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Author Topic: Why is carbon dioxide - CO2 - a greenhouse gas?  (Read 3192 times)

Offline chris

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Why is carbon dioxide - CO2 - a greenhouse gas?
« on: 14/06/2015 16:01:18 »
Why is carbon dioxide - CO2 - a greenhouse gas? What about its chemical nature gives it this property?


 

Offline chiralSPO

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Re: Why is carbon dioxide - CO2 - a greenhouse gas?
« Reply #1 on: 14/06/2015 18:45:07 »
carbon dioxide is a greenhouse gas because it absorbs strongly in the infrared region of the electromagnetic spectrum, but is transparent to visible light. This means that it doesn't reduce the amount of energy coming in to the Earth dramatically, but then when the ground warms up and emits infrared, a significant portion of that light is unable to go through the atmosphere.

The chemical reason for this property is the polarity of the carbon-oxygen bonds. CO2 has a structure of O=C=O, and the oxygens have significantly more negative charge than the carbon (though the whole molecule is neutral). This separation of charge allows it to interact with the oscillating electric field of photons. Photons with particular frequencies (which happen to be in the infrared) will interact with the CO2 molecules, causing them to stretch and bend.

Exaggerated diagrams:

Asymmetric stretching mode:
OCO   O-CO   OCO   OC-O   OCO   O-CO   OCO   OC-O
Symmetric stretching mode (doesn't absorb IR directly):
OCO   O-C-O   OCO   OCO   OCO   O-C-O   OCO   OCO
Bending mode:
               O      O
                 \    /
OCO         C              OCO           C
                                                      /    \
                                                    O      O

water (H2O), methane (CH4) and ozone (O3) are also greenhouse gases. However, nitrogen (N2), oxygen (O2), and argon (Ar) which together make up almost 99% of the atmosphere are not greenhouse gases. N2 and O2 are completely symmetrical, with no average dipole moment, so they cannot interact with IR very strongly, if though they are capable of vibrating. Ar is a single atom, so it cannot even vibrate.
 

Offline evan_au

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Re: Why is carbon dioxide - CO2 - a greenhouse gas?
« Reply #2 on: 16/06/2015 12:03:35 »
It's just as well that we have some greenhouse gases in our atmosphere. I saw an estimate that the Earth's average temperature would be around -15C, otherwise. Big snowball, anyone?

Of course, we would like to avoid the excess greenhouse effect on Venus, where the surface temperature is hot enough to melt lead.
 

Offline Fruityloop

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Re: Why is carbon dioxide - CO2 - a greenhouse gas?
« Reply #3 on: 18/08/2015 04:22:17 »
Is the heat we feel from the carbon dioxide due to the vibration of the molecule or infrared photons being absorbed and then re-emitted later? Or a combination of both events?
 

Offline chiralSPO

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Re: Why is carbon dioxide - CO2 - a greenhouse gas?
« Reply #4 on: 18/08/2015 17:08:23 »
Is the heat we feel from the carbon dioxide due to the vibration of the molecule or infrared photons being absorbed and then re-emitted later? Or a combination of both events?

I would say both, but I don't know the extent of the contribution of each. One thing to keep in mind is that when the CO2 re-emits, the direction the photon goes is essentially random, so it may go back towards the ground, or out towards space. So, ultimately, eventually all of the IR gets out into space. It's just that the rate of energy coming in from the sun is currently greater than the rate that energy is leaving, so the planet is warming up--until it gets warm enough that it emits the IR at the same rate as the incoming energy, and a new equilibrium temperature will be established.
 

Offline Fruityloop

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Re: Why is carbon dioxide - CO2 - a greenhouse gas?
« Reply #5 on: 19/08/2015 04:01:41 »
Thanks for the response.  It was something I had been thinking about.  I know the global average temperature has been going up, 2014 was the hottest year on record and it looks like 2015 will be hotter still.
 

Offline alancalverd

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Re: Why is carbon dioxide - CO2 - a greenhouse gas?
« Reply #6 on: 20/08/2015 16:44:45 »
How was global average temperature defined? How measured? Since when?
 

Offline lightarrow

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Re: Why is carbon dioxide - CO2 - a greenhouse gas?
« Reply #7 on: 22/08/2015 11:12:24 »
Quote from: chiralSPO link=topic=57671.msg460327#msg460327
...
This separation of charge allows it to interact with the oscillating electric field of photons.
...
Ok, Chiral ispo, good post, I just have to make a little correction here: don't talk of "oscillating electric field of photons" or anything which laymen are induced to think photons are oscillations or are electric or electromagnetic field.
Either you talk of electromagnetic field/radiation (and here that would be enough) or you talk of photons.
Even talking of "photon frequency" is incorrect because it's the electromagnetic field which has a frequency.

--
lightarrow
« Last Edit: 22/08/2015 11:15:27 by lightarrow »
 

Offline alancalverd

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Re: Why is carbon dioxide - CO2 - a greenhouse gas?
« Reply #8 on: 22/08/2015 13:36:07 »
carbon dioxide is a greenhouse gas because it absorbs strongly in the infrared region of the electromagnetic spectrum, but is transparent to visible light.

The word "strongly" is inapproriate. The CO2 absorption spectrum is negligible in comparison with that of water, which is why "anthropogenic global warming" has fallen out of fashion - it is simply inconsistent with the facts. 

CO2 does make a good scapegoat because its concentration (0.04%) is fairly constant from day to day and its absorption spectrum is sufficiently sharp and narrow to be distinguished from H2O. Water is the only important greenhouse gas in our atmosphere but its concentration varies so much with time and place (between 0.1% and 10%) and is present in so many forms (from monomer to ice) that it cannot be usefully modelled and is therefore ignored by self-styled "climate scientists".

The problem with the CO2 scapegoat is that it doesn't behave as required. In addition to the recent observed upward trend  in CO2 concentration, there is a seasonal variance that peaks in late summer, when human emissions are minimal and plants are absorbing  it at their maximum rate. Thus indicates, at least to an honest scientist,  that CO2 concentration is dependent on temperature, not the other way around.   
 

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Re: Why is carbon dioxide - CO2 - a greenhouse gas?
« Reply #8 on: 22/08/2015 13:36:07 »

 

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