Understanding the Greenhouse (Gas) Effect

[arkie87]

(Before I start, I will repeat my request from my previous thread: please be respectful in this forum; given the subject matter, I imagine it is quite easy to get frustrated and condescending and would like to request that those that do not feel they can be respectful kindly not enter the discussion; I am coming here with questions and want to learn).

So, I am researching the greenhouse gas effect and was reading this article on Skeptical Science and have some questions which i hope this community could (respectfully) answer:

We only have to look to our moon for evidence of what the Earth might be like without an atmosphere that sustained the greenhouse effect. While the moon’s surface reaches 130 degrees C in direct sunlight at the equator (266 degrees F), when the sun ‘goes down’ on the moon, the temperature drops almost immediately, and plunges in several hours down to minus 110 degrees C (-166F).

Since the moon is virtually the same distance from the sun as we are, it is reasonable to ask why at night the Earth doesn’t get as cold as the moon. The answer is that, unlike the Earth, the moon has no water vapour or other greenhouse gases, because of course it has no atmosphere at all. Without our protective atmosphere and the greenhouse effect, the Earth would be as barren as our lifeless moon; without the heat trapped overnight in the atmosphere (and in the ground and oceans) our nights would be so cold that few plants or animals could survive even a single one.

This seems to me to be a flawed argument. One could argue the reason the moon gets so hot during the day and cold during the night is not because it has no greenhouse effect, but because it has no atmosphere. The atmosphere, through convection, dampens temperature swings, and if there is water content in the air, it can rapidly slow cooling if dew point is reached. In dry climates, such as desert, the temperature swings are much larger, since there is no water vapor to slow cooling at night or foliage to absorb the sun's rays. It doesnt necessarily have anything to do with a greenhouse effect i.e. preventing IR radiation from reaching space, but rather, in dampening temperature swings.

The fact that earth is 33C warmer than it would be (on average) without an atmosphere IS evidence of the greenhouse effect.

The most conclusive evidence for the greenhouse effect – and the role CO2 plays – can be seen in data from the surface and from satellites. By comparing the Sun’s heat reaching the Earth with the heat leaving it, we can see that less long-wave radiation (heat) is leaving than arriving

Why shouldnt this be the case? It is actually a requirement of the Planck distribution that at higher temperatures, the intensity at every wavelength is higher (not just the that the peak intensity is higher)

(and since the 1970s, that less and less radiation is leaving the Earth, as CO2 and equivalents build up). Since all radiation is measured by its wavelength, we can also see that the frequencies being trapped in the atmosphere are the same frequencies absorbed by greenhouse gases.

To me, this seems more important than the first part (not sure why it is in parentheses)?

Are they saying that they observed the wavelengths emitted in 1970 and now, and can see noticeable attenuation in the wavelengths absorbed by greenhouse gasses (presumably CO2, methane etc... but not water)?

[AngelLestat]

This seems to me to be a flawed argument. One could argue the reason the moon gets so hot during the day and cold during the night is not because it has no greenhouse effect, but because it has no atmosphere. The atmosphere, through convection, dampens temperature swings, and if there is water content in the air, it can rapidly slow cooling if dew point is reached. In dry climates, such as desert, the temperature swings are much larger, since there is no water vapor to slow cooling at night or foliage to absorb the sun's rays. It doesnt necessarily have anything to do with a greenhouse effect i.e. preventing IR radiation from reaching space, but rather, in dampening temperature swings.

Ok, is not perfect explain there but their point is that if you would not have greenhouse gases, all the heat radiated by the earth on night would escape right away without bounce in the atmosphere.

Of course there is other effect as the thermal mass of oceans which does not counteract this argument, but it will be hard to explain to a newbie if it will be included.

The fact that earth is 33C warmer than it would be (on average) without an atmosphere IS evidence of the greenhouse effect.

That would be the average temperature... cold side and warm side.

If you add 1 bar of atmosphere without greenhouse gases the average temperature would be the same, only that more limited because it helps to distribute the heat (by convection)... thermal mass.

Why shouldnt this be the case? It is actually a requirement of the Planck distribution that at higher temperatures, the intensity at every wavelength is higher (not just the that the peak intensity is higher)

http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod/bbrc1b.gif

???

The only they mention there is that they measure 340 w/m2 reaching the earth, and 339 w/m2 leaving it...

Which is the prove that due the incresing co2 levels, more heat is being trapped.

To me, this seems more important than the first part (not sure why it is in parentheses)?

Are they saying that they observed the wavelengths emitted in 1970 and now, and can see noticeable attenuation in the wavelengths absorbed by greenhouse gasses (presumably CO2, methane etc... but not water)?

it does not said that.. They said that they are sure is due co2 because the radiation comming from earth has missing that exact waveleght signature, the one blocked by co2.

(not all IR is blocked by co2, just to clarify)

Edited August 11, 2015 by AngelLestat

[arkie87]

Always a pleasure AngelLestat

Ok, is not perfect explain there but their point is that if you would not have greenhouse gases, all the heat radiated by the earth on night would escape right away without bounce in the atmosphere.

Of course there is other effect as the thermal mass of oceans which does not counteract this argument, but it will be hard to explain to a newbie if it will be included.

Ok, i would agree if this were an article in a newspaper. But it is on skeptical science, which is supposed to be detailed enough to counter skeptics' arguments. Giving simplified arguments will just add fuel to skeptics who think global warming is a conspiracy or based on "bad" science.

That would be the average temperature... cold side and warm side.

If you add 1 bar of atmosphere without greenhouse gases the average temperature would be the same, only that more limited because it helps to distribute the heat (by convection)... thermal mass.

Not sure what you mean here. Add 1 bar to current atmosphere (i.e. with O2, N2, CO2 etc... in current proportions?) Then, yes, adding more mass of non-greenhosue gasses wont raise average temperature, but will dampen temperature swings in a given location.

???

The only they mention there is that they measure 340 w/m2 reaching the earth, and 339 w/m2 leaving it...

Which is the prove that due the new co2 levels, that more heat is getting absorb

It is my understanding that they are referring only to long-wave radiation. But i guess, you can read "long-wave radiation" as referring only to outgoing radiation and not incoming i.e. specifying long-wave is redundant since all outgoing is long-wave.

[Ralathon]

This seems to me to be a flawed argument. One could argue the reason the moon gets so hot during the day and cold during the night is not because it has no greenhouse effect, but because it has no atmosphere. The atmosphere, through convection, dampens temperature swings, and if there is water content in the air, it can rapidly slow cooling if dew point is reached. In dry climates, such as desert, the temperature swings are much larger, since there is no water vapor to slow cooling at night or foliage to absorb the sun's rays. It doesnt necessarily have anything to do with a greenhouse effect i.e. preventing IR radiation from reaching space, but rather, in dampening temperature swings.

That's exactly what they're saying there. They're discussing the thermal mass of the atmosphere dampening temperature swings. It's poorly worded though. But you're right, the dampening has nothing to do with the greenhouse effect.

