.

    There are limited ways that the heated surface of the Moon or any planet can transfer heat to its atmosphere:

    – Radiation
    – Latent heat (evaporation to convection to condensation)
    – Sensible heat (convection with no phase change
    – Conduction (not relevant to atmosphere)

    The radiation method doesn’t work with nitrogen (N2) which is a symmetric diatomic molecule. N2 molecules won’t be excited by LW radiation from the surface. Photons of certain frequencies are intercepted by specific absorbers called triatomic molecules. These are molecules that vibrate for an instant when they encounter an electromagnetic field (which is what a photon is). Furthermore, they can bend and change shape as they vibrate, and they move into different energy states when changing shape (they are dipoles). Some of these special molecules are H2O, CO2, CH4, N2O and O3. Molecules like N2 and O2 (99% of Earth’s atmosphere) can’t absorb longwave radiation because identical diatomic molecules don’t bend (they are symmetric).

    Latent heat and sensible heat don’t apply either without some condensable liquid present, typically H2O.

    Conduction is such a small effect with gases that it’s regarded as irrelevant. The molecules of a gas are so far apart that the few that come into contact in the surface won’t add much heat to the atmosphere.

    To heat an atmosphere takes molecules that can be excited by radiation. Those molecules then impart kinetic energy to all the other molecules around them (this is heat), or they remit photons that either further warm the surface or excite nearby radiation sensitive molecules (the latter just defers the result).

    You have no effective mechanism to warm an all nitrogen atmosphere.

    Even when you get warming and that warming is spread around, I doubt there is some reduction in heat loss due to the spreading alone. It is true that less heat will be radiated away on the bright side but now more will be radiated away on the dark side. They will net out to zero.

  8. SoundOff Mar 11th 2011

    P.S. I’m a little fuzzy on the sensible heat transfer concept. You might find a mechanism there. More research is needed. Let us know what you find. 🙂

  9. SoundOff Mar 11th 2011

    Here’s some quick research:

    Sensible heat flux is a phenomenon that allows the Earth to exchange heat between a body of water, most often the ocean, and the adjacent atmosphere.

    If above is true, it would not apply to the Moon with an all nitrogen atmosphere.

    “Sensible” heat is that caused by conduction and convection. For example, with a warm surface and a cooler atmosphere, at the boundary layer heat will be conducted into the atmosphere and then convection will move the heat higher up into the atmosphere.

    Then again, maybe conduction in combination with sensible heat might get you some warming.

    I found the following at http://www.physicalgeography.net/fundamentals/7j.html

    CHAPTER 7: Introduction to the Atmosphere

    Global Heat Balance: Introduction to Heat Fluxes

    The redistribution of energy across the Earth’s surface is accomplished primarily through three processes: sensible heat flux, latent heat flux, and surface heat flux into oceans. Sensible heat flux is the process where heat energy is transferred from the Earth’s surface to the atmosphere by conduction and convection. This energy is then moved from the tropics to the poles by advection, creating atmospheric circulation. As a result, atmospheric circulation moves warm tropical air to the polar regions and cold air from the poles to the equator. Latent heat flux moves energy globally when solid and liquid water is converted into vapor. This vapor is often moved by atmospheric circulation vertically and horizontally to cooler locations where it is condensed as rain or is deposited as snow releasing the heat energy stored within it. Finally, large quantities of radiation energy are transferred into the Earth’s tropical oceans. The energy enters these water bodies at the surface when absorbed radiation is converted into heat energy. The warmed surface water is then transferred downward into the water column by conduction and convection. Horizontal transfer of this heat energy from the equator to the poles is accomplished by ocean currents.

  10. SoundOff Mar 11th 2011

    Good question! How can the mean temperature of Moon be lower than its black-body temperature? I don’t think it can without violating the known rules of radiative physics.

    The black-body temperature of the Moon is 270.7 K (-2°C).

    The Moon has an average surface temperature of 250 K (-23°C). The range is 280°C between day and night though (~ from 100 K to 400 K).

