[alancalverd]

Quote from: alancalverd on Today at 13:45:19
Since the CO2 IR absorption lines are all saturated
That's still meaningless.
Why do you keep saying it?

Because the absorption of the entire atmosphere at 15 microns is almost 100% and has been for as long as anyone could measure or calculate it.

Adding more CO2 might increase the absorption in the first 1000 ft or so of altitude but won't affect the total radiative heat exchange of the planet.

[Bored chemist]

15 microns

Other wavelengths are also available.

[Bored chemist]

What I meant was that hotter air can hold more water before it condenses into liquid drops, which is pretty obviou

which is pretty obvious.

It was obvious and wasn't in any way in dispute.
Not sure why you bothered to say it.
But I'm glad I only had to point out your error twice before you saw it. That's good going.

[Bored chemist]

For the record, it was Alan's point about the absorption being saturated that got my attention and not the historical record. I do accept that there has been an unprecedented rise in co2 due to use of fossil fuels.

Hi Paul.

I think it might have been before you joined the forum that I explained why "saturation" isn't the killer that Alan thinks it is.
https://www.thenakedscientists.com/forum/index.php?topic=65677.msg485680#msg485680

I have had to explain this to him more than twice...

[paul cotter]

BC, I had a quick look at that thread but my brain is not in gear( rotten broken sleep last night ). I will digest it tomorrow and thanks again.

[Bored chemist]

BC, I had a quick look at that thread but my brain is not in gear( rotten broken sleep last night ). I will digest it tomorrow and thanks again.

TLDR version is "there are always wavelengths at which CO2 is a bad enough absorber for the spectrum not to be saturated, but not so bad that the absorbance is zero."
So adding more CO2 will always increase the amount of IR absorbed.

But the links here
https://www.thenakedscientists.com/forum/index.php?topic=65677.msg485704#msg485704
 also make a different point.

Imagine we just pick a wavelength - say 15 μm
Alan nearly gets the point when he says
"Adding more CO2 might increase the absorption in the first 1000 ft or so of altitude but won't affect the total radiative heat exchange of the planet.".

In order for heat to leave the earth, it has to go through that first 1000 ft.
Then the 2nd 1000 ft
Then the third ...
And so on.

Now imagine we double the CO2 concentration.
Now the 15 micron radiation will only travel 500 feet.
So, it takes twice as many steps before it can leave.

And, if you make it harder for the heat to leave, more heat builds up.

So, there are two problems with people getting excited about saturation.
First it doesn't strictly happen.
Second, even if a particular absorption is (very nearly) saturated, that doesn't mean the effect on warming is saturated.

[alancalverd]

You forget that there is a lot of vertical movement in the atmosphere. It doesn't matter where the 15 micron component is absorbed: if the lower layers heat up a bit more, the upper layers heat less (because there is less 15μm radiation reaching them), you get more convection, and the result is the same. More weather, perhaps, but no change in the overall heat balance between insolation and radiation.

[alancalverd]

15 microns

Other wavelengths are also available.

But of little consequence. The blackbody emission spectrum  at  280 K peaks around 15 μm and CO2 has no other absorption bands in the significant region
https://667-per-cm.net/2016/05/28/absorption-of-long-wave-or-thermal-radiation-by-co2-at-667-per-cm has some neat graphics. Sadly, they imply that 99.5% of the 15μm radiation is absorbed in the first meter of the atmosphere, never mind 500 ft - and this paper is by a Believer!

[Bored chemist]

Sadly, they imply that 99.5% of the 15μm radiation is absorbed in the first meter of the atmosphere,

I guess you mean metre.
Fine; I apologise for believing your figure without checking it.

In order for heat to leave the earth, it has to go through that first 1000 3 ft.
Then the 2nd 1000 3 ft.
Then the third ...
And so on.

Now imagine we double the CO2 concentration.
Now the 15 micron radiation will only travel  500 1.5 ft.
So, it takes twice as many steps before it can leave.

That argument is still valid. The harder you make it for the heat to reach space, the warmer the surface gets.