Why shouldnt this be the case? It is actually a requirement of the Planck distribution that at higher temperatures, the intensity at every wavelength is higher (not just the that the peak intensity is higher)

http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod/bbrc1b.gif

Again, that article is very poorly worded. What they mean is that when you look at the spectrum of the earth in infrared you don't see a perfect black body. You see this:

As you can see the spectrum doesn't follow the perfect black body radiator. There are all sorts of gaps and valleys in the emission. This is where greenhouse gasses come in. These gasses absorb those spectra of light and scatter them. Because the energy keeps getting scattered the radiation only leaks out of the atmosphere very slowly. That's what's causing all those gaps.

So what happens is that visible light from the sun comes down through the atmosphere. Hits the surface and gets converted into heat. Then the heat gets radiated as infrared, but because the atmosphere is opaque to infrared the heat can't escape. The more CO2 and water vapor the more opaque the atmosphere and the slower the heat escapes the atmosphere. And thus the higher the temperature on the surface.

To me, this seems more important than the first part (not sure why it is in parentheses)?

Are they saying that they observed the wavelengths emitted in 1970 and now, and can see noticeable attenuation in the wavelengths absorbed by greenhouse gasses (presumably CO2, methane etc... but not water)?

Yes. They detect more CO2 in the air and they see the valleys in the spectra due to CO2 and methane deepening. Water gets out of the atmosphere pretty quickly. All the water we inject by burning fossil fuels rains down pretty quickly. So we don't see any increase in the greenhouse effect due to water.

[AngelLestat]

Always a pleasure AngelLestat

Same, but I did some "english" corrections just before you reply me.

After I post it for first time I realize how many english mistakes I had, so try to read it again.

Ok, i would agree if this were an article in a newspaper. But it is on skeptical science, which is supposed to be detailed enough to counter skeptics' arguments. Giving simplified arguments will just add fuel to skeptics who think global warming is a conspiracy or based on "bad" science.

It´s just a normal site, its not managed by any organization, you may find it first in your searchs because that site is ready to deal with keyword usually made by people who try to find counter arguments against climate change. (is a clever idea, but they need better data and arguments)

But it does not mean is the best site to look for real data.. In fact the moon temperatures are also wrong.

Not sure what you mean here. Add 1 bar to current atmosphere (i.e. with O2, N2, CO2 etc... in current proportions?) Then, yes, adding more mass of non-greenhosue gasses wont raise average temperature, but will dampen temperature swings in a given location.

Lets make an exercise, imagine that the moon has the same material than earth in all its surface.

This mean the same albedo and thermal mass.

If they rotate at the same speed then both would have the same average temperatures.

By average I mean in each location of the whole sphere over the whole day.

Now.. lets add an atmosphere to one of them (I guess the pressure does not matter if they have not greenhouse gases, which is hard because all gases trap a little bit of IR always), then in theory they should have the same average temperature even if now one has an atmosphere.

In fact the moon has 123 C on the day side and minus 153 C at the night side. Which is close to those -33 degree (earth average temperature= 16c) that the earth should have without greenhouse gases.

And if we add the albedo of the moon, then it has more sense.

Edited August 11, 2015 by AngelLestat

[kiwi1960]

I'm going to take a different approach... our poles are like magnets, it produces the van Allen radiation belt.

This means that most of the heat will be getting in through the poles, there is ice there, which reflects the heat back into space... the oceans, get hot, but cool down when the water is at the poles...

They call the poles the refrigerator of the Earth.. The Moon doesn't have any ice, not on the surface anyhow, so of course will be hotter...

They would have been better off comparing earth to Mars... both have poles, ice (even if its not both water based) ... and given the distance we are from Mars to the sun... Mars is still very much colder... because it has no Green house gasses.

[Bill Phil]

I'm going to take a different approach... our poles are like magnets, it produces the van Allen radiation belt.

This means that most of the heat will be getting in through the poles, there is ice there, which reflects the heat back into space... the oceans, get hot, but cool down when the water is at the poles...

They call the poles the refrigerator of the Earth.. The Moon doesn't have any ice, not on the surface anyhow, so of course will be hotter...

They would have been better off comparing earth to Mars... both have poles, ice (even if its not both water based) ... and given the distance we are from Mars to the sun... Mars is still very much colder... because it has no Green house gasses.

Actually the radiation belt traps radiation, not produces it.

[PB666]

The fact that earth is 33C warmer than it would be (on average) without an atmosphere IS evidence of the greenhouse effect.

Why shouldnt this be the case? It is actually a requirement of the Planck distribution that at higher temperatures, the intensity at every wavelength is higher (not just the that the peak intensity is higher)

http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod/bbrc1b.gif

To me, this seems more important than the first part (not sure why it is in parentheses)?

Are they saying that they observed the wavelengths emitted in 1970 and now, and can see noticeable attenuation in the wavelengths absorbed by greenhouse gasses (presumably CO2, methane etc... but not water)?

Let me simplify the argument for you. Its called dynamic equilibrium and you have to think that carbon dioxide increases the complexity of subvisible radiation absorption and thus changes the equilibrium, simple as that.

Imagine you are in a room and there is a door. In this system you can go many directions but as soon as you begin in the direction of the door inevitably you go outside. That is the fate of vibrational energy on the surface of the moon. Once it reaches a IR frequency that directed to space, its gone.

Now imagine the same room but in the path of the door going out are N circus hoops you need to jump through each time you hesitate a little to jump the next hoop. Once you start heading in the direction of the door you will go out, but stuff will interact, you will slow down, you might have a different direction, momentum.

Vibrational energy here on the earth can vibrate air, it can induce state change (water to vapor), it can then radiate, and be recaptured (many gases with many absorption lines) and so on. A cyclone for instance, takes vibrational energy in water, the wind and turbulation helps get the energy into vapor, it travels up close to the eye of the cyclone, at the top it releases its radiation into space and thin-air above, the resulting water droplets that form fall back to earth. The latent energy in the water drives the engine. But the energy is trapped in the water because suppressed evaporation and radiation (water being semi-transparent absorbs light more deeply but is neither fluorescent or phosphorescent) rates at the surface, so the cyclone is another very complex way for energy to get out of the system.

The radiation that comes from the sun is both visible and UV, but once it hits earth and is not reflected it is converted into vibrational energy and eventually converted to microwave and infrared radiation. So any gas added to the air that can absorb radiation, heat up, and release it at a lower frequency filling in a gap that is not already present in N2, O2, H2O and argon, will have the effect of raising air temperature. Eventually the radiation reaches the upper atmosphere with gases at -70'C or so, and alot of radiation occurs but at much lower frequency than the rocks here on earth, and the radiation has to trickle up through a waterfall of obstacles to get out. It keeps the ground warm at night, and gives the ground someplace to send its wobbles to in the morning.

