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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 October 2017:

Here is a walk-through explanation of the Greenhouse Effect for bunnies, by none other than Eli, over at Rabbit Run.

Last updated on 14 February 2023 by John Mason. View Archives

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Further reading

  • Most textbooks on climate or atmospheric physics describe the greenhouse effect, and you can easily find these in a university library. Some examples include:
  • The Greenhouse Effect, part of a module on "Cycles of the Earth and Atmosphere" provided for teachers by the University Corporation for Atmospheric Research (UCAR).
  • What is the greenhouse effect?, part of a FAQ provided by the European Environment Agency.

References

Denial101x video

Comments

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Comments 1 to 25 out of 405:

  1. I'm not a scientist so forgive me if I'm off topic There is a lay explanation of the physics underlying climate alarmism. KE Research, a German public policy consultancy firm, prepared the report based on interviews and editing assistance from noted German theoretical physicists Ralf D. Tscheuschner & Gerhard Gerlich, authors of the peer-reviewed paper Falsification of the Atmospheric CO2 Greenhouse Effects within the Frame of Physics, and numerous other climatologists, physicists, and scientists at http://hockeyschtick.blogspot.com/2010/06/rescue-from-climate-saviors.html Conclusions of the report include: The terms “greenhouse effect” and “greenhouse gas” are misnomers and obstruct understanding of the real world. Earth has a natural “cooling system”. If the planet warms, it will automatically raise its cooling power. An increase of earth temperatures is only achievable if the heating power is stepped up: first to “load” matter with more energy (i.e. to raise temperatures) and then to compensate for the increasing cooling, which results from the increase of IR radiation into space. CO2 and other IR-active gases cannot supply any additional heating power to the earth. Therefore, they cannot be a cause of “global warming”. This fact alone disproves the greenhouse doctrine. The “natural greenhouse effect” (increase of earth temperatures by 33°C) is a myth. IR-active gases do not act “like a blanket” but rather “like a sunshade”. They keep a part of the solar energy away from the earth’s surface. IR-active gases cool the earth: 70% of the entire cooling power originates from these molecules. Without these gases in the air, the surface and the air immediately above the ground would heat up more. The notion that a concentration increase of IR-active gases would impede earth’s cooling is impossible given the true mechanisms explained above. As a consequence the very foundation of the “Green Tower of Climate Dogma” crumbles. Computer models alleging to forecast warming based on “greenhouse effects” are worthless, and any speculation about the “impact of climate change” accordingly dispensable. Since the greenhouse hypothesis has been disproven by the laws of physics, it is only a matter of time until the truth becomes public opinion. Does anyone have any comments on the contents of that report?
  2. val majkus, you're not at all "off topic" -- this thread is basically explaining why Gerlich and Tscheuschner are completely wrong. The report you cite mostly just repeats the same errors and misinformation from G & T's original claims.
  3. Val, the responses to those incorrect claims are in this page above your comment, including the items referenced by the post. If you have questions after reading those, then please do ask here. But please read those first.
  4. "noted German theoretical physicists" - noted for an imaginary second law? Note also posts on Science of doom and Halpern et al above for a formal response.
  5. Fascinating to see Roy Spencer getting annoyed at denialists who won't allow radiation from the atmosphere to warm the surface of the earth. Click...
  6. To whom it may concern, "This is possible only because most of this radiation is absorbed in the atmosphere, and what actually escapes out into space is mostly emitted from colder atmosphere." Are you stating that the primary method of transferring energy from the surface to the Troposphere is via radiation absorption rather than via conduction? When you say 'colder', are you referring to a region or a temperature?
  7. To whom it may concern, "This absorption is due to trace gases which make up only a very small part of the atmosphere." So are you saying that IR from the surface heats the GHG's and then they transfer the energy to the primary gases N2 and O2? If so, how is that transfer made? Conduction from molecules colliding?
    Response: Yep.
  8. To whom it may concern, Forgive me, but this post is ripe with errors, from my understanding of physics. Would you prefer I ask questions regarding the supposed errors, or would you prefer I explain my reasoning outright?
    Response: Whichever is shorter.
