B Why would an atmosphere without greenhouse gasses be colder than with them?

[Graeme M]

TL;DR

How would a hypothetical atmosphere without greenhouse gasses cool?

Quick question that I haven't been able to find the answer to. Greenhouse gasses both warm and cool the atmosphere by slowing heat loss to space. But what would happen without GHGs?

I read that the earth would be colder (though still relatively warm), but why? Without GHGs the atmosphere would still be a similar mass and still warmed by conduction from the surface, yet without a means to radiate that heat to space.

Why wouldn't the atmosphere accumulate heat over time, becoming warmer? How is this hypothetical GHG-less atmosphere reaching thermal equilibrium colder than today?

 

[Drakkith]

Without GHGs the atmosphere would still be a similar mass and still warmed by conduction from the surface, yet without a means to radiate that heat to space.

I'm not sure what this last part means. Greenhouse gases absorb IR radiation that would otherwise be radiated away from the planet and into space. This heats the GHG's which then pass the heat to the rest of the surrounding air or radiate it away. Of the energy radiated away, about half of it is radiated back towards the surface where it is absorbed. The net effect is to heat the surface and air to a higher temperature than they would otherwise be without the GHG's.

Why wouldn't the atmosphere accumulate heat over time, becoming warmer? How is this hypothetical GHG-less atmosphere reaching thermal equilibrium colder than today?

Without GHG's much of the thermal radiation that is currently absorbed and redirected back to Earth's surface or atmosphere is lost to space. Sort of like removing all the glass from a greenhouse. Without the ability to trap all that energy, the surface and atmosphere are cooler.

 

[Graeme M]

Without GHG's much of the thermal radiation that is currently absorbed and redirected back to Earth's surface or atmosphere is lost to space.

That's what I'm struggling with. I thought the point of the GHGs is that they can emit IR, so non-GHG gasses transfer heat via conduction (molecular collisions) and the GHGs then radiate that out to space. But if non-GHGs cannot radiate IR AND there are no GHGs, how does the atmosphere lose its heat to space?

The process without GHGs would be: incoming insolation heats the earth's surface, the surface warms oxygen and nitrogen etc via conduction, convection transfers heat through the atmospheric column, and with no GHGs to radiate that heat away, the atmosphere seems like it should simply grow ever hotter until some other process is involved. I can't see the mechanism for thermal equilibrium at -18C.

 

[Dale]

With GHG in the atmosphere the surface of the earth is in radiative “contact” with a reservoir that is at the temperature of the atmosphere. Without GHG in the atmosphere the surface of the earth is in radiative “contact” with a reservoir that is a few degrees warmer than absolute zero.

 

[sophiecentaur]

with no GHGs to radiate that heat away,

Sorry if I'm stating the obvious here
The GHGs are in a long vertical column ('cylinder' of air) of the atmosphere. This is a diurnal effect so it's not simple; you can't really have an equilibrium state. BUT the GHGS near the ground will be in temporary equilibrium with the ground. They do not absorb the higher frequencies wavelengths so that (most of) solar energy gets to the ground and the GHGs will also absorb some incident energy. The ground radiates IR (solids will end up at a black body temperature of about 300K). This IR is absorbed by the nearby GHGs which then warm up and radiate IR upwards and downwards. A (brief) equilibrium will occur. The upward radiation is absorbed by the higher GHG layers etc. etc. and the column is then in equilibrium with a temperature gradient. The ground heats up a lot more than if there were no GHGs in order to radiate incoming and outgoing energy. The presence of clouds is also a similar effect. On clear nights the ground cools very much faster than with clouds.

With no GHGs, most of the ground's radiant IR will exit to space (even more than on a clear cloudless night) so the average ground temperature would end up at about 10C with huge diurnal / annual variations.

From https://space.stackexchange.com/que...temperature-of-a-satellite-orbiting-the-earth

 

[Bandersnatch]

The OP thinks that only GHGs can radiate heat. - is the issue here.

 

[sophiecentaur]

Why wouldn't the atmosphere accumulate heat over time, becoming warmer?