Don't trouble yourself too much with global warming, we learned yesterday that in 100 billion years there will be no radiation left in the Universe for carbon dioxide to trap. Since we will all be dead by then anyway (Run-a-way greenhouse caused by sun going red-giant), not necessary to fret about global warming. Have I caught the intent of your post wrong?

[magnemoe]

That's exactly what they're saying there. They're discussing the thermal mass of the atmosphere dampening temperature swings. It's poorly worded though. But you're right, the dampening has nothing to do with the greenhouse effect.

Again, that article is very poorly worded. What they mean is that when you look at the spectrum of the earth in infrared you don't see a perfect black body. You see this:

http://i165.photobucket.com/albums/u43/gplracerx/PettyFig8-2.jpg

As you can see the spectrum doesn't follow the perfect black body radiator. There are all sorts of gaps and valleys in the emission. This is where greenhouse gasses come in. These gasses absorb those spectra of light and scatter them. Because the energy keeps getting scattered the radiation only leaks out of the atmosphere very slowly. That's what's causing all those gaps.

So what happens is that visible light from the sun comes down through the atmosphere. Hits the surface and gets converted into heat. Then the heat gets radiated as infrared, but because the atmosphere is opaque to infrared the heat can't escape. The more CO2 and water vapor the more opaque the atmosphere and the slower the heat escapes the atmosphere. And thus the higher the temperature on the surface.

Yes. They detect more CO2 in the air and they see the valleys in the spectra due to CO2 and methane deepening. Water gets out of the atmosphere pretty quickly. All the water we inject by burning fossil fuels rains down pretty quickly. So we don't see any increase in the greenhouse effect due to water.

This is true, however I wonder about the temperature / CO2 relationship, on the other tread about this the graphs had source for the gaps.

Ignore water its mostly dependent on humidity and temperature, the later mostly depend on season.

CO2 had only an tiny potion, it looks like even if the atmosphere was pure CO2 it would not block more than its wavelength, yes an full block would probably kick in long before this and that then you started getting past 50% absorption adding more CO2 would have an diminishing effect as you are getting closer to 100%. Now the CO2 potion looked like it was narrow and also the height of the graph was low something I assume is little energy.

Now my reasoning has to be wrong or other would have used it far more in this debate, yes if the upper limit is +5 degree it would not reduce the worst case scenario in 2100 much but still.

[peadar1987]

I'm going to take a different approach... our poles are like magnets, it produces the van Allen radiation belt.

This means that most of the heat will be getting in through the poles, there is ice there, which reflects the heat back into space... the oceans, get hot, but cool down when the water is at the poles...

They call the poles the refrigerator of the Earth.. The Moon doesn't have any ice, not on the surface anyhow, so of course will be hotter...

They would have been better off comparing earth to Mars... both have poles, ice (even if its not both water based) ... and given the distance we are from Mars to the sun... Mars is still very much colder... because it has no Green house gasses.

That's not really how it works. The van Allen belts trap charged particles in the earth's magnetic field, but the belts of radiation don't block heat from the sun. The overwhelming majority of solar energy is electromagnetic radiation, not charged particles, so the heat hitting earth from the sun is pretty much uniform over the entire disc of the earth. The poles heat up less because of the angle the energy hits at, it has to pass through more atmosphere to reach the surface, and then when it hits the surface, one square metre of disc is projected over a larger area of surface.

This is compounded by the fact that frozen water has a high albedo, reflecting more of the energy back into space.

You are right in that the earth has a higher albedo than the moon. It reflects abut 30% of solar radiation that hits its surface, whereas the moon reflects only about 12%

[YNM]

Here, I'll, once again, say a few things that you should understand. Ask via PM for more explanation on these.

1. Blackbody radiation spectrum profile is governed by Planck's equation, which he have gracefully derived out of a lot of things.

2. No object in this Universe is a perfect black body.

3. Albedo (or, in reverse, transmittance, or if you prefer, optical depth), varies over the whole EM spectrum.

----------

Now, let's get started.

Greenhouse Gas Effect (my preferred naming for it), or as most people like to name it Greenhouse Effect, is a term that is used to explain these phenomena :

- Gas(es) are available on top of Earth's surface.

- Any atoms will absorb some EM radiation (resulting in these things), that differs at most by difference in electron configuration and at longer wavelength by molecular arrangement, that results in increased albedo (in revere, reduced transmittance) of that particular EM spectrum. Even larger than a molecule particles causes scattering and dispersion, which contributes to that. In the interest of explaining Greenhouse Gas Effect, some gas are absorbing heat rays.

- Absorption of heat rays, will induce increased emitance of said heat rays of that particle, or organization of matter; this results in a lengthened 'stay time' of said heat rays, creating increased temperature of both elements, for a time.

Greenhouse Gas Effect DO have good effects (I'll just quote previous posters) :

The problem, now, is these facts :

- The Sun is still shining at Earth.

- These 'Greenhouse Gases' are present on Earth, mostly in the form of CO2, CH4, or H2O. (proof for slight neutrality of O2 & O3, proof of CH4, CO2 and H2O contribution, proof of N2 neutrality isn't available, sorry for that)

- These 'Greenhouse Gases' does not leave the Earth, least to space. Yet.

- As can be seen from these datas, the amount of these gases more or less goes linearly (anything but inverse, inverse square, inverse cube and so on) with temperature.

Some of my statements :

1. We're OK with the presence of Greenhouse Gas Effect.

2. We're NOT OK with Too High Temperature.

3. We're TOTALLY NOT OK with Runaway Greenhouse Gas Effect.

Hope that clears everything out.

[arkie87]

Here, I'll, once again, say a few things that you should understand. Ask via PM for more explanation on these.

1. Blackbody radiation spectrum profile is governed by Planck's equation, which he have gracefully derived out of a lot of things.

2. No object in this Universe is a perfect black body.

3. Albedo (or, in reverse, transmittance, or if you prefer, optical depth), varies over the whole EM spectrum.

----------

Now, let's get started.

Greenhouse Gas Effect (my preferred naming for it), or as most people like to name it Greenhouse Effect, is a term that is used to explain these phenomena :

- Gas(es) are available on top of Earth's surface.

- Any atoms will absorb some EM radiation (resulting in these things), that differs at most by difference in electron configuration and at longer wavelength by molecular arrangement, that results in increased albedo (in revere, reduced transmittance) of that particular EM spectrum. Even larger than a molecule particles causes scattering and dispersion, which contributes to that. In the interest of explaining Greenhouse Gas Effect, some gas are absorbing heat rays.

- Absorption of heat rays, will induce increased emitance of said heat rays of that particle, or organization of matter; this results in a lengthened 'stay time' of said heat rays, creating increased temperature of both elements, for a time.

Greenhouse Gas Effect DO have good effects (I'll just quote previous posters) :

The problem, now, is these facts :

- The Sun is still shining at Earth.

