How does Carbon Dioxide increase global temperatures?

[SteamyTea]

Interesting problem came up today while I was having a coffee with a mate.

Knowing that I was interested in climate change (actually it is weather I like) and being a curious fellow, he asked how atmospheric CO₂ actually increased temperatures.

A good question and one I found very hard to answer.

Once, while at university, it was described as such:

"It is the shape of the molecule and the degrees of freedom that it can rotate when hit by a photon."

Now I quite liked that as it is a very mechanical description and can be used to describe different molecules having different properties.  Hitting one end of a dumbbell is not the same as hitting a bicycle (a complex shaped molecule).

So when I was chatting to a proper chemist (she has a PhD in it, tall and willowly, so someone worth hitting on), her response was:

"don't talk to me about degrees of freedom, I hate them".

She then went onto explain that it was do do with frequencies and resonance.  This seems to be the usual description i.e. the EM radiation coming in is at a high frequency, so not much wave height/length, so misses the CO₂ molecule.  After hitting the ground, the EM radiation is reduced in frequency, increasing the wave height/length, so more chance of hitting the molecule.  Which also happens to have a a 'gap' between the nucleus and the electrons that just happens to match that frequency, so it resonates.

 

Now to be honest, I don't really understand how something resonates, even though I have seen it happen.

So what clever things are happening there then?

 

And is there a better description to how it happens? preferably a 'mechanical' one, as people seem to understand that better than sub-atomic voodoo.

 

 

Edited December 29, 2019 by SteamyTea

[Ed Davies]

CO₂ (subscript)

 

I'm a bit vague and hand wavy about this, too. First search result for “co2 radiation absorption” seems a reasonable place to start: https://scied.ucar.edu/carbon-dioxide-absorbs-and-re-emits-infrared-radiation linking to https://scied.ucar.edu/molecular-vibration-modes

[SteamyTea]

5 minutes ago, Ed Davies said:

CO₂ (subscript)

Bloody phone, I put in an autocorrect superscript, seems to want to do it every time I use the numbers 2 and 3.

[Simplysimon]

or this https://www.nasa.gov/topics/earth/features/co2-temperature.html

[SteamyTea]

Bit more hunting around and I found this, but going to have to look up Raman to see what it is all about.

https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Vibrational_Spectroscopy/Vibrational_Modes/Number_of_Vibrational_Modes_in_a_Molecule

I think that I may have been confused in my thinking.  Seems that the rotations and vibrations are between the atoms, not within the atoms, which kind of makes sense.

Would be nice to work out a simple, but realistic, description or I shall be just like 

3 hours ago, Ed Davies said:

I'm a bit vague and hand wavy about this, too.

I think the first part is to treat incoming photons as particles, but the emitted IR as a wave (think one is allowed to do that, if you drop a stone in a pond you get a wave, but a wave does not create a stone).

Thoughts anyone.

Edited December 29, 2019 by SteamyTea

[Ed Davies]

1 hour ago, SteamyTea said:

I think that I may have been confused in my thinking.  Seems that the rotations and vibrations are between the atoms, not within the atoms, which kind of makes sense.

 

It can be either. For short-wave radiation (UV, visible light, near infrared) I think it's mostly a matter of bouncing electrons between energy levels in the shells around the nucleus so “within the atoms” whereas for thermal infrared it's more about making various modes of the molecules vibrate.

 

Short-wave is a lot shorter wavelength than thermal IR. Visible light centres around about 0.5 µm wavelength whereas thermal IR in the atmosphere is nearer 10 µm. Energy is proportional to frequency and frequency is inversely proportional to wavelength (in mediums like a vacuum or air where it's not slowed down much) the ratios of the photon energies are something like 20:1 so it's not surprising that the behaviour is so different.

[SteamyTea]

28 minutes ago, Ed Davies said:

It can be either.............so it's not surprising that the behaviour is so different.

Think I am going to sleep on it.

It is hard to know what is actually happening, and then trying to find a clear and concise explanation for 'the man on the Clapham omnibus'.

 

Probably why:

It is not explained often

Just expect to be taken on trust

People don't believe it

 

 

[Ed Davies]

41 minutes ago, SteamyTea said:

People don't believe it

 

What, exactly, don't people believe? Why would it be any more surprising that some gasses are transparent to thermal IR but other gasses tend to absorb it than it is that steel is opaque to visible light but glass is transparent? This video might help.