Hmm. That's something he needs to sort out.
Nothing 'only' absorbs or loses heat. Emissivity is the same value as absorptivity. Take a bubble of GHG and leave it somewhere and it will end up at an equilibrium temperature; loss and gain will eventually be the same.

The whole process of heat gain and loss from the Earth takes place throughout the atmosphere and at the surface. Take away all the GHGs and the process will be simpler and the diagram above gives a clue about what a final temperature would be at 1AU. Compare the other planets. Mars is in the news a lot and it has very little atmosphere. Its average surface temperature is around -45C, corresponding to its distance from the Sun.

 

[russ_watters]

That's what I'm struggling with. I thought the point of the GHGs is that they can emit IR, so non-GHG gasses transfer heat via conduction (molecular collisions) and the GHGs then radiate that out to space. But if non-GHGs cannot radiate IR AND there are no GHGs, how does the atmosphere lose its heat to space?

The surface radiates and not all of that energy is absorbed by the atmosphere:

https://scied.ucar.edu/learning-zone/how-climate-works/energy-budget

Without greenhouse gasses, the 6% that is directly radiated to space from the surface goes up and the 15% radiated by the surface that is absorbed by the atmosphere goes down.

 

[Drakkith]

That's what I'm struggling with. I thought the point of the GHGs is that they can emit IR, so non-GHG gasses transfer heat via conduction (molecular collisions) and the GHGs then radiate that out to space. But if non-GHGs cannot radiate IR AND there are no GHGs, how does the atmosphere lose its heat to space?

Non-GHG's CAN radiate energy away. They just can't do it the same way that GHG's do. Asymmetric molecules like GHG's can directly absorb and emit IR radiation like that emitted from the Earth's surface. Symmetric molecules like N2 and O2 cannot do this, but they can radiate some of their kinetic energy away when the collide with other molecules.

 

[Graeme M]

The OP thinks that only GHGs can radiate heat. - is the issue here.

Non-GHG's CAN radiate energy away. They just can't do it the same way that GHG's do. Asymmetric molecules like GHG's can directly absorb and emit IR radiation like that emitted from the Earth's surface. Symmetric molecules like N2 and O2 cannot do this, but they can radiate some of their kinetic energy away when the collide with other molecules.

Now we are getting closer to what I'm really after. As is pointed out, symmetric molecules like O2 and N2 transfer heat from molecular collisions. I get that. But actual thermal radiation happens with asymmetric molecules such as GHGs like CO2 and CH4. In the real atmosphere, non-radiating molecules transfer heat to radiating molecules which can then radiate heat as IR to space. That's how, as I understand it, the earth can reach thermal equilibrium (more or less) with incoming solar radiation.

But in a hypothetical atmosphere without GHGs, various sources claim the earth (atmosphere) will be -18C rather than what, 14C or so. In this hypothetical atmosphere, the total mass will be quite similar as GHGs are a small component of the atmosphere and that mass will still be warmed from contact with the hard surface (ie conduction and convection). Heat will transfer from the surface throughout the column.

BUT it cannot radiate to space - there are no GHGs to radiate IR. So, how is that hypothetical atmosphere cooling to -18C and not growing ever hotter if it cannot radiate accumulating heat to space?

 

[Dale]

BUT it cannot radiate to space - there are no GHGs to radiate IR.

IR will radiate directly from the ground into space.

 

[Graeme M]

IR will radiate directly from the ground into space.

Sure, just as it does now. But the ground also warms the atmosphere from conduction and convection which dominate at lower near-surface levels. Why would that no longer happen in an atmosphere without GHGs?

 

[Dale]

Sure, just as it does now.

No. Substantially more than it does now. That is the point.

 

[Graeme M]

No. Substantially more than it does now. That is the point.

"Substantially more" isn't changing the fact that IR from the surface right now radiates to space in the end. What's relevant to my question is whether or not with an O2/N2 only atmosphere, the warmed surface of the planet will warm the atmosphere via conduction and convection, and how that warmed atmosphere will lose heat to space...

 

[russ_watters]

"Substantially more" isn't changing the fact that IR from the surface right now radiates to space in the end. What's relevant to my question is whether or not with an O2/N2 only atmosphere, the warmed surface of the planet will warm the atmosphere via conduction and convection, and how that warmed atmosphere will lose heat to space...

