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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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The greenhouse effect and the 2nd law of thermodynamics

What the science says...

Select a level... Basic Intermediate

The 2nd law of thermodynamics is consistent with the greenhouse effect which is directly observed.

Climate Myth...

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)

 

At a glance

Although this topic may have a highly technical feel to it, thermodynamics is a big part of all our everyday lives. So while you are reading, do remember that there are glossary entries available for all thinly underlined terms - just hover your mouse cursor over them for the entry to appear.

Thermodynamics is the branch of physics that describes how energy interacts within systems. That interaction determines, for example, how we stay cosy or freeze to death. You wear less clothing in very hot weather and layer-up or add extra blankets to your bed when it's cold because such things control how energy interacts with your own body and therefore your degree of comfort and, in extreme cases, safety.

The human body and its surroundings and energy transfer between them make up one such system with which we are all familiar. But let's go a lot bigger here and think about heat energy and its transfer between the Sun, Earth's land/ocean surfaces, the atmosphere and the cosmos.

Sunshine hits the top of our atmosphere and some of it makes it down to the surface, where it heats up the ground and the oceans alike. These in turn give off heat in the form of invisible but warming infra-red radiation. But you can see the effects of that radiation - think of the heat-shimmer you see over a tarmac road-surface on a hot sunny day.

A proportion of that radiation goes back up through the atmosphere and escapes to space. But another proportion of it is absorbed by greenhouse gas molecules, such as water vapour, carbon dioxide and methane.  Heating up themselves, those molecules then re-emit that heat energy in all directions including downwards. Due to the greenhouse effect, the total loss of that outgoing radiation is avoided and the cooling of Earth's surface is thereby inhibited. Without that extra blanket, Earth's average temperature would be more than thirty degrees Celsius cooler than is currently the case.

That's all in accordance with the laws of Thermodynamics. The First Law of Thermodynamics states that the total energy of an isolated system is constant - while energy can be transformed from one form to another it can be neither created nor destroyed. The Second Law does not state that the only flow of energy is from hot to cold - but instead that the net sum of the energy flows will be from hot to cold. That qualifier term, 'net', is the important one here. The Earth alone is not a "closed system", but is part of a constant, net energy flow from the Sun, to Earth and back out to space. Greenhouse gases simply inhibit part of that net flow, by returning some of the outgoing energy back towards Earth's surface.

The myth that the greenhouse effect is contrary to the second law of thermodynamics is mostly based on a very long 2009 paper by two German scientists (not climate scientists), Gerlich and Tscheuschner (G&T). In its title, the paper claimed to take down the theory that heat being trapped by our atmosphere keeps us warm. That's a huge claim to make – akin to stating there is no gravity.

The G&T paper has been the subject of many detailed rebuttals over the years since its publication. That's because one thing that makes the scientific community sit up and take notice is when something making big claims is published but which is so blatantly incorrect. To fully deal with every mistake contained in the paper, this rebuttal would have to be thousands of words long. A shorter riposte, posted in a discussion on the topic at the Quora website, was as follows: “...I might add that if G&T were correct they used dozens of rambling pages to prove that blankets can’t keep you warm at night."

If the Second Law of Thermodynamics is true - something we can safely assume – then, “blankets can’t keep you warm at night”, must be false. And - as you'll know from your own experiences - that is of course the case!

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!


Further details

Among the junk-science themes promoted by climate science deniers is the claim that the explanation for global warming contradicts the second law of thermodynamics. Does it? Of course not (Halpern et al. 2010), but let's explore. Firstly, we need to know how thermal energy transfer works with particular regard to Earth's atmosphere. Then, we need to know what the second law of thermodynamics is, and how it applies to global warming.

Thermal energy is transferred through systems in five main ways: conduction, convection, advection, latent heat and, last but not least, radiation. We'll take them one by one.

Conduction is important in some solids – think of how a cold metal spoon placed in a pot of boiling water can become too hot to touch. In many fluids and gases, conduction is much less important. There are a few exceptions, such as mercury, a metal whose melting point is so low it exists as a liquid above -38 degrees Celsius, making it a handy temperature-marker in thermometers. But air's thermal conductivity is so low we can more or less count it out from this discussion.