- These 'Greenhouse Gases' are present on Earth, mostly in the form of CO2, CH4, or H2O. (proof for slight neutrality of O2 & O3, proof of CH4, CO2 and H2O contribution, proof of N2 neutrality isn't available, sorry for that)

- These 'Greenhouse Gases' does not leave the Earth, least to space. Yet.

- As can be seen from these datas, the amount of these gases more or less goes linearly (anything but inverse, inverse square, inverse cube and so on) with temperature.

Some of my statements :

1. We're OK with the presence of Greenhouse Gas Effect.

2. We're NOT OK with Too High Temperature.

3. We're TOTALLY NOT OK with Runaway Greenhouse Gas Effect.

Hope that clears everything out.

Not sure what question in the OP you are answering?

The post isnt about greenhouse effect in general; it was to clarify the arguments made by the source in the OP.

- - - Updated - - -

Let me simplify the argument for you. Its called dynamic equilibrium and you have to think that carbon dioxide increases the complexity of subvisible radiation absorption and thus changes the equilibrium, simple as that.

Imagine you are in a room and there is a door. In this system you can go many directions but as soon as you begin in the direction of the door inevitably you go outside. That is the fate of vibrational energy on the surface of the moon. Once it reaches a IR frequency that directed to space, its gone.

Now imagine the same room but in the path of the door going out are N circus hoops you need to jump through each time you hesitate a little to jump the next hoop. Once you start heading in the direction of the door you will go out, but stuff will interact, you will slow down, you might have a different direction, momentum.

Vibrational energy here on the earth can vibrate air, it can induce state change (water to vapor), it can then radiate, and be recaptured (many gases with many absorption lines) and so on. A cyclone for instance, takes vibrational energy in water, the wind and turbulation helps get the energy into vapor, it travels up close to the eye of the cyclone, at the top it releases its radiation into space and thin-air above, the resulting water droplets that form fall back to earth. The latent energy in the water drives the engine. But the energy is trapped in the water because suppressed evaporation and radiation (water being semi-transparent absorbs light more deeply but is neither fluorescent or phosphorescent) rates at the surface, so the cyclone is another very complex way for energy to get out of the system.

The radiation that comes from the sun is both visible and UV, but once it hits earth and is not reflected it is converted into vibrational energy and eventually converted to microwave and infrared radiation. So any gas added to the air that can absorb radiation, heat up, and release it at a lower frequency filling in a gap that is not already present in N2, O2, H2O and argon, will have the effect of raising air temperature. Eventually the radiation reaches the upper atmosphere with gases at -70'C or so, and alot of radiation occurs but at much lower frequency than the rocks here on earth, and the radiation has to trickle up through a waterfall of obstacles to get out. It keeps the ground warm at night, and gives the ground someplace to send its wobbles to in the morning.

Don't trouble yourself too much with global warming, we learned yesterday that in 100 billion years there will be no radiation left in the Universe for carbon dioxide to trap. Since we will all be dead by then anyway (Run-a-way greenhouse caused by sun going red-giant), not necessary to fret about global warming. Have I caught the intent of your post wrong?

I dont see how this example is any simpler than just directly explaining what is actually happening...

I also dont see what question you are answering. It seems like you read the thread title and are trying to explain greenhouse gas effect in general, not having read the OP...

- - - Updated - - -

It´s just a normal site, its not managed by any organization, you may find it first in your searchs because that site is ready to deal with keyword usually made by people who try to find counter arguments against climate change. (is a clever idea, but they need better data and arguments)

But it does not mean is the best site to look for real data.. In fact the moon temperatures are also wrong.

Good to know... what is a better site to use?

Lets make an exercise, imagine that the moon has the same material than earth in all its surface.

This mean the same albedo and thermal mass.

If they rotate at the same speed then both would have the same average temperatures.

By average I mean in each location of the whole sphere over the whole day.

Now.. lets add an atmosphere to one of them (I guess the pressure does not matter if they have not greenhouse gases, which is hard because all gases trap a little bit of IR always), then in theory they should have the same average temperature even if now one has an atmosphere.

Why? I would argue the atmosphere would give the surface more thermal mass and convective cooling, and would dampen temperature swings somewhat...

- - - Updated - - -

Again, that article is very poorly worded. What they mean is that when you look at the spectrum of the earth in infrared you don't see a perfect black body. You see this:

http://i165.photobucket.com/albums/u43/gplracerx/PettyFig8-2.jpg

As you can see the spectrum doesn't follow the perfect black body radiator. There are all sorts of gaps and valleys in the emission. This is where greenhouse gasses come in. These gasses absorb those spectra of light and scatter them. Because the energy keeps getting scattered the radiation only leaks out of the atmosphere very slowly. That's what's causing all those gaps.

So what happens is that visible light from the sun comes down through the atmosphere. Hits the surface and gets converted into heat. Then the heat gets radiated as infrared, but because the atmosphere is opaque to infrared the heat can't escape. The more CO2 and water vapor the more opaque the atmosphere and the slower the heat escapes the atmosphere. And thus the higher the temperature on the surface.

Yes. They detect more CO2 in the air and they see the valleys in the spectra due to CO2 and methane deepening. Water gets out of the atmosphere pretty quickly. All the water we inject by burning fossil fuels rains down pretty quickly. So we don't see any increase in the greenhouse effect due to water.

Thanks for the graphs.

Do these graphs show the valleys and spectra deepening? Do you have other graphs that do?

Also, I assume, that:

(1) when looking up at the atmosphere, we are looking at the intersection of the IR frequencies from the surface and what is absorbed, reflected, and/or scattered by the atmosphere and what is emitted by the atmosphere (since nothing is coming from space)?

(2) when looking down at the land, we see what is emitted from the surface and transmitted through the atmosphere, a fraction of what is emitted from surface and absorbed and then re-emited by the atmosphere, and what is emitted by the atmosphere on its own?

[YNM]

Not sure what question in the OP you are answering?

The post isnt about greenhouse effect in general; it was to clarify the arguments made by the source in the OP.

Whoops, thought it's meant for some extension of your previous thread. But i hope you have the same viewpoint as I do.

Now, I'll point things out :

This seems to me to be a flawed argument. One could argue the reason the moon gets so hot during the day and cold during the night is not because it has no greenhouse effect, but because it has no atmosphere. The atmosphere, through convection, dampens temperature swings, and if there is water content in the air, it can rapidly slow cooling if dew point is reached. In dry climates, such as desert, the temperature swings are much larger, since there is no water vapor to slow cooling at night or foliage to absorb the sun's rays. It doesnt necessarily have anything to do with a greenhouse effect i.e. preventing IR radiation from reaching space, but rather, in dampening temperature swings.

Actually, it have more to do with rotation. Ever tried to roast something without ever turning it over ? Compare with a constantly rolling roasted stuff - pretty much the same.

The fact that earth is 33C warmer than it would be (on average) without an atmosphere IS evidence of the greenhouse effect.

Hmm... correct AFAIK. It is there wherever an atmosphere exist.