 

Edited to add: if people don't believe in the greenhouse effect at all (never mind the human influence on it) then get them to explain why the atmosphere gets cooler with height. If there's no absorption or emission of thermal radiation by the atmosphere, and particularly, no emission from the upper atmosphere to space then a simple 1-dimensional model would result in the whole atmosphere quickly reaching the same temperature and convection stopping. The real world has differences in elevation, surface absorption and, most importantly, latitude so there'd still be advection and convection going on but it would be very different from what we actually see.

Edited December 29, 2019 by Ed Davies

[SteamyTea]

10 hours ago, Ed Davies said:

What, exactly, don't people believe

That such a small molecule, in tiny proportions to the surrounding gases, can make any difference.

 

Right, had a sleep, and a think this morning and I think I shall work on the model in that chem.libretext.org link.

I don't fully understand why the shape of the molecule i.e. linear or non-linear makes much difference, that will need more reading, but as it is only two equations to work out, it is easier to remember.

 

For linear molecules like CO₂ it is equation 1

 

3(N) - 5

 

For non-linear molecules like SO₂ it is equation 2

 

3(N) - 6

 

Where N is the number of atoms.

 

This does rely on knowing (or finding out) the shape of the molecules, and I am not sure how much difference the angles between the atoms makes i.e. H₂) being 104.5°, SO₂ being 119°C.

Or how much difference the mass of a molecule makes.

In classical physics, those two properties should make a difference.

(think I may have asked myself the question to explain why the shape may be important)

 

Some molecules like Sulfur tetrafluoride (SF₄) have a degrees of freedom number of 9, as does CH₄.

SF₄ has two angles of 101.6° and 173.1°

CH₄ only has 1 angle, 109.5°

 

There is also a difference in the orbital distance between the nucleus and the electrons.

SF₄ is 154.5 pm and 164.6 pm depending on the fluorine pair.

CH₄ is 133.9 pm between the carbon atoms and 108.7 pm between the carbon and hydrogen atoms (this assumes that CH₄ is always paired with another).

 

I am starting to remember the real reasons I disliked chemistry at school, I thought it was because the teacher was a religious zealot (a Born Again Christian, whose name a still fully remember) but I think it is really because it was just a memory test. 

Edited December 30, 2019 by SteamyTea

[Temp]

https://chemistry.stackexchange.com/questions/5348/how-do-you-calculate-the-absorption-spectrum-of-water/5349#5349

 

.... It is possible to calculate absorption spectra from molecular structures using quantum mechanical calculation programs (for instance DFT using Gaussian) starting from the Schrödinger equation. These programs only work for a relatively small number of electrons...

 

"Schrödinger's equation"... shudder...reaches for a beer...

[SteamyTea]

1 hour ago, Temp said:

reaches for a beer

Says in that link that it is not very good for calculation the liquid phase.

So shall reach for the laughing gas.

But out of interest, does my part finished explanation above make sense?

[Temp]

Sorry I only did O-level chemistry. Came across Schrödinger's equation doing electronics a long time ago.

 

Google says water vapour (reasonable simple molecule) has at least 37,000 absorption lines so how the maths works im not sure.

[Ed Davies]

6 hours ago, SteamyTea said:

That such a small molecule, in tiny proportions to the surrounding gases, can make any difference.

 

Surface pressure is about 100 kPa (1000 hPa = 1000 mbar). So 1 m² of surface has 100 kN weight of gas above it or about 10.2 tonnes mass. Ignoring the trace gasses and odd isotopes and rounding a bit for easy arithmetic it's made up of 20% O₂ with a mass of 32 g/mol and 80% N₂ at 28 g/mol so an average of 28.8 g/mol. Over a m² there will be 10.2 t / 28.8 g/mol ~= 354167 moles of gas.

 

CO₂ currently makes up just over 410 ppmv of the atmosphere so 354167 × 410 / 10⁶ = 145.2 mol/m². One mole of an ideal gas has a volume of about 22.71 litres at standard temperature and pressure so that's equivalent to a layer of 3297 litres/m² or one 3.297 metres thick if you could persuade all of the CO₂ molecules to move together to the bottom of the atmosphere at one time.

 

I'm in awe of anybody's physical intuition to tell if that's significant or not. John Tyndall showed it was in the 1860s and if anybody's proved any different since you can probably find them quickly enough by looking in the list of Nobel prize winners.

