Simple explanation of greenhouse effect - right or wrong?

[Graeme M]

TL;DR Summary

Is my simple explanation of the greenhouse effect correct?

This is a very simple question. I've read a fair bit about the greenhouse effect and how GHGs affect the Earth's surface temperature and I know it can get very complex. But if I boil it down, it seems to be a very simple thing. Am I right to think of it this way?

Radiation from the sun reaches the Earth where some is reflected back by various surfaces, some is absorbed by the atmosphere, and some is absorbed by the Earth's surface. The Earth's surface is warmed by the absorbed incoming radiation. Because the Earth's atmosphere consists of matter, the atmosphere is subsequently warmed by the Earth's warmed surface. The matter that responds in this way is known as greenhouse gasses (GHGs). At the top of the atmosphere, the warmed atmosphere radiates back to space sufficently energetically (along with that radiation that has been reflected by various surfaces) to reach equilibrium with incoming radiation. The temperature of the warmed GHGs corresponds with the density of GHG molecules in the atmosphere; roughly speaking the atmosphere is warmer closer to the Earth's surface and cooler at altitude. Generally speaking (and ignoring various dynamic atmospheric processes) the more molecules of absorbing GHGs in the atmosphere, the warmer the atmosphere at any given altitude.

 

[DrStupid]

Because the Earth's atmosphere consists of matter, the atmosphere is subsequently warmed by the Earth's warmed surface.

If you include indirect energy transfer by latent heat and thermal radiation from the surface, then yes.

At the top of the atmosphere, the warmed atmosphere radiates back to space sufficently energetically (along with that radiation that has been reflected by various surfaces) to reach equilibrium with incoming radiation.

Not only from the top of the atmosphere but also from lower altitudes or even directly from the ground - depending on the wavelength.

The temperature of the warmed GHGs corresponds with the density of GHG molecules in the atmosphere; roughly speaking the atmosphere is warmer closer to the Earth's surface and cooler at altitude.

The temperature gradient in the troposphere does not depend on green hous gas concentrations. Without greenhouse gases the atmosphere would still be warmer closer to the Earth's surface and cooler at altitude. That is not the result but a prerequisite of the greenhouse effect. But as an increasing surface temperature results in increased temperatures of the tropospheric gas column above (because the isentropic temperature gradient remains unchanged) your summary remains correct:

Generally speaking (and ignoring various dynamic atmospheric processes) the more molecules of absorbing GHGs in the atmosphere, the warmer the atmosphere at any given altitude.

However, it does not explain why the surface temperature increases in the first place.

There are different explanations (which are in fact different views on the same process). The argumentation of the IPCC for example is based on the back radiation from the atmosphere. Greenhouse gases absorb thermal radiation from the ground, heat up and than re-emit thermal radiation themselves. This re-emission goes in both direction, up and down. When the downward radiation reaches the suface it gets absorbed in addition to the sunlight and the surface temperature increases until a new equilibrium is reached.

 

[Klystron]

The simplest explanation based on the electromagnetic spectrum notes that some gases are virtually transparent in visible light but inhibit transmission in infrared. Sol radiates primarily in visible light that warms the Earth's surface. The warm surface radiates primarily in infrared blocked by these 'greenhouse' gases.

 

[Graeme M]

The warm surface radiates primarily in infrared blocked by these 'greenhouse' gases.

Yes, though in my explanation I mean to observe that because the air is made of matter, it is warmed by the sun and the earth, just as any matter is warmed by a heat source. The main contribution in this case is the warmed earth; so as I see it, it isn't so much that the IR is blocked but that it warms the air which then eventually radiates back to space. I avoid worrying too much about the wavelengths, relative absorptivity of different greenhouse gasses or the fact that some wavelengths escape directly from surface back to space. My main point is that the Earth warms the atmospheric matter and that more matter in the atmosphere means warmer air, on average.

However, it does not explain why the surface temperature increases in the first place.

There are different explanations (which are in fact different views on the same process). The argumentation of the IPCC for example is based on the back radiation from the atmosphere. Greenhouse gases absorb thermal radiation from the ground, heat up and than re-emit thermal radiation themselves. This re-emission goes in both direction, up and down. When the downward radiation reaches the suface it gets absorbed in addition to the sunlight and the surface temperature increases until a new equilibrium is reached.

Indeed, though back radiation seems to cause a degree of confusion. My thinking on that is that if I explain that the matter in the air is warmed by the Earth's surface, the air gets warmer. We can see this in effect when on consecutive days in say summer, a clear sky air temperature on one day may be much higher than on the next day. The difference is not directly the amount of solar insolation but the temperature of the air which is a result of the Earth heating the air.

That said, I do not know the relative contributions of conduction and IR. A Stevenson Screen detects the air temperature, so on the hotter day I described, it may record 30C and on the cooler day 20C. But does that thermometer respond pimarily to conduction or radiation within the enclosure?

You can see I am not 100% clear on whether back radiation really warms the Earth's surface or whether we are referring to the near ground air temperature when we say that the surface temperature increases. I assume that some degree of conduction and radiation must take place between warmed air and ground surface, but in explaining the greenhouse effect it seems easier to omit this and simply say that the air gets warmer because the Earth's surface warms the matter in it and that the more matter in the air the warmer it gets, on average.

Put another way, is it important to identify "back radiation" as a significant part of the greenhouse effect?

 

[Tom.G]

Put another way, is it important to identify "back radiation" as a significant part of the greenhouse effect?

Yes.

That is why it is called the Greenhouse Effect.