Why shouldnt this be the case? It is actually a requirement of the Planck distribution that at higher temperatures, the intensity at every wavelength is higher (not just the that the peak intensity is higher)

Most likely they're talking of absorption, which is another yes. Though, on total, it got to be the same without any absorption.

Think of

E = (ÃÆ’((T^4)(R^2)/(d^2)))*(r^2) (d = Earth distance to Sun, R= Sun's radius, r = Earth's radius, T = sun's temperature) -> Total received flux from Sun

vs

L = 4ÃÀ(t^4)(r^2) (t = Earth's surface temperature) -> Total emitted flux of Earth

To me, this seems more important than the first part (not sure why it is in parentheses)?

Are they saying that they observed the wavelengths emitted in 1970 and now, and can see noticeable attenuation in the wavelengths absorbed by greenhouse gasses (presumably CO2, methane etc... but not water)?

This one is covered in my linked graphs. Look up my post.

[arkie87]

Actually, it have more to do with rotation. Ever tried to roast something without ever turning it over ? Compare with a constantly rolling roasted stuff - pretty much the same.

Very good point. Mun... uh... i mean Moon day is 28 times longer than Earth's.

Hmm... correct AFAIK. It is there wherever an atmosphere exist.

Being there doesnt mean its significant at all. Without greenhouse gasses, air (O2 and N2) wouldnt have any significant greenhouse effect.

Greenhouse effect (raising average temperature) is different than atmosphere effect (dampening temperature swings between day and night).

[Alchemist]

The fact that the radiation emitted from the surface is longer wavelength than solar radiation gives one particular effect - most absorption by greenhouse gases happens at these longer wavelengths - that is thermal radiation from the surface is trapped in the atmosphere much stronger tan solar radiation.

As for the temperature without greenhouse effect, the recipe is simple: take the average solar radiation received per unit of surface, multiply it by what fraction gets absorbed, then find the temperature at which the same surface material will emit the same power. And even if it's not blackbody it often gets rather close to blackbody temperature in the same conditions, if there's no strong specific absorption bands (which are not so strong for most common solid materials, but are in effect for quite a few common atmospheric gases).

BTW, found an absorption spectrum of the atmosphere

Now let me explain a bit about the radiation absorption by gases:

1) There are specific absorbtion bands for each gas (which are rather wide for CO2 and H2O), and only these wavelengths are absorbed - therefore you can see some wavelengths for which the atmosphere almost fully transparent and some which are totally absorbed. Changes in particular gas concetntration do not affect wavelengths outside these bands

2) The most change of the actual transmittance with change of particular gas mount is along the edges of the absorption bands, when the radiation gets partially absorbed. Increase in the gas concentration obviously lower the amount of radiation passing through the atmosphere at these wavelengths.

3) What happens to the absorbed radiation? The atmosphere heats up. What does it do with this heat? It emits it. At what wavelengths? Gases mostly emit at the wavelengths they absorb (and the intensity is proportional to the absorbtion coefficient)

4) So, the atmosphere radiates into space. But only small fraction of this radiation (at the partially absorbed wavelengths where emission is not wery strong) gets directly into space. the rest will be reabsorbed on the way up. And reemitted...

5) But it doesn't only emit up! Half of that is emitted downwards, and with the radiation emitted at low altitudes it's much higher chance to reach the surface than to reach space. And here's the real greenhouse effect - most of the thermal radiation from the surface is absorbed by the atmosphere and emmited back down to the surface!

I once compiled this picture from multiple sources:

Now... what's the real effect of change in a particular gas concentration?

There are parts of the spectrum that are almost not absorbed no matter the amount of greenhouse gases, and there are parts of the spectrum that will get 100% absorbed near the surface and emitted back even if the amount of CO2 and H2o in the atmosphere gets multiple times lower. There's some concentration-dependent shift in the ratio between what gets returned to the surface and what gets emitted to space, but it's not linear. (In fact, for CO2 and H2O it may be logarithmic dependance).

On the other hand, appearance of strong absorbtion band in the area not covered by CO2 and H2O (technogenic gases and to some extent NO and CH4) may give a relatively significant effect on the greenhouse effect).

However, note that this is the effect of absorbing this large portion of emmitted radiation and it gives just about 30 degrees over what would happen without the greenhouse effect. It seems extremely unlikely that it would change the equilibrium temperature by more than a few degrees by just increasing amounts of CO2 and H2O in the atmosphere several times.

On the other hand, its difficult to predict how such a global change would affect local climates...

[AngelLestat]

3) What happens to the absorbed radiation? The atmosphere heats up. What does it do with this heat? It emits it. At what wavelengths? Gases mostly emit at the wavelengths they absorb (and the intensity is proportional to the absorbtion coefficient)

4) So, the atmosphere radiates into space. But only small fraction of this radiation (at the partially absorbed wavelengths where emission is not wery strong) gets directly into space. the rest will be reabsorbed on the way up. And reemitted...

5) But it doesn't only emit up! Half of that is emitted downwards, and with the radiation emitted at low altitudes it's much higher chance to reach the surface than to reach space. And here's the real greenhouse effect - most of the thermal radiation from the surface is absorbed by the atmosphere and emmited back down to the surface!

On the other hand, appearance of strong absorbtion band in the area not covered by CO2 and H2O (technogenic gases and to some extent NO and CH4) may give a relatively significant effect on the greenhouse effect).

However, note that this is the effect of absorbing this large portion of emmitted radiation and it gives just about 30 degrees over what would happen without the greenhouse effect. It seems extremely unlikely that it would change the equilibrium temperature by more than a few degrees by just increasing amounts of CO2 and H2O in the atmosphere several times.

In your last conclusion, how do you explain the points 3, 4 and 5 that you made then?

If you increase the co2 values the temperature increase, not because that band was already absorbed it means would not be absorbed again after reemmited by another co2 molecule.

You would not be able to explain venus temperature with that conclusion.

How many times that radiation bounce on earth, and how many times bounce on venus?

Of course on earth the temperature increase is not severe by a small change on co2, but it triggers a chain event releasing more greenhouse gases plus more water.

Not sure what would be the reflective (albedo) effect on clouds due more water on the atmosphere.

Very good point. Mun... uh... i mean Moon day is 28 times longer than Earth's.

I mistake in my first post to add the rotation as it were an effect that can change the outcome, is not.

The rotation speed does not change the average temperature that the planet would have at certain distance from the sun.

Good to know... what is a better site to use?

Not sure, I just said that because the moon data was wrong and the arguments were not carefull enoght.

I am not an expert either, I just try to use my common sense to discriminate between sources.

Also is a good practice to see the two sides of the coin in all topics, like arguments from deniers.

Why? I would argue the atmosphere would give the surface more thermal mass and convective cooling, and would dampen temperature swings somewhat...

Is not about thermal mass, If you have a big ocean which add an incredible amount of thermal mass (more than any atmosphere), your average temperature would be the same, because if not, it means you are releasing more heat that the one you receive, which it is absurd. The only it changes is that your night/day side temperatures will be closer to the average.

