Arguments
The greenhouse effect and the 2nd law of thermodynamics
The skeptic argument...
2nd law of thermodynamics contradicts greenhouse theory
"The atmospheric greenhouse effect, an idea that many authors trace back to the traditional works of Fourier 1824, Tyndall 1861, and Arrhenius 1896, and which is still supported in global climatology, essentially describes a fictitious mechanism, in which a planetary atmosphere acts as a heat pump driven by an environment that is radiatively interacting with but radiatively equilibrated to the atmospheric system. According to the second law of thermodynamics such a planetary machine can never exist." (Gerhard Gerlich)
What the science says...
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| The 2nd law of thermodynamics is consistent with the greenhouse effect which is directly observed. | |||||
Skeptics sometimes claim that the explanation for global warming contradicts the second law of thermodynamics. But does it? To answer that, first, we need to know how global warming works. Then, we need to know what the second law of thermodynamics is, and how it applies to global warming. Global warming, in a nutshell, works like this:
The sun warms the Earth. The Earth and its atmosphere radiate heat away into space. They radiate most of the heat that is received from the sun, so the average temperature of the Earth stays more or less constant. Greenhouse gases trap some of the escaping heat closer to the Earth's surface, making it harder for it to shed that heat, so the Earth warms up in order to radiate the heat more effectively. So the greenhouse gases make the Earth warmer - like a blanket conserving body heat - and voila, you have global warming. See What is Global Warming and the Greenhouse Effect for a more detailed explanation.
The second law of thermodynamics has been stated in many ways. For us, Rudolf Clausius said it best:
"Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature."
So if you put something hot next to something cold, the hot thing won't get hotter, and the cold thing won't get colder. That's so obvious that it hardly needs a scientist to say it, we know this from our daily lives. If you put an ice-cube into your drink, the drink doesn't boil!
The skeptic tells us that, because the air, including the greenhouse gasses, is cooler than the surface of the Earth, it cannot warm the Earth. If it did, they say, that means heat would have to flow from cold to hot, in apparent violation of the second law of thermodynamics.
So have climate scientists made an elementary mistake? Of course not! The skeptic is ignoring the fact that the Earth is being warmed by the sun, which makes all the difference.
To see why, consider that blanket that keeps you warm. If your skin feels cold, wrapping yourself in a blanket can make you warmer. Why? Because your body is generating heat, and that heat is escaping from your body into the environment. When you wrap yourself in a blanket, the loss of heat is reduced, some is retained at the surface of your body, and you warm up. You get warmer because the heat that your body is generating cannot escape as fast as before.
If you put the blanket on a tailors dummy, which does not generate heat, it will have no effect. The dummy will not spontaneously get warmer. That's obvious too!
Is using a blanket an accurate model for global warming by greenhouse gases? Certainly there are differences in how the heat is created and lost, and our body can produce varying amounts of heat, unlike the near-constant heat we receive from the sun. But as far as the second law of thermodynamics goes, where we are only talking about the flow of heat, the comparison is good. The second law says nothing about how the heat is produced, only about how it flows between things.
To summarise: Heat from the sun warms the Earth, as heat from your body keeps you warm. The Earth loses heat to space, and your body loses heat to the environment. Greenhouse gases slow down the rate of heat-loss from the surface of the Earth, like a blanket that slows down the rate at which your body loses heat. The result is the same in both cases, the surface of the Earth, or of your body, gets warmer.
So global warming does not violate the second law of thermodynamics. And if someone tells you otherwise, just remember that you're a warm human being, and certainly nobody's dummy.
Last updated on 22 October 2010 by TonyWildish.
The author who drew this failed to provide any temperature information anywhere. How can he possibly know the energy tranfers (all over the diagram in W/m^2) without indicating the temperatures? Whoever drew this diagram obviously hadn't the slightest knowledge of the 2nd Law of Thermodynamics. Who pays money for 'stuff' like this?
The diagram was about the flow in energy which is expressed in units of W/m^2. That is: energy (watts) over a specific area (a square meter). Are you suggesting that it should be "degrees" per square meter?