    The black-body temperature is confirmed on many science web sites including a fact sheet provided by NASA, so it very likely correct.

    http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html

    That means the average surface temperature is suspect. One problem is without an atmosphere there is no real way to measure an average surface temperature on the Moon. What they do on the Moon is measure the sub soil surface temperature instead (directly on moon missions and using other long-distance methods). The surface can be quite warm (probably at the black-body temperature). But just below the surface is much colder. If you have ever walked on a sand beach on a hot sunny day, you will know that the sand surface is superheated but it is much cooler under that surface. I think we are probably comparing apples and oranges when we compare these two temperatures. I could not find a definitive explanation anywhere, so this is just my semi-educated guess at the reason.

  11. SoundOff Mar 11th 2011

    TIS, sorry about addressing you as TCS. It was an unintentional misreading of the blog address, or perhaps a Freudian slip, I’m not sure. But you got me a bit wrong too.

  12. inconvenientskeptic Mar 11th 2011

    No worries about TCS. I have worked with tri-chloro-silane many times in my career. 😀

    I am not concerned with the average temperature. That isn’t particularly relevant. Since the moon gets sun for basically a week (more, but very direct for at least a week) the high temperature on the surface should by definition be the blackbody.

    If you look at the top post right now the depth would absorb heat at the rate of thermal conductivity and the depths would be more stable, but at an in-between max and mix temps.

    The above was just me thinking the problem out, but I decided to look more and found this paper.

    It explains that NASA used Stefen-Boltzmann to predict moon temperatures, but it didn’t work because of exactly the heat transfer to the region underneath the surface. That is pretty nifty stuff.

    The surface temp is what the astronauts needed to be prepared for because they would be in contact with it. But that much of the energy went inward when hot, then switches outward when cold makes perfect sense.

    Fascinating. This is the kind of discussion I like. Hopefully we are both learning in it. 🙂

  13. inconvenientskeptic Mar 11th 2011

    Also… If you read part 3 of the series, I estimate that the convective heat transfer and the LW absorption should be comparable for the situation of the Earth’s surface.

    I also link to NASA where they state that the convective heat and the thermal absorbed are both 5%. The precise estimates give the LW slightly larger, but very comparable in size.

  14. SoundOff Mar 12th 2011

    TIS, I had already read the link you referenced above earlier. But I concluded that the guy didn’t know what he was talking about, or more likely was intentionally misrepresenting radiative heat transfer science. For example, he says “a real-life blackbody can only be approximated by a hole, a dark cavity”. This is nonsense. The Sun is blackbody while a laser beam is not a blackbody. Blackbody just means something radiates in a standard curve across the frequency spectrum according to its temperature.

    The rest of his argument is just that blackbody calculations don’t produce real surface temperatures. Of course they don’t. He’s making a straw man argument. Blackbody temperature calculations tell us about the planet’s overall temperature according to the radiation it receives. One then needs to adjust for greenhouse gases, huge thermal lags in oceans, small thermal lags in land mass and other things to come to some conclusion about the actual surface temperature. Hopefully, that result equals the same blackbody calculations according to the radiation the planet outputs. In the case of the Moon, if there’s less heat in the day and more at night, this is just a temporal thermal transfer that doesn’t affect the overall output. Greenhouse gases, however, would change the output.

    I have read your 3-part series and more. I completely agree with everything commented by Glenn Tamblyn throughout. I don’t want to be too repetitive. An energy flux is a flow of energy at some rate (regardless of the source/target/direction); it’s not a forcing or feedback. The latter are climate events, like solar variations, orbital changes, volcanoes, greenhouse gases, melting ice caps, clathrate methane releases, etc. – all things that cause an energy flux to change, up or down. It’s deemed a forcing when it is caused by a factor external to the climate system otherwise it is considered an internally induced feedback that automatically results when a forcing nudges things one way or another.

    Even if I disagree with some of your central explanations and conclusions, I do admire you for attempting to be skeptical on a scientific basis. Too many skeptics live in a non-science fantasy world. I was a skeptic once too, mostly because I hadn’t bothered to get the necessary deeper understanding of how climate operates. I estimate I’ve invested some 3000+ hours to become convinced, and still learning.

    I would just add that the latest science has come up with different attributions for the greenhouse effect than you:

    Water Vapor (a feedback only): 50%
    Clouds (a feedback only): 25%
    CO2 (the main forcing that drives the feedbacks): 20%
    Other (minor forcings such as methane, ozone, etc.): 5%

    It’s very late here. Goodnight.