Now remember that in the example I give, the atmosphere would be from a single gas element which would not absorb almost nothing of IR, without changing much the albedo either, something very hard to find.

But in that hipothetical case, the radiation would act like if there was nothing there.

So you have conductive and convective heat transfer between the surface and the gas (which is not much), and this gas will add just a bit amount of thermal mass.

But well, now I am in doubt what would happen by the radiation release by this gas in its same wavelenght absortion spectrum, which it will bounce against other molecures of this same gas, which in theory should increase a bit the surface temperature, but just a bit.

And the purpose of this mind experiment is just isolate cause and effects.

Edited August 12, 2015 by AngelLestat

[arkie87]

The fact that the radiation emitted from the surface is longer wavelength than solar radiation gives one particular effect - most absorption by greenhouse gases happens at these longer wavelengths - that is thermal radiation from the surface is trapped in the atmosphere much stronger tan solar radiation.

This would be true even without greenhouse effect, since higher temperature black bodies emit higher intensity light at ALL wavelengths, not just the peak. For that reason, the graph you show is slightly misleading. Though i suppose distance can reduce intensity...

[arkie87]

You would not be able to explain venus temperature with that conclusion.

How many times that radiation bounce on earth, and how many times bounce on venus?

On Venus, CO2 concentration is 965,000 ppm wheres on earth it is ~400 ppm; one would expect IR radiation to bounce many more times on Venus, though that effect might not matter after a certain point, where the atmosphere is essentially opaque.

However, one thing that has confused me for a bit is this:

Consider a blackbody radiator in the shape of a Kerbal separated by a volume of isothermal greenhouse gas (well mixed) from a IR camera. As the pressure of the gas is increased, the image of the Kerbal will slowly disappear; however, the image will not be dark. It will change from a picture of a Kerbal to a uniform bright light, since the gas absorbs and emits the light. Thus, while the light from the Kerbal will not reach the other side of the detector, plenty of light will reach the IR camera (it will just be diffuse).

Thus, there appears to be a limit on the maximum opacity, since the outer-most layer will always radiate to space (even if the original light waves were absorbed and re-emitted).

However, i suppose what causes greenhouse effect is when the outermost layer is very cold (i.e. when the outer most layer is thermally insulated from the surface), such that the amount of radiation leaving the planet is less than would leave if the surface could radiate to space directly.

Does that sound about right?

Of course on earth the temperature increase is not severe by a small change on co2, but it triggers a chain event releasing more greenhouse gases plus more water.

See response below regarding positive feedback loops involving water.

Also, positive feedback is a very dangerous thing. Even small positive feedback, in the absence of any negative ones, can result in exponential growth until saturation. What doesnt make sense to me is how this positive feedback can be strong enough to raise the earth temperature by only a few degrees but not skyrocket the planet towards Venus conditions.

And the fact that more heat is radiated at elevated temperatures doesnt help, since the heat is still trapped by the CO2 (it would only help if the temperature increases enough to change the outgoing wavelengths significantly such that CO2 no longer absorbs them).

In addition, how does one explain this positive feedback, given that millions of years ago, the CO2 concentration in the planet was ~3000 ppm, but the temperature was only ~10 C warmer (i.e. not Venus)?

Not sure what would be the reflective (albedo) effect on clouds due more water on the atmosphere.

The water vapor cycle is very stable and has many negative feedbacks to take water out of atmosphere (residence time of water molecule of short). Any positive feedbacks of CO2 that act through water vapor might be negated by negative feedbacks of water cycle itself.

I mistake in my first post to add the rotation as it were an effect that can change the outcome, is not.

The rotation speed does not change the average temperature that the planet would have at certain distance from the sun.

Of course not (it doesnt affect average temperature). But it DOES change the temperature difference from night and day!

Not sure, I just said that because the moon data was wrong and the arguments were not carefull enoght.

I am not an expert either, I just try to use my common sense to discriminate between sources.

Also is a good practice to see the two sides of the coin in all topics, like arguments from deniers.

Sounds reasonable!

Is not about thermal mass, If you have a big ocean which add an incredible amount of thermal mass (more than any atmosphere), your average temperature would be the same, because if not, it means you are releasing more heat that the one you receive, which it is absurd. The only it changes is that your night/day side temperatures will be closer to the average.

Now remember that in the example I give, the atmosphere would be from a single gas element which would not absorb almost nothing of IR, without changing much the albedo either, something very hard to find.

But in that hipothetical case, the radiation would act like if there was nothing there.

So you have conductive and convective heat transfer between the surface and the gas (which is not much), and this gas will add just a bit amount of thermal mass.

I'm not arguing an atmosphere (non-greenhouse) will affect average temperature, just temperature difference between day and night. Atmosphere might not be a significant thermal mass compared to the oceans or the whole mass of the earth, but on the time scale of a day, the heat from the sun does not have a change to penetrate through the whole ocean or the earth. With wind and natural convection, the thermal mass of the atmosphere is the one that is mostly used.

[Alchemist]

Comparing Venus to Earth is a bit difficult. How much more CO2 would the radiation have to pass through? It might be like 5 orders of magnitude difference. CO2 absorbance bands would widen proportionally to logarithm of such increase, but with this kind of increase they could cover almost entire IR spectrum, reducing direct radiation to almost zero. Plus add the sulfur oxides that should have rather good IR absorption, too.

As for how to threat very high absorbance cases - here's my thought:

There are basically 3 processes of radiative heat exchange. Two of them (surface-atmosphere and atmosphere-space) can be characterized by maximal and minimal effective altitudes, respectively (and direct or low step-count surface-space radiation is significant only if for given wavelength they intersect). Everything inbetween just can be treated as another path of heat-conductance in the atmosphere (besides, since absorption coefficient affects both the intensity and transmission distance for the given wavelength, this conductivity may be similar for all the absorbed wavelengths).

So, after correction for the direct radiation (such wavelengths would be practically excluded from atmospheric heat exchange) you could integrate these effects and get energy exchange altitude distribution for surface-atmosphere and atmosphere-space radiative exchange (that again can be put in terms of effective altitudes) and the heat conductivity of the atmosphere in-between.

What would increase of the greenhouse gas amount do? Besides slightly widening absorbance bands, it'll change efficient edge altitudes (lower altitude is inversely proportional to it, same with atmosphere amount between higher altitude and space), but effect on the conductivity will be relatively small (again, it works both ways; slight increase due to wider bands, but not compensating for the loss of direct emission path).

For full model add other heat conductance paths and...

surface - atmosphere as insulator - radiative outer edge

[magnemoe]

The fact that the radiation emitted from the surface is longer wavelength than solar radiation gives one particular effect - most absorption by greenhouse gases happens at these longer wavelengths - that is thermal radiation from the surface is trapped in the atmosphere much stronger tan solar radiation.