It is the appearance of dagrams like this in IPCC reports that makes one instinctively mistrust all of 'global temperature increase' figures produced by the IPCC.
Why, because they used the proper of units of measure instead of the ones you suggest which make no sense whatsoever for describing the flow of energy.
Also, why are you criticizing the IPCC when the diagram you are talking about is by Trenberth, Fasullo and Kiel (2009)?
Surely 'elementary' physics asks and answers that question - although expressed more precisely - with the
Carnot Cycle etc. No?
Boy, this is a long thread! Can someone explain me why we are having a discussion about something that has been proved over and over again? - GW is real and it's clear as a daylight that we, humans, are driving it ...
An increase in greenhouse gases directly decreases emissivity by absorption band deepening and widening. This drops emitted energy to space.
A lot of scientists do not actually understand that indeed adding more heat-absorbing gases actually REDUCES emissivity of the atmosphere. I only know a few scientists, who realize that.
Also, see this recent article by prof. R. Pierrehumbert from University of Chicago, who explains very well how the greenhouse effect works making the point that the atmosphere is analogous to a house insulation in the way is prevents heat loss. Now that's real physics!
http://geosci.uchicago.edu/~rtp1/papers/PhysTodayRT2011.pdf
My understanding (and I am not an expert on Climate) is that the fact that the downward flux is greater than the Solar input is due to heat accumulated in the climate system. You can see the results of a simple spreadsheet model of the longwave component only here.
The numbers don't relate to Trenbeths diagram, I picked them so the calculation would reach equilibrium fairly quickly.
Column A is somewhat badly named - it should be the amount of Solar radiation absorbed and re-emitted as longwave radiation.
If you wish to replicate this, the cell formulae are: Cx=Ax+Bx Dx=Cx*0.4 Ex=Cx*0.6 Bx=D(x-1) and so this represents an atmosphere that radiates 40% of its longwave radiation to space and 60% back to the ground.
I hope the spreadsheet is clear, towards the end I turn the Sun off, and calculate the time taken for the planet to dissipate the accumulated heat. At the end the energy in does indeed equal the energy out - which proves that I (and OpenOffice) got the sums right and that the model conserves energy.
The main point is that, in this model at least, the back-radiation does indeed grow to be greater than the incoming Solar.
As I said earlier, I'm not an expert so I would welcome corrections and clarifications !
The atmosphere is pretty deep, so maybe it could store substantial amount of energy? I'm curious what others think?
The atmosphere is pretty much a direct in-out of the energy stored, radiating out as much energy as it receives in IR (396 W/m^2), latent heat of evaporation (80), and thermals (17).
But even a simple single-layer model will show this effect of high energy in the climate system - it has to be that high in order to put 240 W/m^2 back into space.
So will a zero-dimensional emissivity model - surface emissivity is about 0.98 (almost a perfect black body), and to radiate the 240 W/m^2 received back to space would be at a temperature of about -17C. The top of atmosphere (TOA) spectra integrates to an effective emissivity of ~0.612 with all the absorption dips, and to put 240 W/m^2 into space the surface needs to be at about 390 W/m^2.
However, this is largely irrelevant to PhysSci's argument. The reason the atmosphere maintains a very high back radiation is because a very high energy flux into the atmosphere is maintained, both by energy transfers from the Earth (IR, convection and evapo/transpiration) and directly from the Sun. In fact, when one of those (the sun) is removed at night, the back radiation falls very rapidly unless either:
1) The surface IR flux is maintained at high levels by being over ocean; or
2) The humidity is very high (which increases the specific heat of the atmosphere) and there is a low cloud cover (which lowers significantly the average altitude from which the back radiation comes, and significantly increases the heat capacity of the source of the back radiation).
Obviously, there is some regional geographic effects, with warm air over sea water at night helping maintain a higher night time back radiation on the coast than in the interior.
All of this follows because the back radiation is simply the thermal (IR) emissions of the lowest kilometer or so of the atmosphere; and therefore fluctuates with the temperature of that lowest portion of the atmosphere. Because of this, back radiation can fluctuate by more than 145 watts per meter squared in a single 24 hour period (the calculated night/day variation for winter in Mount Isa, Queensland) even without major changes in weather.