  15. inconvenientskeptic Mar 12th 2011

    Sound,

    Mostly I agree with your views about that paper, but I am still trying to find the details of the moon surface temps. Even if his reasons are incorrect, the temperature behavior of the subsurface matches what we see on Earth as well.

    That effect would make the temperature of the surface differ than the blackbody prediction and that would skew the resulting GHE for the Earth.

    This ties in directly to the time lag stuff I am working on. The surface clearly loses energy to the interior during the day and the gains it back at night. On the moon the only mechanism for heat loss is radiative to space and conductive from the interior to the surface.

    So whichever is the bottleneck will determine the total heat loss. I would really like to find the source data that he used for that article, which I agree is not a great article, but he might have found some really interesting data. It looks like he based on this paper.
    http://climaterealists.com/attachments/database/RadiativeNonEquilbrium_BHermalyn_Final.pdf

    The paper itself used to be linked at NASA, but I am still sourcing it.

  16. SoundOff Mar 12th 2011

    Hi TIS,

    Let’s revisit the “Greenhouse_Effect_on_the_Moon.pdf” document now that I’ve studied it more. Its main argument is that idealized blackbody calculations did not correctly predict the Moon’s surface temperatures in the 1960s because other factors besides radiative heat transfer equations actually determine real surface temperatures. Therefore the greenhouse effect is likely being overestimated on Earth because these same “other factors” are at work here too. The Moon is being used as a simplified case to demonstrate this point. The document suggests NASA’s day/night projections for the Moon were off in the 1960s because of unexpected thermal absorption by the rock and soil of the surface. NASA may have also underestimated Earth shine (an input to the calculation).

    However blackbody calculations do not speak to high and low surface temperatures and how these are distributed over cycles in time, across surface space or throughout the atmosphere. They just state an overall temperature for the radiating surface as if it was a non-spinning flat surface without greenhouse gases and other complications. All the various complications need to taken into account to allocate the blackbody temperature appropriately.

    Based on the insolation received by the Moon, allowing for its estimated albedo and assuming little internal core heat, the blackbody temperature of the Moon is 270.7 K (-2°C). The document states “Thus (within the zone in question) the surface of the real moon is roughly 20° cooler than predicted by day and 60° warmer by night, the net result being a surface that is 40° warmer than predicted.”. His graph shows this too. (I agree he cherry-picked most of his analysis from the NASA report you linked to and then just added his own spin, particularly the part that begins “the net result being …”).

    Is he then saying that the mean temperature of the Moon is +38°C (which is his 40° plus the -2°C blackbody prediction)? That’s more than twice as warm as Earth, and without any greenhouse effect. And it disagrees with the mean estimate that I provided earlier of -23°C (see * below). His numbers are not reasonable. Either his has misstated NASA’s estimates or NASA did not properly allocate the 2°C properly “within the zone in question”. It looks to me like he has based his blackbody analysis on maximum equatorial temperatures and minimum polar temperatures on the Moon and then compared them to temperatures actually measured at the landing sites. This is not a very meaningful comparison.

    Based on the insolation received by the Earth, allowing for its estimated albedo and some internal core heat, the blackbody temperature of the Earth is 254.3 K (-18.8°C). The Earth’s measured temperature is about 288 K (15°C). Both numbers are well known. There is about a 33°C delta between these two temperatures that is called the Greenhouse effect. This is a reasonable assumption because surface redistribution of heat does not alter the calculations and the burying of heat was cyclically static until recent times.

    * I’ve since learned that -23°C is just an average of the normal max and min values for the Moon so it is probably not a meaningful temperature depending on the distribution of real temperatures in time and space. For example, Earth’s normal max and min values are +41°C and -46°C and average of these two values is -2.5°C, which is a long way from the properly weighted value of +15°C.

  17. SoundOff Mar 12th 2011

    Hi TIS,

    Here are my thoughts on your time lag topic.

    If you created a fresh cold moon, there would be some heat loss to the interior for a time. If this was a never-ending process, then after 4.5 billion years the continued heat penetration would have melted the Moon to a lava or plasma blob. So clearly heat penetration must have reached its maximum long ago and all that’s occurring now is day/night surface redistribution of heat. Surface redistribution of heat does not affect the overall heat radiated from an object or its blackbody temperature. On the moon the only mechanism for heat loss is radiative to space. There’s no one-way interior heat storage or conductive heat transfer from the interior to the surface to distort blackbody calculations. One side of the Moon stores heat and the other side releases heat equally. Blackbody rules apply.

