[Catalist] Greenhouse effect

Sun Apr 25 14:56:51 AEST 2021

Hi Roy, that's a common argument by the pseudoscience brigade, on 
websites like this one:

https://nov79.com/gbwm/ntyg.html#:~:text=Absorption%20Peaks,heat%20producing%20radiation%20escapes%20it.

But saying that the CO2 lines only cover 8% of 'the infrared spectrum' 
is very misleading, because emission isn't flat across the whole 
infrared spectrum. The peak of the black-body spectrum of the Earth's 
surface, at around 290K, happens to be close to the broad 15um CO2 
absorption band.

And yes, water vapour is extremely efficient at absorbing IR in the 
atmosphere, but there's a broad window in the water vapor absorption 
profile that falls right across the _peak_ in the Earth's black body 
spectrum - and the 15um CO2 absorption band covers a sizeable fraction 
of that window, much more than 8% of it.

What's more, over most of the IR spectrum (where there is no CO2 or 
methane absorption), water vapour absorption is saturated at 100%, so 
changes in the amount of water vapour in the atmosphere will have no 
effect at those wavelengths. It's only in that window from ~8um to 20um 
that changes in water vapour, CO2 or methane concentrations will result 
in any significant change to the total energy absorbed.

There's a good illustration in this paper, showing water, CO2, methane 
and other atmospheric absorption bands in comparison to the incoming 
(~5500K) and outgoing (~290K) black body profiles.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6174548/

More to the point - in the past, the sum (Water + Methane + CO2) was 
stable over thousands of years. In the last hundred years, human beings 
have caused the values of 'Methane' and 'CO2' to become larger, so the 
temperature is increasing. This increase in temperature is much faster 
than any changes the earth has seen in millions of years.

Human beings haven't (directly) changed the amount of water vapour in 
the atmosphere, and we have no (direct) way of reducing the amount of 
water vapour in the atmosphere. The fact that it's one of the parameters 
in the sum total of IR absorption, or even that it's larger than the 
others, isn't a good argument against changing the parameters that we 
CAN control.

Andrew

On 25/04/2021 11:36 am, rsskinner at optusnet.com.au wrote:
> Catalystians – and especially the physicsy-type ones.
> 
> I am struggling to explain the topic of Greenhouse effect with my year 
> 12s. Absorption spectra are on the syllabus – where certain wavelengths 
> of radiation passthrough a gas and certain lines are absorbed due to 
> electron level transitions. But with molecules, electrons are not 
> promoted – the absorption there is due to the uptake of energy mainly 
> from the molecular bonds.
> 
> When we focus on the gas carbon dioxide I have found that the absorption 
> only takes place at the wavelengths 2.7, 4.3 and 15 micrometres, the 
> latter line being a bit broader that the others. These energy levels 
> account for about 8% of the total IR spectrum. Water vapour accounts for 
> about 60% absorption of IR.
> 
> The question is: How can I explain to my year 12 physics students that 
> by removing all the carbon dioxide from the atmosphere (apart from 
> killing most of the life on Earth) this will significantly lower the 
> temperature of the Earth.
> 
> Roy
> 
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