Atmospheric CO2 absorption - actual quantification?

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Andy Resnick

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What I struggle to understand is the part of CO2 in radiative absorption, in relative quantified terms.
This topic is unfortunately prone to unscientific sentiments.

There are publicly available atmospheric transmission codes (HITRAN, MODTRAN and LOWTRAN) that you can run and see what parameters are used for (nearly) any molecule of interest.

Here's atmospheric absorption datasets for a large number of relevant molecules:

If you need a higher-resolution dataset:
 

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No, they're saying any added water vapour rapidly condenses out of atmosphere, so the overall content stays constant for a given temperature. The content is already in equilibrium, as if the atmosphere could hold more vapour, it would (there's plenty liquid water to evaporate).
That's why something else needs to first drive the temperature change, with water acting as a positive feedback.
But that is evidently and clearly not true, else it'd be 100% RH all the time and raining.

Sometimes it doesn't rain in winter and sometimes it does. Sometimes it rains in summer and sometimes it doesn't. What has temperature got to do with whether the air reaches 100% RH?
 

phyzguy

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No, they're saying any added water vapour rapidly condenses out of atmosphere, so the overall content stays constant for a given temperature. The content is already in equilibrium, as if the atmosphere could hold more vapour, it would (there's plenty liquid water to evaporate).
That's why something else needs to first drive the temperature change, with water acting as a positive feedback.
What I was arguing with was the following statement:

But hey, when the temperature on Earth is high enough that water never condenses, then we'll count it as a greenhouse gas.
Water vapor is a greenhouse gas, and in fact is the dominant greenhouse gas in the Earth's atmosphere.
 

Bandersnatch

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But that is evidently and clearly not true, else it'd be 100% RH all the time and raining.

Sometimes it doesn't rain in winter and sometimes it does. Sometimes it rains in summer and sometimes it doesn't. What has temperature got to do with whether the air reaches 100% RH?
I don't think I wrote anything about the air being 100% saturated with water vapour.
 

phyzguy

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I don't think I wrote anything about the air being 100% saturated with water vapour.
How else should we interpret your statement below?
The content is already in equilibrium, as if the atmosphere could hold more vapour, it would (there's plenty liquid water to evaporate).
 

Bandersnatch

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How else should we interpret your statement below?
As the atmosphere (the global system, not some parcel or air) being in equilibrium (between the amount of water evaporating and condensing, not being at 100% humidity).
 

russ_watters

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As the atmosphere (the global system, not some parcel or air) being in equilibrium (between the amount of water evaporating and condensing, not being at 100% humidity).
I guess I also don't see how overall/average water vapor content could be constant in a changing climate. Indeed my understanding was that one of the fears is that water vapor content would rise with temperature, creating a positive feedback.

In either case, the original statement that water vapor should not be counted as a greenhouse gas is clearly wrong. It shouldn't be necessary anymore, but I'll point out "the greenhouse effect" includes but is not limited to climate change. The greenhouse effect increases and moderates Earth's temperature, making it habitable. Climate change happens because of changes in, not creation of the greenhouse effect. So all greenhouse gases must be included in the analysis if their composition is changing.
 

Bandersnatch

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I guess I also don't see how overall/average water vapor content could be constant in a changing climate. Indeed my understanding was that one of the fears is that water vapor content would rise with temperature, creating a positive feedback.
I must be having a really bad day at expressing myself.
I was pointing out that what @hmmm27 was saying in his comment was that water vapour acts as a feedback, not as a forcing. Which shouldn't be contentious and in any case everybody here seems to agree, as far as I can tell. But posts somehow keep being misread as saying water vapour is not a greenhouse gas.
 

Lord Jestocost

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As the atmosphere (the global system, not some parcel or air) being in equilibrium (between the amount of water evaporating and condensing, not being at 100% humidity).
To avoid misunderstanding regarding the term "equilibrium", one should say that "the earth/atmosphere system is a non-equilibrium system in a steady state."
 
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I don't want to get stuck in the politics of this issue, but... I'm a statistician, and I was first asked to look at this issue back in the 1970s. Then the concern was about falling global temperatures. Two things jumped out. The first was the shift from turbojets to turbofans. The net effect was that turbofans were more efficient, and actually burned up the particulate carbon left by the turbojets. There are still some turbojets in use, but the stratosphere has been cleaned up, resulting in several decades of global warming. The second is that if you look at global temperatures over a long period of time, in most years the temperature creeps up. Then there are major explosive volcanic eruptions that almost instantly drive the temperature lower. Some of you may remember Mt. Pinatubo, a VEI 6 eruption in the 1990s that drove global temperatures down about a degree. In the 19th century there were two still remembered eruptions, Krakatoa was a VEI 6, and Mt. Tambora was a VEI 7, that resulted in a several degree temperature drop, also known as the year without summer, or 1816 and froze to death. Most of the deaths from the Mt. Tambora eruption were from starvation though, not cold weather.