As for the temperature without greenhouse effect, the recipe is simple: take the average solar radiation received per unit of surface, multiply it by what fraction gets absorbed, then find the temperature at which the same surface material will emit the same power. And even if it's not blackbody it often gets rather close to blackbody temperature in the same conditions, if there's no strong specific absorption bands (which are not so strong for most common solid materials, but are in effect for quite a few common atmospheric gases).

BTW, found an absorption spectrum of the atmosphere

http://s015.radikal.ru/i333/1508/58/3f2af6214db7.jpg

Now let me explain a bit about the radiation absorption by gases:

1) There are specific absorbtion bands for each gas (which are rather wide for CO2 and H2O), and only these wavelengths are absorbed - therefore you can see some wavelengths for which the atmosphere almost fully transparent and some which are totally absorbed. Changes in particular gas concetntration do not affect wavelengths outside these bands

2) The most change of the actual transmittance with change of particular gas mount is along the edges of the absorption bands, when the radiation gets partially absorbed. Increase in the gas concentration obviously lower the amount of radiation passing through the atmosphere at these wavelengths.

3) What happens to the absorbed radiation? The atmosphere heats up. What does it do with this heat? It emits it. At what wavelengths? Gases mostly emit at the wavelengths they absorb (and the intensity is proportional to the absorbtion coefficient)

4) So, the atmosphere radiates into space. But only small fraction of this radiation (at the partially absorbed wavelengths where emission is not wery strong) gets directly into space. the rest will be reabsorbed on the way up. And reemitted...

5) But it doesn't only emit up! Half of that is emitted downwards, and with the radiation emitted at low altitudes it's much higher chance to reach the surface than to reach space. And here's the real greenhouse effect - most of the thermal radiation from the surface is absorbed by the atmosphere and emmited back down to the surface!

I once compiled this picture from multiple sources:

http://s020.radikal.ru/i701/1508/5d/983b6390ef4a.jpg

Now... what's the real effect of change in a particular gas concentration?

There are parts of the spectrum that are almost not absorbed no matter the amount of greenhouse gases, and there are parts of the spectrum that will get 100% absorbed near the surface and emitted back even if the amount of CO2 and H2o in the atmosphere gets multiple times lower. There's some concentration-dependent shift in the ratio between what gets returned to the surface and what gets emitted to space, but it's not linear. (In fact, for CO2 and H2O it may be logarithmic dependance).

On the other hand, appearance of strong absorbtion band in the area not covered by CO2 and H2O (technogenic gases and to some extent NO and CH4) may give a relatively significant effect on the greenhouse effect).

However, note that this is the effect of absorbing this large portion of emmitted radiation and it gives just about 30 degrees over what would happen without the greenhouse effect. It seems extremely unlikely that it would change the equilibrium temperature by more than a few degrees by just increasing amounts of CO2 and H2O in the atmosphere several times.

On the other hand, its difficult to predict how such a global change would affect local climates...

Thanks, it was much as I imagined it had to be, also explains well why deserts are so cold during the night.

CO2 probably has more impact than it looks like because of the upper atmosphere, here its little water vapor and the low pressure make the absorption less than 100% so an increase will increase the absorption.

[GeneralVeers]

Me again.

One could argue the reason the moon gets so hot during the day and cold during the night is not because it has no greenhouse effect, but because it has no atmosphere.

Bingo. But not for the reasons people expect:

The atmosphere, through convection, dampens temperature swings, and if there is water content in the air, it can rapidly slow cooling if dew point is reached. In dry climates, such as desert, the temperature swings are much larger, since there is no water vapor to slow cooling at night or foliage to absorb the sun's rays.

There's more to it than that. By way of experiment? Step outside on a sunny day and put your hand on the sidewalk. In a non-desert climate such as the one I live in, the white sidewalk is uncomfortably hot to the touch--and the asphalt in the street will give you a burn if you put your hand on it for more than a few seconds. But these surfaces are still significantly cooler than the ground in a desert. The surface temperature in a desert is blistering hot during the day, proving that convection alone doesn't cut it.

The only thing that keeps the Earth's surface from reaching the boiling point of water (convection or not!) is the fact that greenhouse gases reflect a fairly large percentage of the Sun's energy before it reaches the surface. Again, we need only look to deserts for the hero gas that keeps the non-desert parts of the world cool: water vapor.

Water vapor is the single thing missing from a desert, which makes it a desert. Water vapor is a strong absorber in the infrared range, and it's also lighter than air. Its absence is the single reason why a desert bakes during the day and a coastal town does not.

Whereas the higher CO2 levels in a city produce a very small, barely measurable heating effect that has several possible explanations.

[arkie87]

The only thing that keeps the Earth's surface from reaching the boiling point of water (convection or not!) is the fact that greenhouse gases reflect a fairly large percentage of the Sun's energy before it reaches the surface. Again, we need only look to deserts for the hero gas that keeps the non-desert parts of the world cool: water vapor.

Water vapor is the single thing missing from a desert, which makes it a desert. Water vapor is a strong absorber in the infrared range, and it's also lighter than air. Its absence is the single reason why a desert bakes during the day and a coastal town does not.

You say water vapor is the reason deserts are hotter, and then cite how water is a strong absorber in the IR range. The fact that it is lacking in the deserts should make deserts colder during the day, since the radiated IR heat isnt trapped...

If by water vapor, you meant cloud coverage, which can reflect a large portion of visible light, then cities should get just as hot as deserts on cloudless days.

Clearly, it is more complicated than just one factor.

[AngelLestat]

Consider a blackbody radiator in the shape of a Kerbal separated by a volume of isothermal greenhouse gas (well mixed) from a IR camera. As the pressure of the gas is increased, the image of the Kerbal will slowly disappear; however, the image will not be dark. It will change from a picture of a Kerbal to a uniform bright light, since the gas absorbs and emits the light. Thus, while the light from the Kerbal will not reach the other side of the detector, plenty of light will reach the IR camera (it will just be diffuse).

Yeah, that will be correct. Of course co2 does not block all the IR wavelenght, and if it does, it will be at big concentrations or big distances.

Thus, there appears to be a limit on the maximum opacity, since the outer-most layer will always radiate to space (even if the original light waves were absorbed and re-emitted).

Yeah, not sure if called a limit, but yes.. So the second factor here would be the height and density of the atmosphere.

However, i suppose what causes greenhouse effect is when the outermost layer is very cold (i.e. when the outer most layer is thermally insulated from the surface), such that the amount of radiation leaving the planet is less than would leave if the surface could radiate to space directly.

Does that sound about right?

mmm, is late so not sure if my brain works fine, but if the greenhouse effect already reach an equilibrium between the energy received and the energy released, then it means those radiations are equal.

But they are equal because the atmosphere heat up a lot more until reach its equilibrium.

And that would happen when the last atmosphere layer; atmosphere from certain height to top, which average temperature is equal to the average surface temperature it will need to emit the same radiation back to space.

