Simple explanation of greenhouse effect - right or wrong?

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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.
 

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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.
 
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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.
 
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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?
 
  • #5
Tom.G
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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
 
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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.
 
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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.
 
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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).
 
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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. Lets 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.
 
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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.
 
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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.
 
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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.
 
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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.
 
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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.
 
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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 modelled 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.
 
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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)
 
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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?
 
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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.
 
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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.
 
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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.
 
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  • #21
Buzz Bloom
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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.
 
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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.
 
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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
 
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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)?
 
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pbuk
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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.
 

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