With the "warmed surface" radiating more heat directly into space it will be cooler and thus will heat the atmosphere less via conduction and convection as well as radiation.

 

[Dale]

I read that the earth would be colder (though still relatively warm), but why?

This was your question.

"Substantially more" isn't changing the fact that IR from the surface right now radiates to space in the end.

But it is changing the amount of heat that transfers through that mechanism. To change the temperature (your original question) does not require a mechanism that doesn’t occur right now. A change in the amount of an existing mechanism suffices to answer the question.

 

[Graeme M]

I really worry that continuing to ask this question is going to make me seem sillier than I fear I am. Maybe if I try to be clear about the bit I want to understand - the rest I more or less grasp. Respondents appear to be missing what I am asking and focussing on the way that IR to space is how the earth loses heat to balance incoming insolation. That is NOT what I am asking about.

With the "warmed surface" radiating more heat directly into space it will be cooler and thus will heat the atmosphere less via conduction and convection as well as radiation.

You seem to be saying that yes, the atmosphere without GHGs will be warmed via conduction. So, to clarify the first part of my question.

When incoming solar insolation heats the earth's surface, some of that heat is transferred to O2 and N2 molecules by direct physical contact with the earth (conduction) and rise through the column via convection.

Question: Is that correct?

If yes, which is what I have read, would that continue to happen in an atmosphere composed solely of O2 and N2? It doesn't matter how much or how little, I'm simply chasing the mechanism. Would an atmosphere without GHG gasses be warmed by conduction and convection? I think it would, and russ_watters seems to be agreeing above.

If not, why not?

 

[russ_watters]

You seem to be saying that yes, the atmosphere without GHGs will be warmed via conduction.

Of course.

When incoming solar insolation heats the earth's surface, some of that heat is transferred to O2 and N2 molecules by direct physical contact with the earth (conduction) and rise through the column via convection.

Question: Is that correct?

Yes.

If yes, which is what I have read, would that continue to happen in an atmosphere composed solely of O2 and N2?

Yes, of course.

Are these self-evident questions really what you want to know?

 

[Graeme M]

Are these self-evident questions really what you want to know?

No, because that's what I understand to be the case. However, at this point we have an atmosphere being warmed that has no means to radiate that heat away to space. As Drakkith said above, O2 and N2 cannot radiate heat via thermal radiation, all they can do is transfer heat to other O2 and N2 molecules.

So, why does this hypothetical atmosphere not keep getting hotter?

My best guess is this. The surface warms the air which convects that warmed air upwards. As it does so, it warms higher layers via further conduction and convection. At nights though, the surface will cool rapidy via IR emission to space. If so, then the air will lose heat back to the surface via conduction. In other words, convective processes would reverse at night. But this will depend on a net equal or negative balance - that is the heat loss to the surface must equal or exceed heat gain from the surface. If it's even just the tiniest bit positive, the atmosphere will gain heat over the long run. I think the -18C hypothesised temperature is only possible if the loss equals the gain. That's my guess, what I'm after is the true physical explanation.

 

[PeroK]

My best guess is this.

A lot of what has been posted in this thread was already posted in your previous thread from four years ago:

Thread 'Simple explanation of greenhouse effect - right or wrong?'

Apr 26, 2021

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...

• Graeme M
• Explanation Greenhouse Greenhouse effect
• Replies: 51
• Forum: Earth Sciences

 

[Graeme M]

A lot of what has been posted in this thread was already posted in your previous thread from four years ago:

Yes, which is what is frustrating. In that post, I was more trying to get to a simple explanation of how the real atmosphere is affected by growing concentrations of CO2/CH4 etc. It got complicated real fast, but that's OK. I don't really need a rehash of that. What I am asking with this post was not covered in that previous thread.

 

[sophiecentaur]

TL;DR: How would a hypothetical atmosphere without greenhouse gasses cool?

