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Comment by poorjohnmacafee | Hacker News Reader

They are talking about the atmospheric window - the band of EM/heat that can be freely radiated out to space unblocked by our atmosphere.

This is an important concept in global warming/ghg that you almost never see discussed. The band that CO2 absorbs (15 microns mainly) is not in the atmospheric window. The 15 microns band is already absorbed by water vapor, and is almost 99% saturated as is.

(This is where some particle physics students stop talking so they don't upset people)

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If you are a particle physics student, you will understand the concept of mean free path. Adding more greenhouse gases (including water vapour) decreases the distance a photon can travel before being absorbed again, and thus increases the amount of time it takes IR radiation to escape the atmosphere. The fact that basically no photons manage it in one go is sort of irrelevant.

There is nil escape to space from that band with today's water vapor level which is why it's not considered part of the atmospheric window.

Do you have any research you can link to about your opinions on the 15 micron band?

An ivy league physics professor was the one who insisted in discussion adding CO2 will only increase warming by some minuscule overall amount due to the band saturation. This was over 15 years ago when the academic climate was a little different.

You are correct that 15um radiation will not escape directly to space from the ground. It will be absorbed by by the atmosphere. The atmosphere heats up and re-radiates the energy. Some of that energy ultimately escapes to space. The atmosphere warms up until the energy escaping to space equals the energy input by the sun. By adding more greenhouse gases, you are decreasing the distance a photon in the 15um band can travel before it is reabsorbed. This causes the atmosphere to heat up more than it would otherwise. Sure, water dominates the greenhouse effect. If it weren't there, the Earth would be frozen. That doesn't meant that adding more greenhouse gas has no effect. It most certainly does. It sounds like your professor was simply wrong.

This is not a bad article, but I am mainly linking it for the graph which shows that the Earth does indeed radiate at 15um:

https://www.giss.nasa.gov/research/briefs/2010_schmidt_05/

Though, again, that is a little irrelevant. What matters is at what temperature the total energy escaping balances the incoming energy regardless of the wavelength.

Hey late reply but thanks for taking some time for a thoughtful response. I'm going to dig back into research on this topic and see if I do indeed need to update my views.

The article does bring up another key point that has also bothered me about the ghg/carbon debate, and I'm curious if you also have an opinion on it: Adding co2 also ends up adding a significant multiple of water vapor ghg to the atmosphere, which is a more potent ghg, since water vapor increases as a function of temp. If you then remove that co2, how would it end up removing that same multiple of water vapor that got released? Our fossil fuel global warming problem actually just because a much larger water vapor warming problem. The solution then is how to also remove the water vapor, arguably a bigger more urgent problem than the carbon.

This is only 'not discussed' in the popular literature.

The scientists and modelers are definitely take this into account. But its is a tricky thing to get right.

https://www.acs.org/content/acs/en/climatescience/climatesci...

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