Absent the greenhouse effect, the surface IR radiation would equal the incoming solar radiation. The green house effect lifts that till it is much higher, and therefore the back radiation is higher. It is as simple as that.
It sounds like you all are telling me that there is no error in the reported intensities of back radiation, and that it is larger than the absorbed solar flux. Correct?
I'm an ecologist and from working with observed met data (to drive my ecosystem models), I noticed that the diurnal variation in back radiation is much smaller than solar radiation. In fact, back radiation is nearly invariant (+- 20W m-2 or so) between day and night. This is true for sites in the NH. I'm bringing this up in response to Tom Curtis's remark that back radiation varies diurnally quite a bit. I've seen the opposite in the data sets I worked with ...
Anyway, I understand that the lower atmosphere, where most of the back radiation is coming from, is heavily heated by the surface through surface-atmosphere exchange of various energies. But I'm still not clear what's making the temperature of the surface to increase beyond the black-body temperature. I guess this is the chicken-and-egg question; which is heating which?
The reported intensities in the diagram have significant margins of error (around 5% I believe), but are the best available estimates.
If that is the case, then the lower atmosphere must contain kinetic energy that is larger than solar radiation, IF back radiation is really larger than the absorbed solar flux. This suggests that the atmosphere must be able to somehow store significant amount of energy. So, we come again to the atmospheric heat storage question. Correct?
Something here does not add up - either we have a large atmospheric heat storage or the back radiation numbers must be wrong (or I'm profoundly misunderstand something). Do you agree?
One way to really visualize this is to think of the system as left-to-right. Sunlight enters from the left, heating the surface. To maintain conservation of energy in a steady state solution, the surface must dump that heat to outer space (far right). But there's an insulating atmosphere in between.
The temperature that the Earth reaches is that which is necessary to pump 240 W/m^2 through the radiatively insulating atmosphere into space. Moving this energy through the atmosphere requires an energy differential!
The Earth radiates 396 W/m^2 to the atmosphere (some 40 or so going straight through), the atmosphere acquires a temperature equal to the surface at the surface and radiates a large amount back down, and the upper colder atmosphere radiates 240 W/m^2 to space.
It's insulation, pure and simple. And the more insulation between a constant input and a cold sink, the hotter the final temperature of the heated object.
Total flow through the system is still 240 in, 240 out. There's just a buildup of energy at the thin link through the Earth/atmosphere interface.
Your will notice the range of the back radiation is around 150 w/m^2 over just a few weeks, and that on a single night it can be as much as 100 w/m^2. This is less than Mount Isa's, mostly because of differences in climate and surface cover(Mount Isa is semi-desert,Omaha is Prairie). Greater plant cover in Omaha means a high stored water content in the plants an soil, resulting in less variation in the temperature of the surface, which coupled with greater humidity in the atmosphere will account for most of the difference.
The second shows the annual variation at several sites at different latitudes:
Again there are significant differences in the variation depending on location and climate. The stark contrast between the tropical atoll, Kwajalein (which because of its effectively non-existant land mass will vary with changes in Sea Surface Temperatures) and that for the inland, and arid Boulder Colarado, is very informative. (I would have preferred a tropical comparison, but do not know the location of Tateno.)
Radiation does not contain kinetic energy. However the Solar radiation is a function of kinetic energy on the surface of the Sun. That is much higher than that on the surface of the Earth, but because of our distance and the inverse square law, the portion of that energy that the Earth receives is very low. Further, because the Earth is a sphere and always half in darkness, the average energy per meter squared received on the surface of the Earth is only a quarter of that received by a flat plate perpendicular to the sun in the same orbit.
So, the conclusion that the Earth's atmosphere's kinetic energy is large compared to that on the sun is unwarranted, and fails to account for the scaling factors.
Do greenhouse gases also work to trap heat other than IR radiation? They have to in order to achieve the the significant warming of the surface above the black body ...
You cannot compare these quantities in the way you are attempting. Solar flux is a rate, measured as units of energy per units of time. The total kinetic energy of the atmosphere is the total amount of energy stored at a specific point in time.