I read that the earth would be colder

Which bit of the Earth are you referring to? It's a thick sandwich of gas and solids (water too!) Go back to a greenhouse; which is useful by getting warmer when the glass is added? The temperature of the surface of a double glazed roof will be much the same as the temperature of the floor with no glass. (Assuming that equilibrium is reached.) You can get frost on the outside of a cosy conservatory when there's frost on the lawn.

You have a choice; you either accept the very basic model and work with it, or go the whole hog and use all the mechanisms present in a real planet with an atmosphere. Any apparent contradictions you get from the basic model are not a surprise and are mostly explicable when using the more full model. Also, beware of reading any single verbal explanation and assuming that you have understood exactly what the wording is exactly match to your understanding. The Maths is a more way of presenting the situation but, in this case, it's fearsomly complicated.

 

[Graeme M]

You have a choice; you either accept the very basic model and work with it, or go the whole hog and use all the mechanisms present in a real planet with an atmosphere.

Just to be clear, I'm not trying to "disprove" the greenhouse effect of how GHGs affect the atmosphere. I am reasonably comfortable with how it all works at a relatively basic level. What I am struggling with is the suggestion that without GHGs, the atmosphere will be at equilibrium at -18C. As far as I can see, unless there is some way for the atmosphere to lose heat, it will simply get hotter over time. I am just trying to find out what that "some way" is, to complete my basic understanding. I don't care what the mechanism is, or what temperature the atmosphere might be, it just doesn't make sense to say that if it can't radiate to space to lose heat it will not heat up. In the real atmosphere, while GHGs might act to slow heat loss, in the end they are a significant mechanism for heat loss which allows the atmosphere (and the planet as a whole) to reach thermal equiibrium with incoming insolation.

 

[PeroK]

Just to be clear, I'm not trying to "disprove" the greenhouse effect of how GHGs affect the atmosphere. I am reasonably comfortable with how it all works at a relatively basic level. What I am struggling with is the suggestion that without GHGs, the atmosphere will be at equilibrium at -18C. As far as I can see, unless there is some way for the atmosphere to lose heat, it will simply get hotter over time. I am just trying to find out what that "some way" is, to complete my basic understanding. I don't care what the mechanism is, or what temperature the atmosphere might be, it just doesn't make sense to say that if it can't radiate to space to lose heat it will not heat up. In the real atmosphere, while GHGs might act to slow heat loss, in the end they are a significant mechanism for heat loss which allows the atmosphere (and the planet as a whole) to reach thermal equiibrium with incoming insolation.

Do you mean that if the atmosphere started at a temperature of +15C, say, and all the GHG were removed, then you don't understand why the atmosphere would lose any of its heat?

You think it should stay at a constant +15C forever?

Or, gradually heat up and just get hotter and hotter?

 

[Graeme M]

Do you mean that if the atmosphere started at a temperature of +15C, say, and all the GHG were removed, then you don't understand why the atmosphere would lose any of its heat?

You think it should stay at a constant +15C forever?

Or, gradually heat up and just get hotter and hotter?

Pretty much, yes. If the atmosphere were at +15C and all the GHGs removed (leaving about 99% of the total mass), I'd assume it could not lose that heat to space because the GHGs are the mechanism by which the atmosphere radiates heat to space. So unless there is some other mechanism (like conduction back to the surface) for losing heat, it seems to me that it would then get warmer over time. Clearly I have some major misunderstanding here, but I don't know what.

 

[russ_watters]

However, at this point we have an atmosphere being warmed that has no means to radiate that heat away to space.

You've been told several times already that that isn't true. I don't understand why you keep saying it.

As Drakkith said above, O2 and N2 cannot radiate heat via thermal radiation,

No he didn't. You're trying to get that by reading between the lines while ignoring explicit statements that it isn't true:

Non-GHG's CAN radiate energy away.

 

[Graeme M]

You've been told several times already that that isn't true. I don't understand why you keep saying it.

OK. Drakkith clearly said this:

Non-GHG's CAN radiate energy away. They just can't do it the same way that GHG's do. Asymmetric molecules like GHG's can directly absorb and emit IR radiation like that emitted from the Earth's surface. Symmetric molecules like N2 and O2 cannot do this, but they can radiate some of their kinetic energy away when the collide with other molecules.

