Simple explanation of greenhouse effect - right or wrong?

  • Thread starter Graeme M
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  • #26
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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?
 
  • #27
hmmm27
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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}##.
 
  • #28
Buzz Bloom
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@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.
 
  • #29
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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.
 
  • #30
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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 Iν in a given propagation direction θ impinges on a slab from below, a fraction aν will be absorbed, with aν << 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 eνB(ν,T); here eν << 1 is the emissivity of the slab (see figure 1). Both aν and eν 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 aν = eν. 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 ΔIν = eν [−Iν + B(ν,T)], assuming eν ≪ 1. The radiation decays exponentially with rate eν, but it is resupplied by a source eνB. The stable equilibrium solution to the flux-change iteration is Iν = 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 Iν. 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.
rayhumbert.JPG

It comes from his conference paper titled "Infrared Radiation and Planetary Temperature"
 
  • #31
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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.
 
  • #33
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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:

-Energy-System-satellite-infrared-radiation-fluxes.jpg
 
  • #34
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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 transfered into the atmosphere.

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

https://geography.name/how-does-earth-maintain-an-energy-balance/
 
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  • #35
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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 transfered 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.
 
  • #36
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Hi everyone,I am brand new to this forum however I have been participating in other forums for 14 months.My interest in AGW/CC began when a young girl whose house I was working on confidently declared."CO2 is like a blanket in the sky and its making the planet hotter."I had no clue as to the reality of this and have been investigating the effects of CO2 in the atmosphere since then and feel i have learned a lot.I purchased a CO2 meter and have been taking measurements at sea level and up in the Darling ranges at altitude and it is constant between 390ppm and 420ppm.This is a miniscule amount and as much as I understand the theory of CO2 reflecting light and it should make the surface warmer it simply is not.I have found it very difficult to find out from anywhere what the global average temperature should be or what it actually is.All I can find is its warming from some unknown quantity to another unknown quantity.This could be because we have no way of taking a snapshot of the global average at any given time.There are vast areas of Australia and Africa that have no measuring devices and I have studied how Satellites are supposed to measure surface temperatures and its sketchy at best.There is a lot of homogenisation which is tech speak for making it up.NASA spent huge sums of money putting instruments on AQUA and TERRA to read the UV light being emmitted from the Earth no one is going to say it did not work.
 
  • #37
pbuk
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I have found it very difficult to find out from anywhere what the global average temperature should be or what it actually is.All I can find is its warming from some unknown quantity to another unknown quantity.This could be because we have no way of taking a snapshot of the global average at any given time.There are vast areas of Australia and Africa that have no measuring devices and I have studied how Satellites are supposed to measure surface temperatures and its sketchy at best.

For all these reasons and more we do not use a 'global average temperature' to measure global climate change, we measure the 'mean global temparature anomaly'. There is a good explanation of the what, why and how of this here.

This is a miniscule amount and as much as I understand the theory of CO2 reflecting light and it should make the surface warmer it simply is not.

There is a good explanation of why CO2 and other greenhouse gasses matter, despite their low absolute concentrations in the atmosphere here.
 
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  • #38
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Thank you
 
  • #39
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Satellites do not measure temperature directly. They measure radiances in various wavelength bands, which must then be mathematically inverted to obtain indirect inferences of temperature.The resulting temperature profiles depend on details of the methods that are used to obtain temperatures from radiances. As a result, different groups that have analyzed the satellite data have produced differing temperature datasets.

GISS results are usually double CRU East Anglia.The results are given in .00 increments.Is this reliable science?
12 physical stations in the Alps at around 2400 meters have been recording temperature in the Alps since 1970 and some seasons are cooler some are warmer.The overal trend is minor.Who to trust?
 
  • #40
hutchphd
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12 physical stations in the Alps at around 2400 meters have been recording temperature in the Alps since 1970 and some seasons are cooler some are warmer.The overal trend is minor.Who to trust?

As has been pointed out, such questions are not all that relevant to the question at hand, and are often used as a red herring by those who prefer inaction and demand simple explanations to a complex problem . The analysis of the young girl is probably correct but incomplete , and the ice caps are in fact melting .
 
  • #41
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I feel actual ground measurements taken at the same location over 50 years to be very relevant.The young man who did the study runs a short video on where the stations are and how he compliled the data.Its across 3 nations and I have a high confidence that the Swiss can measure stuff.
 
  • #42
hutchphd
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I have a high confidence that the Swiss can measure stuff.

And those measurements may well be true locally.
But the polar ice caps are most certainly melting.
They are large and phase change is most assuredly accurate, inconvenient though that may be.
.
 
  • #43
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I am concerned about going off topic and I am not going to post links or name sources.When I started my journey of enlightenment I dared question sea levels and was blocked within a few hours by one site and have permanently left another site recently as I was abused for agreeing with Atmospheric Thermal Effect.My understanding of ATE is .Gravity makes the Atmosphere denser at the surface
.The Atmosphere has a mass
.The Atmoshere takes time to heat/cool
.This keeps the planet habitable
.I have learned there are many factors which affect this.
 
  • #44
BillTre
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You got periods at the wrong end of your sentences!
Better watch out for the punctuation police.

If you wanted to do bullets you could use these:
Screen Shot 2021-05-08 at 8.18.00 PM.png
 
  • #45
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  1. Got it
 
  • #46
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I would like to apply a physical side to this debate.This morning I went outside to load my jeep with pipe for tomorrows job.At 9.00am it was cold enough to make my hands uncomfortable and when I moved my jeep to the trailer inside the car was comfortable.At 11.00 am we still had 100% cloud cover and it was still cold however now it is 1.00pm and the clouds have dissipated and the sun is shining through and it is comfortably warm.From this is it reasonable to assume heavy water filled black clouds do not allow as much energy through as clear skies and how can this possibly be calculated?
 
  • #48
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I was testing my ability to post stuff and did it twice.Ooopps
 
  • #49
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.Gravity makes the Atmosphere denser at the surface
.The Atmosphere has a mass
.The Atmoshere takes time to heat/cool
.This keeps the planet habitable
If Earth's atmosphere wouls consist of pure nitrogen only, then
  • Gravity would make it denser at the surface.
  • The Atmospere would have a mass.
  • The Atmosphere would take ime to hat/cool.
  • This would not keep the planet habitable.
 

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