The total kinetic energy of our atmosphere is higher than it would be if the greenhouse effect was not present. This is because greenhouse gases slow the rate at which energy can be dissipated into space via thermal radiation (lower emissivity), resulting in more energy accumulating within the atmosphere.
If you look at the Trenberth et al (2009) energy budget diagram (which Tom Curtis kindly linked to at 491), you'll see that the atmosphere absorbs 396-239 = 157 W m-2 of OUTGOING long-wave radiation. This flux is only 47% of the downward IR flux (333 W m-2) shown on the same diagram. So, 53% of the back radiation must be coming from other sources (solar, evapo-transpiration etc.) indicating that GH gases also trap other types of heat. Is this correct?
This also makes me wonder, why the greenhouse effect is only attributed to interception of surface IR radiation, when more than 50% of the net energy input to the atmosphere comes from sources other than surface IR flux ... Is this a legitimate conclusion assuming that Trenberth's diagram is correct?
There must be some sort of high energy storage in the atmosphere that is maintained by the absorption of heat by GH gases. At least, that's the logical conclusion (in my view) from the discussion so far ... Or, there might be another source of energy we have not considered yet. Is this possible?
The atmosphere doesn't have to hold the energy in thermal mass (and it doesn't have much thermal mass to start with) - just re-emit it back down and prevent it from leaving. This is a lot like those shiny "space blankets" - they have almost no thermal mass, but via IR reflection and blocking convection they insulate quite well.
Without GHG's the Earth could radiate all 240 W/m^2 out to space directly. If you look at just the atmospheric layer, ~240 reach space through various pathways, driven by the 17+80+396 coming up from Earth, the 78 incident on the atmosphere from the sun, minus the 333 going down as "backradiation" = 238 W/m^2 (239 without rounding, with a 0.9 calculated imbalance warming things).
To make it clear, what matters is total energy entering and leaving the system, and a stable system will reach the temperatures and internal energy balances necessary for that. The atmosphere reduces the emissivity of the planet, and if emissivity reduces, temperature is the only free variable to return power outflow to the level of the incoming energy. And the stable state of the climate will move towards that point.
Internal energy levels are a function of input/output and energy transfer rates, but if there's a narrow point in the energy flow (like the atmosphere) local energy levels will, must, go considerably higher to reach that throughput level. That's the first law of thermodynamics - conservation of energy, what goes in must come out.
Why does it need to be in the atmosphere? The ocean provides a handy, accessible, and huge, energy storage facility right next door. (I'm assuming that the question really does reflect some difficulty here.)
If I remember correctly, those blankets have very low emissivity and therefore high reflectivity with respect to IR, and that's how they prevent thermal radiation from escaping and cooling the body. So, greenhouse gases must reflect thermal radiation as well, correct? But then why they say that greenhouse gases have high IR absorptivity especially water vapor. Is it that greenhouse gases have high reflectivity and high absorptivity both at the same time?. But then you mention that "the atmosphere reduces the emissivity of the planet". Do you implay that GH gases reduce the emissivity of the atmosphere, which would mean that they themselves have low emissivity. So, I guess I got confused again ... Sorry, for not being able to follow you completely.
Also, from your analogy with the space blanket, does it mean that GH gases also affect (reduce) convection? That would support my earlier conclusion (suspicion) that those GH somehow trap convective heat as well.
This is a very interesting discussion to me. Thank you to all for participating.
"'d like to clarify why I thought that GH gases may also trap heat other than IR radiation:
If you look at the Trenberth et al (2009) energy budget diagram (which Tom Curtis kindly linked to at 491), you'll see that the atmosphere absorbs 396-239 = 157 W m-2 of OUTGOING long-wave radiation. This flux is only 47% of the downward IR flux (333 W m-2) shown on the same diagram. So, 53% of the back radiation must be coming from other sources (solar, evapo-transpiration etc.) indicating that GH gases also trap other types of heat. Is this correct?
This also makes me wonder, why the greenhouse effect is only attributed to interception of surface IR radiation, when more than 50% of the net energy input to the atmosphere comes from sources other than surface IR flux ... Is this a legitimate conclusion assuming that Trenberth's diagram is correct?"