You probably haven't actually been in a Greenhouse, but you may have noticed, especially in the Summer, that the room in your house that has the Sun shining directly in windows is hotter than a room on the other side of the house. Ordinary window glass passes the visible light from the Sun rather well but blocks the infrared heat radiation from the cooler room contents.

That is exactly the same thing that happens with the Earth and the Greenhouse gasses like Carbon Dioxide, various Fluorocarbons, and Methane.

Now this gets a bit involved to explain, I hope I can do it 'good enough' that it is clear.

The wavelength of light or thermal radiation gets shorter as the temperature of the source increases. Think of it as Blue light (hotter, short wavelength) versus Red light (not as hot, long wavelength).

Now, say you have two lights, Red and Blue, and two colored filters also Red and Blue. If you look at the lights thru the filters, you will see they are brighter thru the filter that matches the light color.

The Greenhouse gasses act like the Blue filter, they let thru the light from the hot Sun (shorter wavelength). The Sun then heats the Earth and everything on it to what we consider a more comfortable, but lower, temperature.

The lower temperature Earth then radiates light that is Redder. This Red light is both absorbed and reflected by the Greenhouse gasses, which favor the Blue light.

What helps to keep the Earth at a 'comfortable temperature' is that it radiates thermal energy out to deep space, which is very cold. The Greenhouse gasses block some of this radiation and you end up just like that room in your house with the direct Sun shining in, hot.

Well, that is sort of long-winded but I hope it helps.

Cheers,
Tom

 

[Graeme M]

Tom, I am not sure that is how a greenhouse really works. When I think about it, it would appear that the main reason it works is down to containing the air rather than allowing it to disperse upwards to be replaced by cooler air. That is, it seems to me that a greenhouse works more by blocking convection than it does by blocking outgoing IR. The real atmosphere on the other hand is not constrained and so convection is free to transport heat to higher altitudes (well, at least within the troposphere).

As I understand it, the over time average near-surface air temperature is a very close proxy for actual surface temperature. This suggests back radiation does little to raise the atmosphere's average temperature over time, though of course it must have an effect at short time scales. But when we talk about global warming we are largely talking long term averages. This is why in my simple explanation I didn't mention back radiation and instead focused on the fact that more molecules of IR absorbing gasses raise the long term average temperature of the atmosphere. Simply said, the heated Earth warms the matter in the air and the more matter the warmer the air.

However, all of what I just said may be wrong - that's just how it looks to me after having read a few sources to get my head around the whole thing. Back radiation just doesn't make sense to me as a long term effect.

 

[pbuk]

Tom, I am not sure that is how a greenhouse really works. When I think about it, it would appear that the main reason it works is down to containing the air rather than allowing it to disperse upwards to be replaced by cooler air. That is, it seems to me that a greenhouse works more by blocking convection than it does by blocking outgoing IR.

But you do agree that at least part of the effect is due to blocking outgoing IR? Open the door and roof vent on your greenhouse, it will still be hotter in there than outside.

The real atmosphere on the other hand is not constrained

Yes it is, by gravity.

and so convection is free to transport heat to higher altitudes (well, at least within the troposphere).

Which is still inside the Earth's 'greenhouse' so the heat is not lost.

If you want to stick with a simple glass greenhouse analogy, stop there.

If you want to consider convective and radiative heat transfer within the atmosphere, as well as conductive, radiative and evaporative heat transfer to and from the Earth's surface then things get a lot more complicated and cannot be simplified into 'the Earth's surface gets warmer because of conductive heat transfer from a warmer atmosphere as a result of greenhouse gases' as you are trying to do.

 

[hmmm27]

Two cars parked in a lot on a sunny day with the windows up. One has white upholstery, the other black.

The white upholstery just reflects the sun's rays back out of the windows ; the light isn't absorbed, the interior of the car doesn't get much warmer.

The black one... well, the upholstery absorbs incoming light, and shares that energy with the interior of the upholstery, all of which gets hotter. The surface of the upholstery emits more thermal radiation than usual, but that just gets bounced back from the window and reabsorbed.

a.k.a. the "greenhouse effect".

Scaling up, the atmosphere is acting like window glass. Light comes in and much of it is absorbed by the ground. The ground gets hotter, emitting more thermal radiation than is originally coming in from the Sun, but the GHG's just bounce it back into the ground.

Convection and conduction don't have much to do with it. Mostly it's just non-thermal-range radiation being absorbed by the ground which changes it into internal heat ; escaping heat radiation from the surface doesn't get far and comes back, to be reabsorbed.

(pedantry: the glass (or atmosphere) does block the IR incoming directly from the Sun, but that's not much compared to the stuff bouncing around inside. Also, the phrase "thermal radiation" doesn't mean much in the grand scheme of things, but a near-infrared peaking blackbody more or less corresponds to the temperature range we're used to).

 

[DrStupid]

You can see I am not 100% clear on whether back radiation really warms the Earth's surface or whether we are referring to the near ground air temperature when we say that the surface temperature increases.

The back radiation has about the power of the influx from the Sun. That means that the energy absorbed by the surface is doubled compared to an Earth without greenhouse effect. That results in an increased steady state temperature. That's the natural greenhouse effect. When we say that the surface temperature increases due to the green hosue effect we are referring to an even higher steady state temperature resulting from even more back radiation emitted from additional green house gases.

I assume that some degree of conduction and radiation must take place between warmed air and ground surface, but in explaining the greenhouse effect it seems easier to omit this and simply say that the air gets warmer because the Earth's surface warms the matter in it and that the more matter in the air the warmer it gets, on average.