    If warming did increase for some reason (e.g. a brighter Sun), then interior heat penetration would also increase for a short time until equilibrium is again reached and the Moon’s blackbody temperature would consequently rise a bit. This interior heat penetration is a very minor effect when dealing with solid surfaces which don’t store heat well. Solids absorb and release heat quickly because their closely packed molecules tend to vibrate in unison (vibration = heat) – so there’s little time lag. In the case of soil and rock, only the molecules closest to the radiation surface will warm due to all the gaps between the aggregate and crystals, conduction is impaired. It’s also a minor effect with gases. Gas molecules are far apart so won’t vibrate in unison but they move around very quickly and so they quickly redistribute their heat by randomly colliding with nearby molecules (collisions = heat) – again little time lag. Liquids happen to work best but the Moon lacks any of these. Some very odd things happen to the rules at extremely low temperatures but I don’t have enough knowledge to comment on this. Physics is a nonlinear subject.

    The same idea applies on Earth with respect to heat loss to the interior of solid planet – solids have a minor heat storage role when warming increases. But the presence of oceans really changes things. Water has a very high specific heat, much higher than nearly anything else. This means it can store more heat for a given volume. Another factor is convection currents in water allow heat to move into the depths and oceans are very deep. The oceans can absorb a large part of any increase in warming so that the heat becomes buried in the ocean. Therefore, there is a significant lag (of decades) between the warming (the forcing) and when it becomes visible as an ocean surface temperature increase. But the oceans will eventually respond if the forcing is sustained. Until they do, you will see a delta between incoming and outgoing TOA radiation of Earth. If the new forcing is actually continually increasing then a new equilibrium will never be reached (e.g. as is happening with CO2 emissions). Ocean surface temperatures will always lag behind the forcing.

    Specific heats http://hyperphysics.phy-astr.gsu.edu/hbase/tables/sphtt.html#c1

  18. inconvenientskeptic Mar 12th 2011

    Sound,

    In general I agree with your assessment of the GHE on the moon paper, but we were discussing the question of why the moon is colder than the blackbody predication would have it be.

    I am putting together a new article on this. Should be up tomorrow.

    I have a NASA document that discusses far more detail, but I don’t have data yet. I will put together what I have though. While his blackbody was off base, there is a useful piece of information in there.

    It would not molten anything though. The energy starts to dissipate as soon as the sun sets.

  19. SoundOff Mar 13th 2011

    TIS, I’ll watch for your new article.

    On the question of why the Moon is colder than its blackbody predication, I thought that I had made myself clear. It isn’t. The Moon’s blackbody temperature is -2.5°C but the mean temperature I quoted from elsewhere (-23°C) is just a meaningless average of some published max and min values (I learned this fact after you pointed out this oddity).

    Let me demonstrate what I mean using the max and min values from my bank account over the last complete month. I had $107 there at the start of the month (this is my max value). My balance decreased through the month as I paid my bills until I used my overdraft privileges on the last day before payday. On that day my balance became -$153 (this is my min value). Therefore, (max+min)/2 = ($107-$153)/2 = -$23 was my mean balance for the month. This calculation says that on average my account was overdrawn by $23 but it was actually overdrawn only on the final day. (Coincidently, my max/min account balances roughly match some min/max temperatures quoted for the Moon.)

    My point is that this kind of average of two extremes is a meaningless number, perhaps used when daily balances aren’t known. The Moon’s “mean temperature” I gave you is also a meaningless number and is used (by some) only because daily values aren’t available across a wide area of the Moon’s surface to determine a true number. NASA does not publish any “mean temperature” in their Moon fact sheet since the true number is not yet known. Indeed, using Earth-based max+min values gives a wildly inaccurate result for Earth. Without a true number, there’s no way to say the blackbody equation gives a wrong result.

  20. inconvenientskeptic Mar 13th 2011

    Sound,

    I agree that the mean value isn’t helpful for the Moon, but the blackbody max and min are different from observed. That is important.

    The article is up with more info.

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