No, because all the energy emitted from (and into) the planet is radiative, which means the net effect of all the other components has to be zero. Remember also, incoming solar energy absorbed by the atmosphere and radiated down has yet to reach the surface and this energy is included in the post albedo of about 239 W/m^2.
Here's a very simple example describing emissivity. First, start with the Stefan Bolzmann equation. Power radiated depends on temperature (to the 4th power) and emissivity. In order for the energy of the Earth to be stable, it must radiate as much energy as it receives.
It receives 240 W/m^2 from the Sun. If there were no GHG's, the S-B equation indicates that the temperature of the Earth, with an emissivity of 0.98, would be about -17°C (256.15°K), or chilly.
For toy purposes, assume the Earth has an emissivity of 1.0 (close enough to 0.98). Add an atmosphere of greenhouse gases. Assume the GHG's absorb (say) 80% of that, and re-radiate it. Half goes up, half goes down - only 60% total of the 240 W/m^2 goes to space, or 144. That's an imbalance of 96, an effective emissivity of 0.6 rather than 1.0. Energy builds up on the surface, emitting more IR.
In order to emit 240 W/m^2 with an effective emissivity of 0.6, the surface must go to
( (256.15°K)^4 * 1.0 / 0.6 ) ^ 0.25 = 291°K, or over +17°C; a 34°C difference
And the surface radiation will be about 240 / 0.6 = 400 W/m^2. Pretty close for off the cuff numbers and a zero dimensional model! Real numbers are a 33°C difference, 14°C surface temperature, 396 W/m^2.
Greenhouse gases insulate by absorbing then re-emitting IR, sending half the energy back down, reducing effective emissivity to space. It doesn't take a lot of thermal mass, just absorbing/emitting in the IR.
"It's insulation, pure and simple. And the more insulation between a constant input and a cold sink, the hotter the final temperature of the heated object."
Except the "insulation" is not made up of one substance or even a constant mix of all the individual substances. If one of the substances changes, it could trigger changes in the other substances - making the net effect very difficult to determine. You're assuming atmospheric opacity will increase and the rate at which energy can escape will decrease with rising CO2 levels. It may, but it also may not.
Ah, but basic physics tells us that it does. Emissivity (not opacity) decreases as GHG's increase, widening and deepening absorption bands, the observed satellite emission spectra matching the physics predictions, temperature must increase in response for a stable state emitting as much power as comes in, QED.
That's an observation, not an assumption. An unsupported statement of "...but it also may not" is not science.
The level of feedbacks is another topic entirely, Climate Sensitivity. This thread is on the existence of the radiative greenhouse effect (RGE), hence the direct forcing, and the foolishness of claiming that the RGE violates thermodynamics.
"To finish, Trenberth, Khiel and Fasullo are not obfusticating by indicating some SW radiation is absorbed in the atmosphere. They are describing an indirect emperical result, and one that is more easily determined than, for example the proportion of SW light reflected from clouds, or from the surface. The method is to measure downward SWR at the Top of the Atmosphere, upward SWR at the TOA, and subtract the later from the former. You then measure downward SWR at the surface, and subtract that result from the difference; giving you the amount of SWR absorbed. (Clearly the measurements need to be made at a large number of points and times to determine a global average.) T,K, & F (2009) list a summary of such mesurements on table 2b.
It is a telling indictment of your "science" that you cannot use standard definitions correctly, and have to dismiss observational results as "obfustications".
Your missing the point, and that is of the 396 W/m^2 radiated from the surface, Conservation of Energy dictates that 239 W/m^2 of if has to come from the Sun because the atmosphere cannot create any energy of its own. Yes, all 239 W/m^2 does not get to the surface as direct SW, but it gets there one way or another. Ultimately, this means the remaining 157 W/m^2 of the 396 W/m^2 emitted at the surface has to come from 'back radiation' from the atmosphere. Trenberth's depiction obfuscates this, which - along with total transmittance, is the most crucial aspect of the entire greenhouse effect. The net effect of all the other components is zero.