But that does not explain how additional green house gases result in an increased surface temperature. Let's look at the ground and neglect heat conduction and convection as well as daily and seasonal variations of the solar irradiance. In order to make it easy let's start with an atmosphere without green house gases. The ground absorbs radiation from the sun and emits thermal radiation with the same power when it reaches the steady state temperature.

Not we add green house gases. The solar irradiance is almost the same (due to absorbtion by the greenhouses gases it is actually a little bit reduced) and the steady state temperature would not increase if there wouldn't be an additional source of energy. That's where the back radiation comes into play. The greenhouses gases emit thermal radiation. A part of this radiation is absorbed by the ground and increases its temperature. Now the ground warms the air above, the green hous gases emit even more thermal radiation which increases the ground even further and so on, until a new steady state is reached.

The new steady state is much more complex compared to an Earth without green house effect - not only because the ground has an additional source of energy but also because the green house gases emit in different altitudes with different power (due to different temperatures), both up and down and also absorb thermal radiation coming from green house gases in other altitudes.

Put another way, is it important to identify "back radiation" as a significant part of the greenhouse effect?

Yes it is, because it is the only link from increasing green house gas concentrations to increasing surface temperatures.

 

[DrStupid]

The surface of the upholstery emits more thermal radiation than usual, but that just gets bounced back from the window and reabsorbed.

It would be bounced back if the window would be a mirror for thermal radiation. But it is not that simple. The window absorbs the energy and re-emits it both, inside and outside. That's an important difference because the effect is reduced with increasing outer temperature. If the outer temperature would reach the same temperature as the upholstery the window wouldn't block the radation anymore. The temperature gradient is a major factor in this process and within the troposphere it is not dominated by radiation but by convection.

 

[Genava]

Summary:: Is my simple explanation of the greenhouse effect correct?

This is a very simple question. I've read a fair bit about the greenhouse effect and how GHGs affect the Earth's surface temperature and I know it can get very complex. But if I boil it down, it seems to be a very simple thing. Am I right to think of it this way?

Historically, the "Greenhouse Effect" is a simple thing emerging from a few observations and to explain divergences from an intuitive model that the Earth's temperature is simply dependent on the Sun.

From Historical Overview of Climate Change Science, from IPCC 2007 report, WG1

The realisation that Earth’s climate might be sensitive to the atmospheric concentrations of gases that create a greenhouse effect is more than a century old. Fleming (1998) and Weart (2003) provided an overview of the emerging science. In terms of the energy balance of the climate system, Edme Mariotte noted in 1681 that although the Sun’s light and heat easily pass through glass and other transparent materials, heat from other sources (chaleur de feu) does not. The ability to generate an artificial warming of the Earth’s surface was demonstrated in simple greenhouse experiments such as Horace Benedict de Saussure’s experiments in the 1760s using a ‘heliothermometer’ (panes of glass covering a thermometer in a darkened box) to provide an early analogy to the greenhouse effect. It was a conceptual leap to recognise that the air itself could also trap thermal radiation. In 1824, Joseph Fourier, citing Saussure, argued ‘the temperature [of the Earth] can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, than in repassing into the air when converted into non-luminous heat’. In 1836, Pouillet followed up on Fourier’s ideas and argued ‘the atmospheric stratum…exercises a greater absorption upon the terrestrial than on the solar rays’. There was still no understanding of exactly what substance in the atmosphere was responsible for this absorption.

I think it is important to remember that it started from this, a simple understanding that the atmosphere itself plays a role. I presume you already know the major breakthrough from Tyndall but I want to also point out another important contribution that shaped the modern approach:

From Radiative Forcing of Climate: The Historical Evolution of the Radiative Forcing Concept, the Forcing Agents and their Quantification, and Applications

Experimental developments, along with advances in conceptual thinking on the heat balance of the planet, began to provide the platform for quantifying the radiation budget, for example, solar irradiance determination by Abbott and Fowle (1908), and an early estimate of Earth’s global-average energy budget by Dines (1917). Dines’s effort was a remarkable intellectual attempt given there was very little then by way of observations of the individual components.

The work of William Henry Dines about "The heat balance of the atmosphere" is fascinating because it makes the link between the role of the atmosphere and the radiative forcing, the two concepts that are grounding the idea of the greenhouse effect.

The term "greenhouse" itself seems to emerge a bit earlier with John Henry Poynting comment about Lowell's contribution to planetary temperature estimates:

Prof. Lowell’s paper in the July number of the Philosophical Magazine marks an important advance in the evaluation of planetary temperatures, inasmuch as he takes into account the effect of planetary atmospheres in a much more detailed way than any previous writer. But he pays hardly any attention to the “blanketing effect,” or, as I prefer to call it, the “greenhouse effect” of the atmosphere.”

Which triggered this article from Frank Washington Very with the explicit title "The Greenhouse theory and planetary temperatures".

The simple definition is still actively used, even recently,
From Attribution of the present‐day total greenhouse effect:

The global mean greenhouse effect can be defined as the difference between the planetary blackbody emitting temperature (in balance with the absorbed solar irradiance) and the global mean surface temperature. The actual mean surface temperature is larger (by around 33°C, assuming a constant planetary albedo) due to the absorption and emission of long‐wave (LW) radiation in the atmosphere by a number of different “greenhouse” substances.

In this regard, the theory and concept around the "Greenhouse Effect" are very similar to the theory of evolution in biology: it started simple, and it provided straightforward answers. Explaining the causal mechanisms is another task, much more difficult.