Also, positive feedback is a very dangerous thing. Even small positive feedback, in the absence of any negative ones, can result in exponential growth until saturation. What doesnt make sense to me is how this positive feedback can be strong enough to raise the earth temperature by only a few degrees but not skyrocket the planet towards Venus conditions.

Yeah the effect is kinda unknown yet, but some of the conditions might be quite different than Venus starting by radiation.

I dont know, extra water will produce clouds, and depending its height, those clouds are ice which may counter the greenhouse effect of water.

Maybe it takes much time that process, and only 1 meteorite or a big eruptions in that period, may cover the sky with dust and block enoght sunlight to reset the climate.

Maybe someone knows.. I never search that in deep.

And the fact that more heat is radiated at elevated temperatures doesnt help, since the heat is still trapped by the CO2 (it would only help if the temperature increases enough to change the outgoing wavelengths significantly such that CO2 no longer absorbs them).

In addition, how does one explain this positive feedback, given that millions of years ago, the CO2 concentration in the planet was ~3000 ppm, but the temperature was only ~10 C warmer (i.e. not Venus)?

We dont know what might be the co2 trigger in those times, co2 only last 30 years, maybe something trigger but before the temperature rise enoght, the amount of co2 decay.

The water vapor cycle is very stable and has many negative feedbacks to take water out of atmosphere (residence time of water molecule of short). Any positive feedbacks of CO2 that act through water vapor might be negated by negative feedbacks of water cycle itself.

yeah, somebody should search for that to see if they find something new.

But the possibilities are many, from life cycles, ocean cycles and who knows..

Of course not (it doesnt affect average temperature). But it DOES change the temperature difference from night and day!

Yeah, but we were talking about the -33 degrees that earth should have without greenhouse.

I'm not arguing an atmosphere (non-greenhouse) will affect average temperature, just temperature difference between day and night. Atmosphere might not be a significant thermal mass compared to the oceans or the whole mass of the earth, but on the time scale of a day, the heat from the sun does not have a change to penetrate through the whole ocean or the earth. With wind and natural convection, the thermal mass of the atmosphere is the one that is mostly used.

If you have a planet with sea and you dont have atmosphere (lets pretend the water remains liquid XD), if you place a thermometer 1 to 10 meters above the sea even in the night side, then it will have the same temperature than the sea, because it receives heat by radiation.

So I imagine that you imagine the atmosphere helps in keep us warm when the sun will stop bright.

That is true with real atmospheres, but with this hypotetical atmosphere it will not help much.

CO2 probably has more impact than it looks like because of the upper atmosphere, here its little water vapor and the low pressure make the absorption less than 100% so an increase will increase the absorption.

Sand is a thermal insulator, animal dig in the sand during day to escape from the heat, because heat only reach few centimeters. That is the opposite to water, which is a good thermal conductor and sun light can reach several meters.

So when the night fall, the little heat trapped in sand is released in a short time, that is the first or second main effect of why deserts are cold on night.

Comparing Venus to Earth is a bit difficult. How much more CO2 would the radiation have to pass through? It might be like 5 orders of magnitude difference. CO2 absorbance bands would widen proportionally to logarithm of such increase, but with this kind of increase they could cover almost entire IR spectrum, reducing direct radiation to almost zero. Plus add the sulfur oxides that should have rather good IR absorption, too.....

....What would increase of the greenhouse gas amount do? Besides slightly widening absorbance bands, it'll change efficient edge altitudes (lower altitude is inversely proportional to it, same with atmosphere amount between higher altitude and space), but effect on the conductivity will be relatively small (again, it works both ways; slight increase due to wider bands, but not compensating for the loss of direct emission path).

For full model add other heat conductance paths and...

surface - atmosphere as insulator - radiative outer edge

Yeah I agree. (I guess), is kinda late here

I will read it again tomorrow.

[GeneralVeers]

You say water vapor is the reason deserts are hotter

Call me paranoid, but just to make sure no misunderstanding is happening: I said LACK OF water vapor is the reason deserts are hotter.

and then cite how water is a strong absorber in the IR range.

Exactly. Scientific fact.

The fact that it is lacking in the deserts should make deserts colder during the day, since the radiated IR heat isnt trapped...

And, of course, that's not the case.

Radiated IR heat isn't trapped on the Moon, either. Yet the Moon breaks the boiling point of water during the day. A desert gets so hot during the day for the same reason the daytime side of the Moon does: because more solar radiation is hitting the ground unimpeded. Then, of course, the hot ground heats the air just above it via direct contact. IR radiated from the ground doesn't need to be trapped by a greenhouse gas envelope in order for either a desert or the Moon to bake.

Conversely--per your statement above, water vapor should make a coastal town warmer during the day, right? Since the radiated IR heat is being trapped? But, of course, this is also not the case. Water vapor is a strong absorber of infrared (much stronger than carbon dioxide)--yet for some reason a coastal town is cooler during the day. We know the water vapor is absorbing heat, but it's somehow not warmer at ground level.

Even on a cloudless day, the California coastline is (usually!) a lot cooler than the Mojave Desert, so obviously clouds can't be the primary factor. There's only one other explanation: greenhouse gases absorbing infrared on its way DOWN.

And whenever heat is absorbed by a gas before reaching the ground, where does it go? Same direction a hot-air balloon does. Up.

If by water vapor, you meant cloud coverage

I didn't. I meant water vapor below cloud level, in its transparent (and invisible) form. Non-cloud water vapor absorbs strongly in the infrared range--it's the Chuck Norris of greenhouse gases, roundhouse kicking carbon dioxide in the head and sending it home crying for mommy.

Cloud coverage is certainly a big factor when it happens, though.

[peadar1987]

That's not how greenhouse gases work. The a greenhouse gas is something that absorbs strongly in the IR range, but is a weak absorber in the visible. If it blocks as much incoming radiation as outgoing, it ain't a greenhouse gas, by definition.

The reason deserts have higher temperatures isn't because of water vapour in the air, it's because of a lack of liquid water on the ground. Liquid water is an excellent heat sink, it has a specific heat capacity of 4180 J/kgK, whereas sand, rock or soil are all of the order of 1000 J/kgK, often less: http://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html

As Arkie correctly pointed out in the previous thread, the phase change of water vapour does have some moderating effect, but let's take a case study of three cities with similar average temperatures, but varying humidity and proximity to the coast:

Jeddah: Coastal, desert: http://en.tutiempo.net/climate/2014/ws-410240.html

New Delhi: Continental, tropical: http://en.tutiempo.net/climate/2014/ws-421820.html

Bangkok: Coastal, tropical: http://en.tutiempo.net/climate/2014/ws-484550.html

Delhi actually has bigger temperature swings than Jeddah, in spite of being far more humid. Bangkok and Jeddah have similar temperature swings in spite of Jeddah being less humid.

Clearly water vapour in the atmosphere isn't the sole driving factor behind temperature or the rate at which it swings, as you're trying to make out.

Edited August 13, 2015 by peadar1987