"Ah, but basic physics tells us that it does. Emissivity (not opacity) decreases as GHG's increase, widening and deepening absorption bands, the observed satellite emission spectra matching the physics predictions, temperature must increase in response for a stable state emitting as much power as comes in, QED."
I don't really want to argue. All I'm saying is an increase in CO2 concentration will increase absorption in the CO2 absorbing bands, yes, but this does not necessarily mean all of the other absorbing bands will remain constant, especially at various levels of the atmosphere where their concentrations can vary (H20 in particular). That increased CO2 will decrease total transmittance through the whole atmosphere not definite by any means - just seemingly probable.
The reason the GH effect is attributed to intercepting IR radiation from the surface is because simplified models are used to explain the concept. It is correct, but only provides part of the more complete explanation. The complete explanation is that GHG intercept some of the IR radiation from the surface, but replace it with their own IR radiation. Because the gases are cooler than the surface at their level of effective radiation to space, the IR radiation they produce is less energetic than that which they intercepted. The difference between the energy emitted by the atmosphere to space (composed of energy drawn from a variety of sources, as per the diagram) and the the energy from the surface makes the radiation of energy to space less efficient, thus warming the surface.
So, the key terms from the diagram for the greenhouse effect are:
1) The surface IR radiation intercepted by the atmosphere (356 w/m^2); and
2) The IR radiation from the atmosphere to space (199 w/m^2).
The difference between these two, a reduction of IR emission to space of 157 w/m^2, represents the loss of efficiency in reradiating energy to space, which in turn results in a warmer surface.
Please note that I am not disagreeing with KR (@538 & 543). He, however is using two analogies which I dislike because they are misleading if pushed, and people always push analogies. The problem with the blanket analogy is obvious. For example, your natural push of the analogy to suggest GHG reflect IR radiation is natural given the analogy, but incorrect in fact.
With regard to emissivity, increasing GHG increases the emissivity of the atmosphere, but decrease the emissivity of the Earth as a whole because the atmosphere is colder than the surface so the net radiation is initially reduced. So talking about reduced emissivity of the Earth is not wrong, but is very prone to confusion.
@ 537: The proportion of GHG in the atmosphere excluding water vapour is very small. The largest proportion of it is CO2 with an atmospheric concentration of approx 0.4%, with other GHG having much smaller concentrations. Water vapour on the other hand can very between around 6% (from memory) and 0.01% of the atmosphere. Crucially, water vapour concentrations at high altitudes are very low, so at those altitudes in the frequencies in which CO2 absorbs IR, CO2 is the dominant GHG. Outside those frequencies, H2O tends to dominate.
Sorry, for asking these additional questions, but I'm really trying to understand this.
1) I remember reading somewhere that this 240 W m-2 planetary absorbed solar flux includes the effect of a 30% albedo. This is Earth's actual reflectivity. So, if we are talking about Earth without an atmosphere (with emissivity of 0.98) as you suggest, then isn't it more appropriate to consider a much lower albedo in our calculations, because removing the atmosphere (or even only the water vapor) means getting rid of all clouds as well? And, I think clouds were contributing well over half of the earth albedo (not quite sure). So, an Earth without an atmosphere (or without water vapor in the atmosphere) would have an albedo of say 12-15%, correct? This implies a significantly higher absorption of solar radiation, hence higher 'black-body' temperature in the absence of water vapor (or an atmosphere). How would that change your calculations?
2) I did not quite understand, why the energy emitted back to space would be only 60% of the total 240 W m-2 originally absorbed by the system? I thought, in equilibrium, the amount of absorbed solar energy must equal the amount of emitted IR radiation to space. What happens with the 96 W m-2 imbalance? Where does this energy go?
Also, in 545 you say that atmospheric opacity is not the same as emissivity, and opacity would stay constant while emissivity decreased with the addition of GH gases. This really cinfuses me, because I thought that emissivity/absorptivity is a measure of opacity - higher absorptivity resulting from more GH gases would cause higher opacity to long-wave radiation. What I did not understand?
Analogies are useful for explaining forward, not reasoning/disproving backwards. That can only be done in the original, complex system with all the interactions.
Still, it's a bit faster to give an analogy than to attempt a first semester thermodynamics course in a blog post...