  9. Regarding molecular collisions and heat distributions: At surface temps and pressure each air molecule (CO2, O2, N2, argon, etc.) collides with another molecule roughly one billion times per second (thanks, Ned). The relaxation time for an energized CO2 molecule is 100ns or more, depending on the vibrational state. That means that an IR energized CO2 molecule has on average a minimum of 100 collisions with other molecules before it has a chance to emit IR. CO2 _will_ maintain thermal equilibrium with the rest of the air mass, whether the air mass as a whole is cooling or heating by IR. (Or conduction, convection, latent heat changes, etc.)
  10. To whom it may concern, Well put, thank you :)
  11. So is CO2 a good reflector while a poor absorber or is it a good absorber but a poor reflector ? It seemed to me, that the 2nd law put the final nail in the coffin of this (CO2) debate. Yet, it's still alive. How is it possible ? The theory that man made CO2 is the cause of Global Warming has had so meny holes punched through it - I don't understand how it's survived this long. It's been de-bunked by simple sciance on so meny levels, it should have been dead long ago. All the while, people far smarter than myself keep pushing it along. What is it that I'am not understanding? I'm a simple man, thats probably a bit more inquisitive than most, looking for answers. Which is what lead me here.
    Response: Please don't post the same comment repeatedly. I deleted your second one.
  12. Re: KnuckleDragger (11) I'm glad you're inquisitive and I'm glad you're here.
    "It seemed to me, that the 2nd law put the final nail in the coffin of this (CO2) debate. Yet, it's still alive. How is it possible ?"
    This is a common objection from people who do not understand the greenhouse effect (or the 2nd law of thermodynamics, for that matter). A simple understanding of the greenhouse effect:
    Longwave radiation from the earth’s surface is absorbed by many trace gases, including water vapor and CO2. The absorption causes these gases to heat up and energy is radiated back out – both up and down. The upward radiation is effectively “no change”. The downward radiation adds to the energy received from the sun and heats up the surface of the earth more than if this downward radiation did not occur.
    The 2nd "law", simply put:
    "Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature"
    What this means is this:
    No net energy can flow from a cold body to a hot body. In the case of the real “greenhouse” effect and the real 2nd law of thermodynamics, net energy is flowing from the earth to the atmosphere. But this doesn’t mean no energy can flow from the colder atmosphere to the warmer ground."
    It simply means more energy flows from the warmer surface to the colder atmosphere than in the reverse direction. Sources: Here and here.
    "The theory that man made CO2 is the cause of Global Warming has had so meny holes punched through it - I don't understand how it's survived this long...What is it that I'am not understanding?"
    The first part of your statement is wrong on every level, but I can understand the confusion you must feel. If it's so wrong, why does every scientific body in the world support it? Why does Shell Oil, of all things, support it? KnuckleDragger, I'm a simple man too. If someone is telling you that CO2 doesn't warm the Earth or that it's the sun or that it's cooling, or it's a natural cycle, then you have 2 possible answers:
    1. They don't understand quite a bit about science, physics, the greenhouse effect or pay attention to developments in the natural world... OR... 2. They're lying to you...
    The greenhouse effect is quite well understood. Here's a quick backgrounder on the GHE, CO2 and AGW (the important bit is the response to Question 1 & the 8 steps outlined). Basically, it come down to this: No-one has been able to come up with a physics-based alternative to the observed & predicted effects of CO2 and GHG's that explains what we can see and measure that ALSO explains why CO2 derived from fossil fuels DOESN'T act as a GHG. At this point, being an inquisitive man, you probably will have more questions. Feel free to ask; the kind people here will be glad to help you gain an objective understanding. The Yooper
  13. KnuckleDragger, to explicitly address your first question on absorption/reflection, thinking in this terms could be confusing. In everyday life we call reflection the "bouncing back" of light from a solid or liquid surface. In a gas there's no surface and it could be hard to understand how "reflection" may occur. I think it's easier to think in terms radiation absorption/emission. Quoting from the post: "Any substance that absorbs thermal radiation will also emit thermal radiation; [...]. The atmosphere absorbs thermal radiation because of the trace greenhouse gases, and also emits thermal radiation, in all directions." The backward emitted radiation is what you (and others) call reflection. So, the answer to your question is that CO2 at high concentration is a good absorber and reflector.
  14. KnuckleDragger, it's best to not use the term "reflect" at all, even as a convenient shorthand, because as Riccardo wrote it has a particular meaning that is different from absorption-emission. CO2 does effectively zero reflection at the wavelengths we are concerned with here, at CO2's concentrations in our atmosphere.