The temperature gradient in the troposphere does not depend on green hous gas concentrations.

The existence of a temperature gradient by itself, no. But the contribution from greenhouse gases is probably contributing a lot to the convection and to the lapse rate in the atmosphere. Although it seems it was the major point highlighted by the earliest radiative-convective models.

 

[DrStupid]

But the contribution from greenhouse gases is probably contributing a lot to the convection and to the lapse rate in the atmosphere.

No, it doesn't. The adiabatic laps rate in the troposphere does not depend on the IR spectra but on the thermodynamic properties of the atmosphere only.

 

[Genava]

No, it doesn't. The adiabatic laps rate in the troposphere does not depend on the IR spectra but on the thermodynamic properties of the atmosphere only.

Temperature is a thermodynamic property of the atmosphere. So indirectly it is related to the greenhouse gases. The simple fact that the convection is assumed as adiabatic is due to the role of the greenhouse effect, making the lower troposphere mostly opaque to IR and reducing the loss of thermal energy through thermal radiation.

 

[DrStupid]

Temperature is a thermodynamic property of the atmosphere. So indirectly it is related to the greenhouse gases.

That applies in some degree to the moist adiabatic lapse rate but the dry adiabatic lapse rate is independent from temperature. It is not affected by IR emission or absorption - neither directly nor indirectly.

The simple fact that the convection is assumed as adiabatic is due to the role of the greenhouse effect, making the lower troposphere mostly opaque to IR and reducing the loss of thermal energy through thermal radiation.

The simple fact that the convection is assumed as adiabatic is due to negligible heat conduction at atmospheric scale. Synthetic air consisting of pure N₂ and O₂ has the same dry adiabatic lapse rate as air with 400 ppm CO₂ even though it is transparent for IR.

Greenhouse gases would change the lapse rate if the pure radiation dominated temperature gradient would remain below the adiabatic lapse rate. But that is not the case. The greenhouse gases "try" to increse the temperature gradient above the adiabatic lapse rate. But as soon as that happens the atmosphere gets instable and convention starts which reduces the gradient. More greenhouse gases speed up that process but do not change the dry adiabatic lapse rate.

The only greenhouse gas that has a significant effect on the lapse rate is H₂O - but not because it is a greenhouse gas. A substance with the same molar mass, boiling point, heat capacity and heat of vaporisation but without IR absorption would have the same effect.

 

[Genava]

The greenhouse gases "try" to increse the temperature gradient above the adiabatic lapse rate. But as soon as that happens the atmosphere gets instable and convention starts which reduces the gradient. More greenhouse gases speed up that process but do not change the dry adiabatic lapse rate.

The only greenhouse gas that has a significant effect on the lapse rate is H2O - but not because it is a greenhouse gas. A substance with the same molar mass, boiling point, heat capacity and heat of vaporisation but without IR absorption would have the same effect.

I think you are right. It does have an effect on the convection but not really on the temperature gradient (or the lapse rate). The expected changes I have seen in the literature are due to the role of humidity and moist air motions. Not the greenhouse effect directly. Example:

Lapse-rate changes have been shown to contribute to the Mediterranean amplification (Kröner et al. 2017). The projected future lapse rates are characterized by a stronger upper-level warming in comparison to the ground (Byrne and O’Gorman 2013a). This is caused by moist adiabatic vertical mixing that maintains the thermal stratification close to a moist adiabat in summer over continental regions (Schneider 2007). As the atmosphere warms, it can carry more moisture because of the Clausius–Clapeyron relation (Allen and Ingram 2002; Held and Soden 2006). During moist adiabatic vertical motions, more latent heat can be released into the upper troposphere, which leads to the enhanced warming (Schneider 2007). This process increases the atmosphere’s dry static stability, and its influence on the equilibrium climate sensitivity is referred to as lapse-rate feedback (Bony et al. 2006). Regionally different lapse-rate changes can influence the spatial pattern of climate change. Most notably, differing lapse-rate changes have been suggested as a driver of the land–sea contrast of warming (Byrne and O’Gorman 2013a,b; Sherwood and Fu 2014). The moisture contrast between land and sea leads to different lapse-rate changes in a way that, if upper-level warming is uniform, the temperature over land warms more than over ocean (Joshi et al. 2008; Byrne and O’Gorman 2018). However, it is less clear how lapse-rate changes influence the Mediterranean amplification.

Even by looking at the different hypothesis modeled in Manabe, S., & Strickler, R. F. (1964), the lapse rate doesn't vary that much in the atmosphere, excepted at the ground (with differences of several dozen of degrees C from a purely radiative model). But those are extreme postulates they used to test their model and estimate the contribution of each part.

 

[willem2]

Greenhouse gases would change the lapse rate if the pure radiation dominated temperature gradient would remain below the adiabatic lapse rate. But that is not the case. The greenhouse gases "try" to increse the temperature gradient above the adiabatic lapse rate. But as soon as that happens the atmosphere gets instable and convention starts which reduces the gradient. More greenhouse gases speed up that process but do not change the dry adiabatic lapse rate.

The warming of greenhouse gases, is mostly at high altitude and they "try" to reduce the temperature gradient below the adiabatic lapse rate. This doesn't increase convection but reduces it. Since convection cools the surface, reducing convection will make the surface warmer. This is one way the heat captured by greenhouse gases ends up at the surface. (and an increase in backradiation is the other way)

 

[DrStupid]

The warming of greenhouse gases, is mostly at high altitude and they "try" to reduce the temperature gradient below the adiabatic lapse rate.