[Bored chemist]

You forget that there is a lot of vertical movement in the atmosphere.

And if it happens at nearly the speed of light , then it matters to radiative heat transfer.

if the lower layers heat up a bit more, the upper layers heat less

Yes! Finally! you realise why it matters, with the outermost layers unheated, they can't radiate the excess heat into space.

Not sure why you described this as "

It doesn't matter

but maybe we can work on that later- after the celebration.

[Bored chemist]

More weather, perhaps, but no change in the overall heat balance between insolation and radiation.

Exactly.
The incoming and outgoing heat must balance.
But, if you put better lagging round the earth, the only way for it to lose the same amount of received heat is for its temperature to rise.
That's the whole point of the greenhouse effect as a cause of global warming.

[alancalverd]

Nobody is denying the greenhouse effect. I'm only questioning whether increasing the CO2 concentration above 200 ppm has any significant influence on it.

As I see it, you might create slightly stronger turbulence close to the surface but convective mixing will still dissipate the heat throughout the atmosphere. The key to net heat gain or loss is the transmissivity and emissivity of the upper layers, and these parameters are dominated by water vapor and ice. Water vapor is fairly transparent to incoming shortwave radiation but  strongly absorbent (and not saturated at atmospheric concentrations) in the outgoing infrared, and ice is of course reflective across most of the spectrum.

[Bored chemist]

I'm only questioning whether increasing the CO2 concentration above 200 ppm has any significant influence on it.

Still?
Even after I pointed out why it clearly is?

As I see it, you might create slightly stronger turbulence close to the surface but convective mixing will still dissipate the heat throughout the atmosphere.

It's not an "either or" thing.
How fast can air travel?
How fast can radiant heat travel?

The key to net heat gain or loss is the transmissivity and emissivity of the upper layers,

How do you define "upper"?
In this context it's roughly the mean free path of a photon- because, if a molecule below that level emits radiation that radiation doesn't escape.

and not saturated at atmospheric concentrations

If your "upper" layer isn't optically thick enough to be nearly saturated, it's not "upper"

So, adding more CO2 makes the layer thinner.
And that means more of the atmosphere is in the way of getting heat to that outermost layer which can actually dump heat into space.
And that retards the process by which heat is lost.

[Bored chemist]

Incidentally, "In the thermal infrared, ice is moderately absorptive, so snow is nearly a blackbody, with emissivity 98?99%. The absorption spectrum of liquid water resembles that of ice from the ultraviolet to the mid-infrared. "
From
https://royalsocietypublishing.org/doi/10.1098/rsta.2018.0161
It also absorbs hard UV.
It's only really shiny in the visible and UV

[alancalverd]

Depends on your definition of "mid-infrared" but my interest in high altitude ice is its albedo (the clue is in the name!) compared with wet air.

[Bored chemist]

Neither ice nor air reflects much of the IR at wavelengths where liquid or vapour phase water absorb.
One's black; the other is clear.

Neither is white- unlike clouds observed with visible light.
Having said that, the clouds I can see from my window are a bit too much like black bodies.

[alancalverd]

But the clouds I see from the plane or satellite images are white, and so is snow. Thus high level cloud reduces surface insolation, exactly as observed - you very rarely see cumulus (convection cloud) forming under even a thin layer of cirrus. It is also the case that you rarely get radiation fog forming at night under cloud cover - the surface doesn't cool as quickly as under a clear sky.

[Bored chemist]

Thus high level cloud reduces surface insolation

Across the visible range.
But it's black as coal at other wavelengths.
Try to be less anthropocentric about your em radiation.

You seem to be missing your own point. Water vapour is a powerful greenhouse gas because it absorbs lots of IR.
It still does that if you cool it down until it liquefies or freezes.

[alancalverd]

Pity that meteorology and common observation doesn't agree, but the essence of climate science is not to let the facts get in the way, so I won't bother to argue.

[Bored chemist]

Pity that meteorology and common observation doesn't agree, but the essence of climate science is not to let the facts get in the way, so I won't bother to argue.

In what way do "common observation" and "meteorology " tell you much about IR spectroscopy?