  15. If I get it right the greenhouse effect works as an insulator and diminishes the heat loss of the earth. So far I am unaware that any unidirectional insulator exists or is even possible. Therefore the greenhouse effect has also to diminish the incoming energy from the sun which heats the earth. As a portion of the incoming energy gets converted into forms of energy that are not radiative (kinetic, chemical, electric) that is not trivial the logical conclusion is that the incoming radiative energy needs to exceed it's outgoing counterpart. As incoming and outgoing radiation is (more or less) equally effected by the insulation it is quite hard to see how the result could be a warming of the earth.
  16. h-j-m, almost all incoming solar irradiance is at short wavelengths where the atmosphere is (mostly) transparent. In contrast, almost all outgoing emitted radiation is at longer wavelengths, portions of which are absorbed by CO2, water vapor, CH4, and other greenhouse gases. This difference between incoming and outgoing radiation is essential to understanding how the greenhouse effect works. If you want to know more, a good place to start is Science of Doom, which has an excellent series of posts explaining the fundamentals of the greenhouse effect.
  17. "So far I am unaware that any unidirectional insulator exists or is even possible." (sound of a vinyl record scratching to a halt) 1. Infrared radiation is emitted by the surface. 2. The radiation--or, rather, radiation at certain frequencies--eventually makes it into the stratosphere and is absorbed and emitted in all directions by molecules of CO2, H20, and CH4. 3. Some of the radiation is eventually emitted into space, because that's one direction. 4. Other directions include all versions of "sideways"--and perhaps right into another molecule of CO2. 5. Down is also another direction. The radiation eventually reaches the top of the atmosphere and is emitted into space (the only way it can leave). The visual, though not physical, analogy is the pinball machine. The atmosphere is a huge pinball machine, and GHG are bumpers (gravity has little effect in this machine). The more bumpers the machine has, the longer, on average, the ball takes to reach the boundary. Another analogy is the dam. The atmosphere is a dam. It doesn't block water, because the water eventually reaches the top and flows over, but when the water reaches the point of flowing over, the same amount of water that flows into the lake behind the dam equals the same amount of water that flows over the top. Yet there is the fact of the lake. And if we build the dam higher, then the lake gets deeper, but eventually the same amount of water will once again begin to flow over the top. We live in the lake. CO2 does not absorb UV radiation, so incoming solar radiation isn't slowed by it.
  18. Re #12 Daniel Bailey, as you say "No net energy can flow from a cold body to a hot body." And "It simply means more energy flows from the warmer surface to the colder atmosphere than in the reverse direction." It is this "net energy flow" from hotter place to colder one that means that it is losing energy in and cooling down, not just 2nd Law but 1st Law also. Just incase you thinking of saying that CO2 slows down the heat transfer rate like an insulator (which it isn't); you can have any insulator you like but it cannot reverse the direction of energy tranferring from a warm surface to colder one.
  19. Re damorbel: You have me a bit mystified; reading over my comment at 12 and then yours at 18 I fail to see what point you're trying to make. If you have one, please rephrase it so that my slow gray matter can understand it. Thanks!
  20. Damorbel #18: "...you can have any insulator you like but it cannot reverse the direction of energy tranferring from a warm surface to colder one." Which would only be relevant if there wasn't this thing called the Sun constantly transferring energy to the planet's surface. Decrease the rate at which that energy leaves the system (by adding greenhouse gases) and you get incoming energy + retained energy... which is obviously greater than incoming energy alone.
  21. Ned and DSL, Before writing my post I checked the irradiative composition of sunlight. I would advise you to do the same before posting a reply.
  22. h-j-m, only a tiny fraction of exoatmospheric solar irradiance is in the thermal infrared range. The vast majority of it is visible and near-IR. I have no idea what you think you're seeing, but if it differs from what I just said, then you're probably misunderstanding something.
  23. Ned, what I am seeing is this: Solar Spectrum Though I don't know what you are looking at.
  24. h-j-m, that graph doesn't even show the longwave infrared region -- it only goes to 2.5 micrometers. The wavelengths corresponding to emitted thermal radiation from the Earth are in the 8-14 micrometer range.
  25. Ah, perhaps you were confused by the label "infrared" on that graph? It's referring to the near-IR and shortwave IR range. Not the thermal part of the spectrum where the Earth and its atmosphere emit radiation.

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