Will this pattern continue? Of course. The 19th century ended colder, the 20th century, in spite of Pinatubo, warmer. This lack of trend, which statisticians call a drunkard's walk, will continue. But what worries me is that the emphasis on the warming side is ignoring the cold side. And the last VEI 8 Toba, almost wiped out the human race. We need to be ready for either one, and the eruptions take days not decades.
 
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As I tell people that ask - its a complex issue The chemical society's write-up is just a start. What I hate is something I saw on a program discussing this. There was a climate scientist on the panel. He was asked about the opinions of a political commentator. They are wrong, even I saw they were wrong, but superficially they look good. What was his answer - I am a climate scientist and I am telling you they are wrong. That is not science - he could have easily explained why they are wrong - but didn't. As people into science we have an obligation to explain what we do know and what we do not.

Thanks
Bill
 
As an aside, we are in a long-term ice age that started around 2.6 million years ago. An informal geologic definition of an ice ace is a period of time when ice exists year round in nature, such as Greenland. We are between glacial periods within the long-term ice age in a warm interglacial period. In the last interglacial it seems like it was warmer than it is projected to get even with current CO2 production. https://en.wikipedia.org/wiki/Ice_age We are in the coldest part of the earth's history for the past 500 million years. https://en.wikipedia.org/wiki/Geologic_temperature_record
 

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As the atmosphere (the global system, not some parcel or air) being in equilibrium (between the amount of water evaporating and condensing, not being at 100% humidity).
I am at risk of a thread deviation of my own thread, but ... errrr, so just for clarity what do you think causes rain?

If the air is always at some constant equilibrium % humidity and doesn't reach 100% occasionally, do you think rain is created by some other process than the dew point being reached? Like, rain dancing or something?
 

Bandersnatch

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With all due respect, I think you're being intentionally obtuse.
 

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Just say what you think this equilibrium level is, then?
 
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Just say what you think this equilibrium level is, then
The general principle is rather easy. Water vapor from the sun heating oceans, lakes etc - rises - the higher you go the colder it gets and it begins to condense, form droplets and falls to earth. This forms an equilibrium along with other gasses. Now lets add a little bit of another greenhouse gas like CO2 - but there are many others eg O2 and Methane. They have a crucial difference to water - they do not condense out. This little bit raises temperature a little bit which causes more evaporation, more water vapor, and hence by the greenhouse effect the temperature rises, so you get a positive feedback loop. This is the runaway greenhouse effect some worry about. But to counter this more clouds form which absorbs some of the sunlight reaching earth so less water vapor and you have the opposite effect - a lowering in temperature. It will form a new equilibrium. To try and figure the equilibrium out we have climate models - and of course there are obviously other factors involved - they too must be taken into account.

The above is just the basics, but I think anyone into science should know it and be able to explain it to people. There are people that think the small amount of CO2 added by man cant possibly make any difference - and they can mount a very persuasive argument - the above explains why that is wrong.

But the models do not all produce the same result eg MIT researchers has the runaway effect not happening until we reach 152%:

Slightly worrying though is it does not contain a link to the associated peer reviewed paper.

Others say doom is only about 11 years away.

This is the sort of thing that makes discussing climate science - how to put it - problematical. I am fairly sure of the basics - but beyond that - shrug.

Thanks
Bill
 

cmb

706
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The general principle is rather easy. Water vapor from the sun heating oceans, lakes etc - rises - the higher you go the colder it gets and it begins to condense, form droplets and falls to earth. This forms an equilibrium along with other gasses. Now lets add a little bit of another greenhouse gas like CO2 - but there are many others eg O2 and Methane. They have a crucial difference to water - they do not condense out. This little bit raises temperature a little bit which causes more evaporation, more water vapor, and hence by the greenhouse effect the temperature rises, so you get a positive feedback loop. This is the runaway greenhouse effect some worry about. But to counter this more clouds form which absorbs some of the sunlight reaching earth so less water vapor and you have the opposite effect - a lowering in temperature. It will form a new equilibrium. To try and figure the equilibrium out we have climate models - and of course there are obviously other factors involved - they too must be taken into account.

The above is just the basics, but I think anyone into science should know it and be able to explain it to people. There are people that think the small amount of CO2 added by man cant possibly make any difference - and they can mount a very persuasive argument - the above explains why that is wrong.

But the models do not all produce the same result eg MIT researchers has the runaway effect not happening until we reach 152%:

Slightly worrying though is it does not contain a link to the associated peer reviewed paper.

Others say doom is only about 11 years away.

This is the sort of thing that makes discussing climate science - how to put it - problematical. I am fairly sure of the basics - but beyond that - shrug.

Thanks
Bill
I didn't spot the number there. Did I miss it?
 
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I didn't spot the number there. Did I miss it?
No. Unless you want to actually get the measurements, and I have no idea where they are, you can search for them as easily as I can - if they exist - then you must consult the models. My point is they give wildly different answers.

We know the general mechanism but specifics seem in short supply. If others know the exact numbers please post them. But I have to ask what good would they be? We do not know the tipping point or even which of the two effects I mentioned will win out.

Thanks
Bill
 

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