Convection

Convection

Figure 1: Severe thunderstorm developing over the Welsh countryside one evening in August 2020. This excellent example of convection had strong enough updraughts to produce hail up to 2.5 cm in diameter. (Source: John Mason)

Hot air rises – that's why hot air balloons work, because warm air is less dense than its colder surroundings, making the artificially heated air in the balloon more buoyant and thereby creating a convective current. The same principle applies in nature: convection is the upward transfer of heat in a fluid or a gas. 

Convection is highly important in Earth's atmosphere and especially in its lower part, where most of our weather goes on. On a nice day, convection may be noticed as birds soar and spiral upwards on thermals, gaining height with the help of that rising warm air-current. On other days, mass-ascent of warm, moist air can result in any type of convective weather from showers to severe thunderstorms with their attendant hazards. In the most extreme examples like supercells, that convective ascent or updraught can reach speeds getting on for a hundred miles per hour. Such powerful convective currents can keep hailstones held high in the storm-cloud for long enough to grow to golfball size or larger.

Advection

Advection is the quasi-horizontal transport of a fluid or gas with its attendant properties. Here are a couple of examples. In the Northern Hemisphere, southerly winds bring mild to warm air from the tropics northwards. During the rapid transition from a cold spell to a warm southerly over Europe in early December 2022, the temperatures over parts of the UK leapt from around -10C to +14C in one weekend, due to warm air advection. Advection can also lead to certain specific phenomena such as sea-fogs – when warm air inland is transported over the surrounding cold seas, causing rapid condensation of water vapour near the air-sea interface.

Advection

Figure 2: Advection fog completely obscures Cardigan Bay, off the west coast of Wales, on an April afternoon in 2015, Air warmed over the land was advected seawards, where its moisture promptly condensed over the much colder sea surface.

Latent heat

Latent heat is the thermal energy released or absorbed during a substance's transition from solid to liquid, liquid to vapour or vice-versa. To fuse, or melt, a solid or to boil a liquid, it is necessary to add thermal energy to a system, whereas when a vapour condenses or a liquid freezes, energy is released. The amount of energy involved varies from one substance to another: to melt iron you need a furnace but with an ice cube you only need to leave it at room-temperature for a while. Such variations from one substance to another are expressed as specific latent heats of fusion or vapourisation, measured in amount of energy (KiloJoules) per kilogram. In the case of Earth's atmosphere, the only substance of major importance with regard to latent heat is water, because at the range of temperatures present, it's the only component that is both abundant and constantly transitioning between solid, liquid and vapour phases.

Radiation

Radiation is the transfer of energy as electromagnetic rays, emitted by any heated surface. Electromagnetic radiation runs from long-wave - radio waves, microwaves, infra-red (IR), through the visible-light spectrum, down to short-wave – ultra-violet (UV), x-rays and gamma-rays. Although you cannot see IR radiation, you can feel it warming you when you sit by a fire. Indeed, the visible part of the spectrum used to be called “luminous heat” and the invisible IR radiation “non-luminous heat”, back in the 1800s when such things were slowly being figured-out.

Sunshine is an example of radiation. Unlike conduction and convection, radiation has the distinction of being able to travel from its source straight through the vacuum of space. Thus, Solar radiation travels through that vacuum for some 150 million kilometres, to reach our planet at a near-constant rate. Some Solar radiation, especially short-wave UV light, is absorbed by our atmosphere. Some is reflected straight back to space by cloud-tops. The rest makes it all the way down to the ground, where it is reflected from lighter surfaces or absorbed by darker ones. That's why black tarmac road surfaces can heat up until they melt on a bright summer's day.

Radiation

Figure 3: Heat haze above a warmed road-surface, Lincoln Way in San Francisco, California. May 2007. Image: Wikimedia Commons.

Energy balance

What has all of the above got to do with global warming? Well, through its radiation-flux, the Sun heats the atmosphere, the surfaces of land and oceans. The surfaces heated by solar radiation in turn emit infrared radiation, some of which can escape directly into space, but some of which is absorbed by the greenhouse gases in the atmosphere, mostly carbon dioxide, water vapour, and methane. Greenhouse gases not only slow down the loss of energy from the surface, but also re-radiate that energy, some of which is directed back down towards the surface, increasing the surface temperature and increasing how much energy is radiated from the surface. Overall, this process leads to a state where the surface is warmer than it would be in the absence of an atmosphere with greenhouse gases. On average, the amount of energy radiated back into space matches the amount of energy being received from the Sun, but there's a slight imbalance that we'll come to.