Interesting. Do you have sources explaining that in detail (because it sounds a bit counterintuitive)? Wouldn't a reduction of the temperature gradient at high altitudes below the adiabatic lapse rate imply a reduction of the altitude of the tropopause? But the tropopause is in fact elevated by global warming (e.g. Santer et.al. 2002). That means that the temperature gradient has been increased to the adiabatic lapse rate where it was below before.

Or is that just a statistical effect of increased probabilities of temperature inversions, resulting in a reduced average temperature gradient over a long period of time?

 

[willem2]

It's just because most of the instantaneous warming from increased CO2 is at an altitude and not at the surface. look at the top left graph at page 6837, the radiation forcing from doubled co2.
https://pubs.giss.nasa.gov/docs/1997/1997_Hansen_ha01900k.pdf
extra c02 doesn't have much effect at the surface.
Note that the flux near the ground is 0. The temperature is predicted to rise everywhere, but more in the troposphere than at the surface.

But the tropopause is in fact elevated by global warming (e.g. Santer et.al. 2002). That means that the temperature gradient has been increased to the adiabatic lapse rate where it was below before.

You get an elevated tropopause, if the temperature increases. I don't think that means that the gradient has to increase

Heat loss by convection. (including latent heat from evaporation of water and rain) can increase if the temperature increases, and more water evaporates. The overall effect on surface temperatures is complicated however.

 

[DrStupid]

It's just because most of the instantaneous warming from increased CO2 is at an altitude and not at the surface.

That's not sufficient. Lok at Figure 2 (d) on page 6835 in https://pubs.giss.nasa.gov/docs/1997/1997_Hansen_ha01900k.pdf. The radiative forcing increases the temperature somewhere in the middle of the troposphere. In the result the temperature gradient below this altitude is indeed decreased. But above it gets increased. If it was at the adiabatic lapse rate before, it should now exceed that limit and make the atmospheric layering instable. The resulting convection would decrease the temperature gradient and that would finally increase the gradient at altitudes below.

As the paper includes convective feedback this doesn't seem to happen. But why? Is the original temperature gradient in the upper troposphere below the adiabatic lapse rate? That would imply it is not convection dominated. Is that correct? I was under the impression that convections stop at the tropopause.

 

[meekerdb]

Such complicated answers to a simple question! The atmosphere is transparent in the visible part spectrum (that's why it's called "visible"). So the Sun's energy reaches the surface. At equilibrium the same energy must be radiated to space. So it has to be transmitted from the surface to space. Given the Earth's distance from the Sun you can easily calculate that the Earth must radiate 342 w/m^2. So as a black body, it must radiate at 258degK to be in equilibrium. But the atmosphere, containing GHGs H2O and CO2 impedes the radiation from the surface. It acts like an insulator. So in order to get 258degK out the surface must be hotter in order to provide the gradient to push 342 w/m^2 up thru the atmosphere. How much hotter depends on the amount of GHG. Right now it's 288degK needed at the surface. Adding more GHG provides more insulation between the surface and space. That's how GHGs warm the surface.

 

[Buzz Bloom]

The following is intended to be a bit over-simplified explanation of how the GHG effect works.

There is one important difference between two concepts of the GHGs affect.

(1) The GHG is warmed, and its warmth re-radiates photons, some towards the Earth to add to its warmth, and some towards the sky and space.

(2) A GHG molecule captures a photon, and after a short while it re-radiates a photon of similar energy. About half of the re-radiated photons will go towards the Earth, and half towards space. This behavior is similar to a GHG acting like a partially transparent mirror: some reflected photons go back to add to the Earth's warmth, and the other photons that go through the mirror continue to the sky and space. It turns out that (2) is much more influential than (1). This process makes no change to the temperature of the GHG.

In order for (1) to occur, the molecule that has acquired a photon from Earth interacts with another GHG molecule before it re-radiates the similar photon, and the energy of the added photon is converted to dynamic energy increasing the average temperature of the GHG. This does not happen anywhere nearly as often as the re-radiating of the photon.

Here is one more detail. A re-radiated photon may be absorbed by a GHG molecule before the molecule hits the Earth or escapes into space.

 

[DrStupid]

But the atmosphere, containing GHGs H2O and CO2 impedes the radiation from the surface. It acts like an insulator.

The answers above are so complicate because they already are a step ahead. They try to explain how the greenhouse gases make the atmosphere act like an insulator. Greenhouse gases not only absorb but also emit IR radiation. Why does the absorption dominate the emission to space? The answer lies in the temperature gradient of the atmosphere. Greenhouse gases cannot increase solar irradiance or reduce black body radiation emitted from the surface. How do they change the radiation balance of the surface in a way that increases the surface temperature? The answer is the back radiation.

