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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 1301 to 1325 out of 1588:

  1. @1297 Yogi, When I talked about the OLR graph in #1290, I said "the radiation coming from the surface", not radiation coming from CO2 near the surface. When you are looking down in the 600-750 band, you are seeing the emission from CO2 at the level near the detector. Using a standard lapse rate of 6.5 degrees, at 3 km, the brightness temperature should be about 19.5 lower than the surface temperature, so the dip will be relatively small. Using the standard atmosphere at 3km looking down, that is about right (it'll be clearer if you turn off water vapour, which tends to distort the top of the spectra). The closer you get to the surface, the less temperature contrast between the surface and the air temperature near the "instrument", and so the dip will be smaller. Now for clouds it appears to radiate as a black body. If you are on the surface looking up, you are effectively seeing the bottom, and if you are using heavy cloud/rain, the cumulus base is at 0.66km. The temperature on the surface is 288K, so lapse rate of 6 degree puts you at 284k at the base. that looks about right from the spectra. If you look closely at the 600-750 band you'll see a slight bump, since there you are actually detaching radiation from CO2 near the ground, which is warmer. When you are looking down at 20km, having cloud is effectively like having a surface that is cooler. If you compare the case without cloud, and altostratus at cloud top at 3km, you'll expect a change in brightness temperature in the 800-1000 band by about 20 degrees, and that is exactly what you'll get.
  2. IanC#1290 "As for your question, in the OLR (20km looking down) graph and at regions without atmospheric absoprtion, you will be seeing radiation coming from the surface, which is ~265K." But the surface is at 287.7K so must be looking at something colder further up. "The closer you get to the surface, the less temperature contrast between the surface and the air temperature near the "instrument", and so the dip will be smaller." #1301 The closer you get to the surface the less absorption there is from the 600-750 band, and the majority of the effect is above 3km.
  3. So what happens in a desert at night, does the CO2 back-radiation turn off when the sun goes down ? and what is warming the ground up quicker through the next morning, the Sun or the back-radiation ?
  4. YOGI (i) Why choose a desert rather than say a rain forest? (ii) the sun of course, the back radiation is only there because GHGs in the atmosphere absorbs IR radiated from the surface that is heated by absorbing largely visible and UV light from the sun. N.B. the surface doesn't stop radiating IR when the sun goes down, as it has considerable thermal inertia.
  5. "(i) Why choose a desert rather than say a rain forest?" its drier so has more temperature range. (ii)..it has considerable thermal inertia. yes, but the desert cools rapidly at night.
  6. YOGI Yes, but *why* does dryness mean there is a bigger temperature range?
  7. Now if I had a greenhouse with glass that was coated with a highly IR reflective layer, it should heat up considerably slower than one with panes made from a material which IR can pass through, according to this flow chart: http://4.bp.blogspot.com/_nOY5jaKJXHM/TDDizFtBw0I/AAAAAAAABMI/Hl_EW6F_-og/s1600/divine.gif
  8. Dikran Marsupial moisture moderates extremes.
  9. YOGI Perhaps it would be better if you finished discussing the previous example you raised before initiating another? So *why* does moisture moderate extremes?
  10. Earth`s average surface temperature is maintained by Ocean heat not 33C of back-radiation.
  11. YOGI Please answer my question, why does moisture moderate extremes (explaining how the dryness of a desert means there is a greater temperature range)?
  12. By slowing the loss of heat at night (unless clouds/air that are warmer move in, then that can radiate and warm a cooler surface below), and in daytime by reducing sunlight, and increasing convection due to the low density of water vapour.
  13. YOGI O.K. so *how* does water vapour slow the loss of heat at night (we can move onto the day later)?
  14. Insulation, it cant warm the ground though unless its warmer than the ground.
  15. YOGI O.K. so how does it insulate the ground? IR is radiated from the surface, then what happens to it?
  16. If I take my cup of tea out in the cold night air it will cool quicker. Same for the ground, but if there is more water vapour above it will absorb IR from the ground, warm up, and reduce the rate at which the ground can then radiate.
  17. YOGI O.K. so the water vapour absorbs the IR radiated from the ground and then warms up. What happens to the heat energy that the water vapour has acquired in this way? n.b. insulators do not work by reducing the rate at which something radiates photons of IR (that depends on its temperature)
  18. YOGI 1302, Our discussion up to post 1290 is related to the figures in the article. If you read the caption that came with the figure, you'll notice that 265K, instead of 287K, will be the more sensible surface temperature. "The closer you get to the surface the less absorption there is from the 600-750 band, and the majority of the effect is above 3km." No. Absorption increases with density. Since the density is the greatest near the surface, the absorption is the greatest there too.
  19. IanC#1318 So from the IR intensity Earth looks a lot colder from 20km up than it really is on the surface. Did you like my greenhouse in #1307 ?
  20. IanC can I suggest we leave YOGIs greenhouse until we get to the conclusion of the discussion of why the desert cools rapidly at night. We won't make any progress in this discussion if it keeps branching out onto new thought experiments before the earlier ones have been resolved.
  21. Dikran Marsupial "O.K. so the water vapour absorbs the IR radiated from the ground and then warms up. What happens to the heat energy that the water vapour has acquired in this way?" It will want to radiate it towards somewhere colder. But while it is warmer, it will mean the ground will lose heat slower, remember my cup of tea ? ~ insulation. "insulators do not work by reducing the rate at which something radiates photons of IR" Paint your roof white and it will do exactly that.
  22. YOGI wrote "It will want to radiate it towards somewhere colder." O.K. so when a molecule of water vapour in the atmosphere emits a photon of IR, how does it know not to emit it in the direction of a warmer body?
  23. #1322 ok there may an exchange but it cant get cool off in the direction of the warmer body, as the warmer body is warming it faster than it can cool off.
  24. YOGI O.K., so you agree that the water vapour re-radiates some IR photons back to the surface. Would you agree that these back-radiated IR photons cause the surface to be warmer than it would be if those molecules of water vapour were not there?
  25. "Insulation, it cant warm the ground though unless its warmer than the ground." The "insulation" effect is radiative. A quick look at the thermal properties of moist air (conductivity, heat capacity) cf dry air, would tell you that conductive insulation isnt at work. Do the math. As to your "greenhouse". Incoming radiation from sun isnt in IR range (that is whole point) so no, it would heat up slower. If you have really read the SoD articles explaining the science here, you are showing little evidence of it.

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