If this system was severely out of balance either way, the planet would have either frozen or overheated millions of years ago. Instead the planet's climate is (or at least was) stable, broadly speaking. Its temperatures generally stay within bounds that allow life to thrive. It's all about energy balance. Figure 4 shows the numbers.

Energy Budget AR6 WGI Figure 7_2

Figure 4: Schematic representation of the global mean energy budget of the Earth (upper panel), and its equivalent without considerations of cloud effects (lower panel). Numbers indicate best estimates for the magnitudes of the globally averaged energy balance components in W m–2 together with their uncertainty ranges in parentheses (5–95% confidence range), representing climate conditions at the beginning of the 21st century. Figure adapted for IPCC AR6 WG1 Chapter 7, from Wild et al. (2015).

While the flow in and out of our atmosphere from or to space is essentially the same, the atmosphere is inhibiting the cooling of the Earth, storing that energy mostly near its surface. If it were simply a case of sunshine straight in, infra-red straight back out, which would occur if the atmosphere was transparent to infra-red (it isn't) – or indeed if there was no atmosphere, Earth would have a similar temperature-range to the essentially airless Moon. On the Lunar equator, daytime heating can raise the temperature to a searing 120OC, but unimpeded radiative cooling means that at night, it gets down to around -130OC. No atmosphere as such, no greenhouse effect.

Clearly, the concentrations of greenhouse gases determine their energy storage capacity and therefore the greenhouse effect's strength. This is particularly the case for those gases that are non-condensing at atmospheric temperatures. Of those non-condensing gases, carbon dioxide is the most important. Because it only exists as vapour, the main way it is removed is as a weak solution of carbonic acid in rainwater – indeed the old name for carbon dioxide was 'carbonic acid gas'. That means once it's up there, it has a long 'atmospheric residency', meaning it takes a long time to be removed. 

Earth’s temperature can be stable over long periods of time, but to make that possible, incoming energy and outgoing energy have to be exactly the same, in a state of balance known as ‘radiative equilibrium’. That equilibrium can be disturbed by changing the forcing caused by any components of the system. Thus, for example, as the concentration of carbon dioxide has fluctuated over geological time, mostly on gradual time-scales but in some cases abruptly, so has the planet's energy storage capacity. Such fluctuations have in turn determined Earth's climate state, Hothouse or Icehouse – the latter defined as having Polar ice-caps present, of whatever size. Currently, Earth’s energy budget imbalance averages out at just under +1 watt per square metre - that’s global warming. 

That's all in accordance with the laws of Thermodynamics. The First Law of Thermodynamics states that the total energy of an isolated system is constant - while energy can be transformed from one component to another it can be neither created nor destroyed. Self-evidently, the "isolated" part of the law must require that the sun and the cosmos be included. They are both components of the system: without the Sun as the prime energy generator, Earth would be frozen and lifeless; with the Sun but without Earth's emitted energy dispersing out into space, the planet would cook, Just thinking about Earth's surface and atmosphere in isolation is to ignore two of this system's most important components.

The Second Law of Thermodynamics does not state that the only flow of energy is from hot to cold - but instead that the net sum of the energy flows will be from hot to cold. To reiterate, the qualifier term, 'net', is the important one here. In the case of the Earth-Sun system, it is again necessary to consider all of the components and their interactions: the sunshine, the warmed surface giving off IR radiation into the cooler atmosphere, the greenhouse gases re-emitting that radiation in all directions and finally the radiation emitted from the top of our atmosphere, to disperse out into the cold depths of space. That energy is not destroyed – it just disperses in all directions into the cold vastness out there. Some of it even heads towards the Sun too - since infra-red radiation has no way of determining that it is heading towards a much hotter body than the Earth,

Earth’s energy budget makes sure that all portions of the system are accounted for and this is routinely done in climate models. No violations exist. Greenhouse gases return some of the energy back towards Earth's surface but the net flow is still out into space. John Tyndall, in a lecture to the Royal Institution in 1859, recognised this. He said:

Tyndall 1859

As long as carbon emissions continue to rise, so will that planetary energy imbalance. Therefore, the only way to take the situation back towards stability is to reduce those emissions.


Update June 2023:

For additional links to relevant blog posts, please look at the "Further Reading" box, below.