 

[Ian J Miller]

I am going to restrict my answer to the effects of radiation. Atmospheric heating involves a lot of other things, but they can be considered a baseline because they are always there. A grey body at a given temperature emits radiation approximately following the Planck black body relationship, and much of the radiation emitted from the Earth's surface is in the infrared part of the spectrum. This would be radiated directly to space unless it is absorbed by the atmosphere, in which case the ground cools, and further heat is conducted from below. As it cools, the wavelength of emitted radiation gradually lengthens. To be absorbed by the atmosphere, the molecule has to have a vibration that involves a change in electric moment, so by symmetry, O2 and N2 are transparent to infrared. CO2 has an asymmetric stretching vibration and a bending vibration, so it can absorb infrared in two reasonably narrow bands. At that point it has an excited vibration but the molecule is not "hotter" - heat is random kinetic energy. If the excited state collides with another molecule it may transfer some energy, in which case heat is generated, but the lifetime of this excited state is sufficiently short that the usual effect is it re-radiates the same energy. However, while the surface flux was directed towards space, that re-emission is in a random direction, so a bit under a half goes back to earth, where it is absorbed and thus removes the need for heat to be transferred from below. Of the re-radiated IR that goes up, when it hits another CO2 molecule it gets absorbed again, and that is also emitted in random directions. The net effect is that radiation at those two frequencies takes longer to get to space. The heating of the atmosphere through collisions obviously depends on pressure, so that will be stronger on Venus, and such energy converted to heat cannot radiate all the energy directly. The effect is a bit like putting a blanket on the bed - it slows heat escape.

Any gas that has a suitable vibration in the infrared will absorb and do the same thing. Water is probably the strongest greenhouse gas, but it condenses so there is negative feedback with its effects. Methane has several vibrations, as does N2O, and many of the gases like SF6 that are now being made commercially for various applications. Since each of these absorb at different frequencies, they are more dangerous, and worse, they are more effective. Methane is supposed to be about 34 times stronger in its effect than CO2, and SF6, and other fluorinated species are extremely strong greenhouse gases. The huge greenhouse effect on Venus is probably partly due to sulphuric acid and sulphur species as these have extremely broad absorptions.

The cooling of the upper atmosphere may be due to CO2 being a better radiator. The thermosphere of Earth is about 1400 degrees C (although the concentration of gas is rather low) while the theermosphere of Venus, thanks to CO2 is, I gather, about 300 degrees C

 

[Graeme M]

OK... that all got very complicated very quickly. Thanks for the many clarifications, however I was trying to keep it to a very simple explanation. My main divergence from the general explanation is not to mention back radiation, but commenters seem certain it must be included. I'm still not clear why. The atmosphere is matter, the warmed ground warms the matter in the atmosphere. The more matter in the atmosphere, the warmer it gets. That seems to be right, surely? For example, taking the existing atmosphere's relative composition, if the near surface density was quite low, then I would think it's temperature would be less than now. Am I right to think this?

In regard to back radiation, are people really saying that this actually makes the ground hotter than it would otherwise be? In other words, if GHGs only re-radiated at angles that did not strike the Earth's surface (the actual ground or water) are you saying that those ground surfaces would be cooler than they are now (averaged over say decadal time scales)?

 

[pbuk]

OK... that all got very complicated very quickly. Thanks for the many clarifications, however I was trying to keep it to a very simple explanation. My main divergence from the general explanation is not to mention back radiation, but commenters seem certain it must be included. I'm still not clear why.

Because if some of the heat energy that is absorbed by GHG was not radiated back to the surface of the Earth then it would be radiated into space and would not be a problem.

The atmosphere is matter, the warmed ground warms the matter in the atmosphere.

By conduction? This effect is negligible. By radiation? Only to the extent that the atmosphere absorbs the radiation. An atmosphere that is high in GHG absorbs more radiation than one that is not.

The more matter in the atmosphere, the warmer it gets.

What do you mean 'warmer': a greater temperature or greater heat energy? In either case it is irrelevent: the total heat capacity of the atmosphere is negligible compared to, say, the oceans.

That seems to be right, surely? For example, taking the existing atmosphere's relative composition, if the near surface density was quite low, then I would think it's temperature would be less than now. Am I right to think this?

No you are wrong.

In regard to back radiation, are people really saying that this actually makes the ground hotter than it would otherwise be?

I don't like the way you keep talking about 'warmer' and 'hotter'. Temperatures are not of great importance to the mechanism here, what matters is heat energy: do you understand the difference?

In other words, if GHGs only re-radiated at angles that did not strike the Earth's surface (the actual ground or water) are you saying that those ground surfaces would be cooler than they are now (averaged over say decadal time scales)?

A molecule cannot choose which direction it radiates in, it radiates equally in all directions. Hypothesising about what what happen if the laws of Physics were different is pointless.

 

[Graeme M]

No you are wrong.

I admit to not really following your comments, in particular this one.

My understanding is that the atmosphere is matter. Some of that matter is warmed by the surface of the earth. Of concern in that regard is CO2, a well mixed gas, which is being emitted by human activity. If we measure the temperature of that matter at a particular place, it is argued that over time - all other things being equal - the average temperature will increase. This is because the number of molecules of CO2 in the atmosphere has increased. It follows that if the number of molecules of CO2 were to decrease, the temperature would decrease. This is the basis for arguing in favour of lowering emissions.

If the concentration of GHG molecules is less, the temperature of the atmosphere is less. So surely it follows that were the Earth's atmosphere less dense but retained the same relative proportions of gasses as today, the temperature at the same place I mention above would be lower.

Why am I wrong to infer this?

 

[hmmm27]

So surely it follows that were the Earth's atmosphere less dense but retained the same relative proportions of gasses as today, the temperature at the same place I mention above would be lower.

You mean if the Earth's gravity were to change ? That would change the density of the atmosphere. The dimension you're looking for is $mass$, not $\frac{mass}{volume}$.

 

[Buzz Bloom]

@Graeme M "Some of that matter is warmed by the surface of the earth."

You seem to be wanting to understand the GHG process and how it works, but are fixated about the warming of the atmosphere. It is true that some such warming takes place, but a very tiny amount compared to the effect of the "reflecting" of photons described in post #21.