Edited December 30, 2019 by Ed Davies

[SteamyTea]

1 hour ago, Ed Davies said:

I'm in awe of anybody's physical intuition to tell if that's significant or not.

I was almost directly quoting something I heard recently. I think it was an aging biker in a supermarket carpark. He started then going on about volcanoes and cows, Chinese and Indians.

It is that sort of conversation that makes me want to have a reasonable and understandable response to.

[Ed Davies]

Yep, amazing how people who take no interest in the subject for decades can suddenly know better than 10s of thousands of scientists almost any of whom could disprove an important theory with a simple lab experiment on a Friday afternoon but for some reason none of them do.

 

In a similar discussion in the past I just said I wasn't sure of the details but thought it was all to do with molecules having three or more atoms being able to bend and rotate in modes in that matched that general frequency range. It seemed sufficient then to allow us to move on to more interesting topics.

 

Or you could invite them to breath air with the “tiny proportion” of 270ppm hydrogen cyanide gas.

[tonyshouse]

Not sure you will want me on this thread. PhD in chemistry, bit pedantic, AGW skeptic, but keen on energy use reduction and reduction in use of all resources anti waste and single use plastics

 

Water is a greenhouse gas too --  get out of that 

[Ed Davies]

Yes, it's well understood that water vapour is a greenhouse gas and causes a bit more than half of the existing greenhouse effect, depending on how you count. What's there to get out of, exactly?

[SteamyTea]

Of cause you are welcome @tonyshouse.

You should be able to explain something I have been pondering.

As atoms have different masses, different nuclei to electron orbital distances and at different angles (and I appreciate that is just one model, it can also be described statistically).

Do they all absorb the same amount of photon energy? Regardless of what they emit afterwards. 

So like a big mattress. Thin and fat people can both lay on one, they just sag different amounts.

Regarding water vapour, climate scientists make a distinction between the condensing and non condensing parts of the atmosphere. That bit is well understood.

[A_L]

10 minutes ago, SteamyTea said:

Do they all absorb the same amount of photon energy? Regardless of what they emit afterwards. 

 

No, atoms can only absorb photons of the same energy as the difference between two of their energy levels. Different atoms have different energy levels. It is why emission spectra of atoms consist of lines at discrete wavelengths.

[SteamyTea]

2 minutes ago, A_L said:

atoms can only absorb photons of the same energy as the difference between two of their energy levels.

Is that set by the distance between nucleus and electrons?

Or does the angle set the frequency?

[A_L]

33 minutes ago, SteamyTea said:

Is that set by the distance between nucleus and electrons?

It is certainly true that electrons closer to the nucleus require more energy to escape from the atom

 

34 minutes ago, SteamyTea said:

Or does the angle set the frequency?

angle ?????

[SteamyTea]

3 minutes ago, A_L said:

angle

The angle between atoms when they form a molecule. See above where I quoted a few (not going to remember them).

[A_L]

@SteamyTea , in simple molecules where we are considering a single bond between two atoms the bond  is free to rotate and the rest of the atoms at either end can be considered as two homogeneous conglomerations and their 3d structures unimportant. If the two atoms are connected by double or triple bonds then the end groups cannot rotate and the 3d structures can produce minor differences in the frequencies absorbed/emitted

[SteamyTea]

Is that the same as saying

On 30/12/2019 at 08:26, SteamyTea said:

For linear molecules like CO₂ it is equation 1

3(N) - 5

For non-linear molecules like SO₂ it is equation 2

3(N) - 6

 

Where N is the number of atoms.

I was hoping to find a simple and clear way to explain why CO₂ causes warming.

 

Just teasing, I shall have to draw it on my table then hit it with a hammer, then look at which bit moves.

Edited January 1, 2020 by SteamyTea

[A_L]

3 hours ago, SteamyTea said:

I was hoping to find a simple and clear way to explain why CO₂ causes warming.

 

Try this :-

 

CO₂ is a strong absorber of infrared radiation. It is then re-emitted. Think of the CO₂ in several layers which absorb the same amount of infrared transmitted through it. The lowest layer re-transmits 50% back to the ground and 50% to the second layer which then absorbs and retransmits 50% back to layer 1 and 50% on to layer three (and so on). So an increase in CO₂ increases retained infrared radiation. Thus the temperature rises.