Last updated on 29 June 2023 by John Mason. View Archives

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Comments 526 to 550 out of 1588:

  1. Climate_Protector - You might find Science of Doom's writeup on the 1st law of thermodynamics useful. 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.
  2. Climate_Protector, here are two charts of back radiation from particular sites. The first shows 14 days of back radiation measurements at Billings, Omaha in October of 1993: 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.)
  3. Climate_Protector @524, in that case I cannot account for the low diurnal variation in back radiation. It may have something to do with your very high altitude, which means the back radiation comes from a greater thickness of atmosphere, and may have something to do with you location near the rockies (which contain a substantial storage of water). But I would be guessing if I gave any explicit explanation. (Science of Doom would probably be able to tell you why.)
  4. CP @523, very briefly because I am out of time: 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.
  5. Tom Curtis, thank you. This helps ...
  6. Tom Curtis 529 - I guess I did not express myself correctly. I did not mean that "Earth's atmosphere's kinetic energy is large compared to that on the sun". I wanted to say that the kinetic energy of the lower atmosphere must higher than the kinetic energy corresponding to the absorbed solar flux, if the back radiation is indeed larger than absorbed solar flux. Does this makes sense?
  7. To KR 526 - I understand your 'insulation' point. But insulation still works through increasing the kinetic energy of air. Which aspect of the atmosphere acts as an insulator? Are those the greenhouse gases? 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 ...
  8. CP @ 532 >kinetic energy of the lower atmosphere must higher than the kinetic energy corresponding to the absorbed solar flux, if the back radiation is indeed larger than absorbed solar flux. 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.
  9. CP @531: Yes, that makes sense. But it is only another way of saying that the surface of the Earth (and hence the lowest portions of the atmosphere) have a higher temperature than you would expect from insolation alone, and ignoring any greenhouse effect.
  10. I'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?
  11. To Tom Curtis 534 - Yes, I agree with you! The question is (again) how can we explain the extra kinetic energy in the lower atmosphere, i.e. the greenhouse effect? 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?
  12. Another (maybe silly) question. Does anyone know the exact concentration of ALL greenhouse gases in the atmosphere? I remember something like 5% but am not sure ... Thanx
  13. Climate_Protector - The greenhouse gases act as insulators by slowing radiant energy from reaching space. At any level half of the energy going into the atmosphere (including the GHG's) gets radiated up and half down (neglecting convective effects, which are relatively small). And hence not all of the IR leaves - the "insulator" slows it down, requiring a larger energy differential between the bottom and top of the atmosphere to drive 240 W/m^2 out to space. 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.
  14. cp#536 "There must be some sort of high energy storage in the atmosphere that is maintained by the absorption of heat by GH gases." 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.)
  15. KB 538 - Thank you. I got the idea about the insulation effect of the atmosphere a while ago. I was just trying to figure out how exactly that insulation process works, and I think you gave me a clue - the analogy with the space blanket! 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.
    Response: [DB] You couldn't pick a better place on the Interwebs for it.
  16. Oops, I misspelled your name in my last posting. It was directed to KR @ 538
  17. Climate_Protector, "'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.
  18. Climate_Protector - The atmosphere receives energy by convection, but that only moves energy from the warm ground to the cooler air. 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.
  19. KR, "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.
  20. RW1 - "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.
  21. Tom Curtis (RE: 497), "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.
  22. KR (RE: 545), "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.
  23. CP @535, the atmosphere does not distinguish between the sources of energy when it radiates. As it happens, the atmosphere radiates 37.4% (199 w/m^2/532 w/m^2) of its energy to space, and 62.6% to the surface (333 w/m^2/532 w/m^2). That means it radiates 37.4% of the energy it receives by convection to space, and 37.4% of the energy it receives by evapo/transpiration to space, and 37.4% of the energy it absorbs directly from the sun to space, and 37.4% receives as IR radiation from the surface to space. It follows also that 33% (175 w/m^2/ 531 w/m^2) of back radiation comes from energy introduced to the atmosphere other than by absorbing IR radiation from the surface. 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.
  24. KR @ 543: 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?
  25. Tom Curtis - You are, of course, quite correct about analogies getting pushed too far. An analogy is find for explaining some aspects of a complex system in terms the listener may understand, but overextending the analogy invariably hits the limits where the analogy no longer corresponds to the complex system. 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...

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