 

[DrStupid]

If the concentration of GHG molecules is less, the temperature of the atmosphere is less. So surely it follows that were the Earth's atmosphere less dense but retained the same relative proportions of gasses as today, the temperature at the same place I mention above would be lower.

Less density but same concentrations means less GHG and it sounds reasonable that less GHG would result in less GHE. Thus, I would agree so far.

 

[Genava]

In regard to back radiation, are people really saying that this actually makes the ground hotter than it would otherwise be?

I don't think the word ground should be taken too literally because the way we define the global temperature is with the "surface temperature" in meteorology, which is actually the very first layer of air close the ground.

Finally the backradiation is not a process from the air to the surface but also a process occurring in the air, in the atmosphere itself. It explains why each layer of air cannot dissipate in the direction of space all the energy in excess. If I can quote Raymond T. Pierrehumbert:

At planetary energy densities, photons do not significantly interact with each other; their distribution evolves only through interaction with matter. The momentum of atmospheric photons is too small to allow any significant portion of their energy to go directly into translational kinetic energy of the molecules that absorb them. Instead, it goes into changing the internal quantum states of the molecules. A photon with frequency ν has energy hν, so for a photon to be absorbed or emitted, the molecule involved must have a transition between energy levels differing by that amount. Coupled vibrational and rotational states are the key players in IR absorption. An IR photon absorbed by a molecule knocks the molecule into a higher-energy quantum state. Those states have very long lifetimes, characterized by the spectroscopically measurable Einstein A coefficient. For example, for the CO2 transitions that are most significant in the thermal IR, the lifetimes tend to range from a few milli-seconds to a few tenths of a second. In contrast, the typical time between collisions for, say, a nitrogen-dominated atmosphere at a pressure of 10⁴ Pa and temperature of 250 K is well under 10⁻⁷ s. Therefore, the energy of the photon will almost always be assimilated by collisions into the general energy pool of the matter and establish a new Maxwell–Boltzmann distribution at a slightly higher temperature. That is how radiation heats matter in the LTE limit [local thermodynamic equilibrium].

According to the equipartition principle, molecular collisions maintain an equilibrium distribution of molecules in higher vibrational and rotational states. Many molecules occupy those higher-energy states, so even though the lifetime of the excited states is long, over a moderately small stretch of time a large number of molecules will decay by emitting photons. If that radiation escapes without being reabsorbed, the higher-energy states are depopulated and the system is thrown out of thermodynamic equilibrium. Molecular collisions repopulate the states and establish a new thermodynamic equilibrium at a slightly cooler temperature. That is how thermal emission of radiation cools matter in the LTE limit. Now consider a column of atmosphere sliced into thin horizontal slabs, each of which has matter in LTE. Thermal IR does not significantly scatter off atmospheric molecules or the strongly absorbing materials such as those that make up Earth’s water and ice clouds. In the absence of scattering, each direction is decoupled from the others, and the linearity of the electromagnetic interactions means that each frequency can also be considered in isolation. If a radiation flux distribution I_ν in a given propagation direction θ impinges on a slab from below, a fraction a_ν will be absorbed, with a_ν << 1 by assumption. The slab may be too thin to emit like a black-body. Without loss of generality, though, one can write the emission in the form e_νB(ν,T); here e_ν << 1 is the emissivity of the slab (see figure 1). Both a_ν and e_ν are proportional to the number of absorber–emitter molecules in the slab. The most fundamental relation underpinning radiative transfer in the LTE limit is Kirchhoff’s law, which states that a_ν = e_ν. Gustav Kirchhoff first formulated the law as an empirical description of his pioneering experiments on the interaction of radiation with matter, which led directly to the concept of blackbody radiation. It can be derived as a consequence of the second law of thermodynamics by requiring, as Kirchhoff did, that radiative transfer act to relax matter in a closed system toward an isothermal state. If Kirchhoff’s law were violated, isolated isothermal matter could spontaneously generate temperature inhomogeneities through interaction with the internal radiation field.

Given Kirchhoff’s law, the change in the flux distribution across a slab is ΔI_ν = e_ν [−I_ν + B(ν,T)], assuming e_ν ≪ 1. The radiation decays exponentially with rate e_ν, but it is resupplied by a source e_νB. The stable equilibrium solution to the flux-change iteration is I_ν = B(ν,T), which implies that within a sufficiently extensive isothermal region the solution is the Planck function appropriate to a blackbody. The recovery of blackbody radiation in that limit is one of the chief implications of Kirchhoff’s law, and it applies separately for each frequency. In the limit of infinitesimal slabs, the iteration reduces to a linear first-order ordinary differential equation for I_ν. Or, as illustrated in figure 1, one can sum the contributions from each layer, suitably attenuated by absorption in the intervening layers. The resulting radiative transfer equations entered 20th-century science through the work of Karl Schwarzschild (of black hole fame) and Edward Milne, who were interested in astrophysical applications; Siméon Poisson published a nearly identical formulation of radiative transfer[3] in 1835, but his equations languished for nearly 100 years without application.

It comes from his conference paper titled "Infrared Radiation and Planetary Temperature"

 

[Graeme M]

Genava, yes I agree that the notion of "surface" is not clear and may not explicitly mean "the ground". But I suspect most people reading about the GHE would indeed draw that inference - that the warmed atmosphere directly warms the ground leading to yet more thermal radiation from the ground. The fact that thermal radiation extends throughout the column in all directions simply means that all of the relevant matter in the column is warmed until sufficient radiation escapes into space to balance incoming radiative energy.