 

[PeroK]

Clearly I have some major misunderstanding here, but I don't know what.

I guess you could start by looking at the temperature gradients within the atmosphere itself. The critical factor for climate change is the surface temperature, as that affects habitats for animals, plants and the ocean environment.

The temperature varies dramatically with height. The temperature at 10,000m altitude is about -50C. That's because the Earth's surface is the main source of heat for the atmosphere.

If there were no GHG, then the Earth's surface would cool (to -18C) and the surface air would cool through conduction if nothing else.

It isn't possible thermodynamically to have a surface ground temperature significantly different from the surface air temperature. Allowing for the complexities of the Earth's weather systems and daily heating and cooling cycles, of course.

 

[russ_watters]

Are you saying that when these symmetric molecules collide, they actually radiate heat as IR?

No, I don't know the mechanism.

But anyway, this is a red herring. The radiation of these molecules is not a major factor here, as surface radiation dominates heat transfer to space if there is limited GHG. You seem to think the atmosphere can only be warmed by the surface, when it is both warmed (during the day) and cooled (at night). In an atmosphere with limited GHG, the primary function of the atmosphere would not be warming/insulation it would be moderation.

 

[Dale]

As far as I can see, unless there is some way for the atmosphere to lose heat, it will simply get hotter over time. I am just trying to find out what that "some way" is, to complete my basic understanding.

The atmosphere is in contact with three other thermal reservoirs, the earth, the sun, and space.

Earth: heat transfer is by conduction, convection, and radiation (ignoring water).
Sun: heat transfer is by radiation only
Space: heat transfer is by radiation only

The sun and space are approximately black bodies so radiative heat can transfer at any wavelength. The atmosphere is not even approximately a black body, so heat transfer will occur at specific wavelengths based on the absorption spectra. This changes the rate of heat transfer, but not the equilibrium temperature (see @sophiecentaur 's plot in post 5).

The interaction with the earth is more complicated. There will be multiple mechanisms driving this heat transfer. Radiative heat transfer will still occur. However, because oxygen and nitrogen are transparent in the infrared range, the radiative heat transfer will be slow. Conduction, convection, and latent heat are not dependent on the absorption spectra, so those will proceed as normal. Because they are independent of the absorption those will be relatively rapid (compared to radiation), and will therefore dominate the rate of heat transfer.

So ALL of the existing mechanisms of heat transfer will still occur. All that changes is the rate of heat transfer, especially the rate of radiative heat transfer. This will mean that the atmosphere will tend towards an equilibrium temperature that is much closer to the earth temperature. And since the ground will be colder the atmosphere will also be colder.

 

[sophiecentaur]

What I am struggling with is the suggestion that without GHGs, the atmosphere will be at equilibrium at -18C.

Without the GHGs, radiation would pass both ways through the atmosphere with relatively little interaction (absorption or radiation and both are equal, for basic reasons and the thermal mass of that atmosphere would be also very small). So the atmosphere would have very little effect on the surface temperature; it would be like a large piece of rock at 1AU.

 

[Ken Fabian]

I wonder if part of the lack of clarity is from the difference between ground heat radiating upwards and the re-radiation in all directions of the portion GHG's absorb out of that. Ground is heated largely by sunlight in parts of the spectrum that are not absorbed by GHG's (unimpeded) but radiates back largely in IR that is absorbed and impeded. - absorbed from one direction but the re-radiation is in all directions, effectively half what is absorbed radiates back down and only half radiates upwards. (Not precisely half - going sideways can reach space but has to travel through more atmosphere and more GHG molecules, yet depth of atmosphere is actually quite thin compared to the diameter of the planet so this is not a big effect).

Some of that upward radiating half gets absorbed again, half of that re-radiates back down. The IR component is relatively small, but the consequences accumulate.

This aspect is not the whole story; what happens higher in the atmosphere where re-radiation upwards meets less GHG molecules and the 'IR fog' clears and IR can reach and be lost to space plays a critical role. As GHG concentrations rise the altitude where the IR fog clears rises. Not only a bit deeper but the temperatures at higher altitude are lower and that slows the re-radiation rate; more of the energy is retained.

Hope this helps.