What I am getting at is more or less what is described in your quote - that thermal IR from the Earth's surface warms the atmosphere because the atmosphere consists of molecules of matter. The molecules are raised to a higher energy state and transfer energy within the local pool via collisions until the local pool equilibrates at a slightly higher temperature. Considered as layers, each layer is warmed and emits to the layer above which warms and so on until energy escapes into space and thermal equilibrium is reached.

Put simply, the warmed ground surface - soil, trees, buildings, water - warms the air because the matter in the air is warmed. The more matter that can be warmed, the warmer the matter will become up to the limit at which escaping radiation cools things enough and we reach thermal equilibrium. So my simple explanation just is that the air is matter and like all matter it can be warmed. And it is warmed by the heat from the Earth's surface.

Back radiation to the Earth's actual surface may warm that surface to some extent, but my guess is that this is a minor contributor and can be ignored when explaining the effect of GHE gasses to everyday people like me.

Over short time scales it may be noticeable but I am not sure how true that is - after all, a sand surface in the direct tropical sun will be very hot in the absence of an atmosphere. With our atmosphere it is significantly cooler. However at night I suspect it is very much warmer than it would be without the atmosphere. The actual effect when considered from a lay perspective seems to be that the atmosphere slows heat loss when solar insolation reduces. Back radiation to the Earth's actual surface may be a feature - however it works - regardless of the relative concentrations of CO2 however its relative effect is only changed by there being more or less molecules of CO2.

All of that said, the thing we are concerned about with global warming is air temperature. And regardless of back radiation, what causes that is that the air is warmed by the ground. The more matter - molecules of GHGs - the warmer it gets. That's about all that needs to be said, it seems to me.

 

[hmmm27]

https://geography.name/how-much-insolation-reaches-the-surface/

 

[DrStupid]

But I suspect most people reading about the GHE would indeed draw that inference - that the warmed atmosphere directly warms the ground leading to yet more thermal radiation from the ground.

That’s how it actually works.

Put simply, the warmed ground surface - soil, trees, buildings, water - warms the air because the matter in the air is warmed.

In order to do that is must be hotter than the air because heat always flows from hot to cold. Without back radiation there would be no warming of the ground and without warming of the ground there would be no significant warming of the atmosphere.

Back radiation to the Earth's actual surface may warm that surface to some extent, but my guess is that this is a minor contributor and can be ignored when explaining the effect of GHE gasses to everyday people like me.

Your guess is wrong. With 2/3 of the total influx at the surface the back radiation is the major contributor:

 

[Graeme M]

I think I shall leave it here. My main aim was to derive a simple explanation for others I talk to who are sceptical about CO2 warming the atmosphere. It's still not clear to me why back radiation is seen as a dominant factor. I guess I am misunderstanding something about the process, however I think it is still correct to maintain that atmospheric warmth is due to matter being warmed by the surface. The more matter the warmer the atmosphere. I remain confused about some of the comments above.

For example, this seems an odd comment:

In order to do that is must be hotter than the air because heat always flows from hot to cold. Without back radiation there would be no warming of the ground and without warming of the ground there would be no significant warming of the atmosphere.

DrStupid, I think you are talking in the context of how increasing GHG concentrations affect existing atmospheric temperatures because it doesn't make sense to me to say that the only way the surface is heated is via back radiation. Indeed as I look at the graphic you've supplied, as far as I can tell the net effect of the exchange between atmosphere and surface is to cool the surface. The rate at which it cools would be a product of the concentration of GHGs. The fewer GHG molecules, the faster the surface cools, the more GHG molecules the slower it cools. Which is a roundabout way of saying that the more GHGs, the warmer the atmosphere.

Interestingly, the site linked to by Hmmm27 above provides an explanation more along the lines of how I've always thought about it. In that explanation, the author writes that about 31% of all incoming insolation is reflected back to space, 20% is absorbed by the atmosphere and 49% is absorbed by the surface. 12% of total incoming insolation is radiated directly to space from the surface via the "atmospheric window". 30% of total insolation is then transported from the surface via transfer of sensible and latent heat. The back-and-forth exchange of radiant energy between the atmosphere and surface (back radiation) results in 7% of total incoming insolation being transferred into the atmosphere.

Anyways, all of this is why I am not a scientist...

https://geography.name/how-does-earth-maintain-an-energy-balance/

 

[DrStupid]

DrStupid, I think you are talking in the context of how increasing GHG concentrations affect existing atmospheric temperatures because it doesn't make sense to say that the only way the surface is heated is via back radiation.

The thread is about the greenhouse effect and that's what I'm talking about - the increase of the temperature by greenhouse gases compared to temperature without greenhouse gases.

In that explanation, the author writes that about 31% of all incoming insolation is reflected back to space, 20% is absorbed by the atmosphere and 49% is absorbed by the surface.

These 49% are just 1/3 of the total influx for the surface. The other 2/3 come from back radiation.

12% of total incoming insolation is radiated directly to space from the surface via the "atmospheric window". 30% of total insolation is then transported from the surface via transfer of sensible and latent heat. The back-and-forth exchange of radiant energy between the atmosphere and surface (back radiation) results in 7% of total incoming insolation being transferred into the atmosphere.

I think it is 7% of the total emission from the ground (and not of the total incoming insolation) that is being transferred into the atmosphere by back and forth radiation. Anyway, this is about the energy balace of the atmosphere. I am talking about the energy balance of the surface. The steady state temperature of the surface is important because it is the upper limit for the temperature of the troposphere. You can't explain the warming of the atmosphere without an explanation for the warming of the surface.