4 ms·

> The complicated part that I have spent many hours trying to understand is how more CO2 increases the greenhouse effect if the the frequencies it absorbs are a

> The complicated part that I have spent many hours trying to understand is how more CO2 increases the greenhouse effect if the the frequencies it absorbs are already 100% absorbed. I'm curious where you're getting the figure that the relevant frequencies are 100% absorbed, I didn't think it was that high, but I'm far from an expert on this. I don't think you can ever get to 100%, just arbitrarily close -- there's always the chance that a given photon will get lucky and escape without hitting any CO2 molecule. But I think that if every IR molecule that was radiated from earth hit a CO2 molecule, what would happen after that first absorption would matter. The photon would get re-radiated with an equal chance to go in any direction. If it goes up, great, it escapes into space -- but the more CO2, the more of a chance that it gets absorbed again and re-radiated back down. So it's not a concrete wall, it's like a field of gopher holes with gophers that will take any golf ball that falls into their hole and chuck it out randomly. Even if the field is dense enough that you can't roll a golf ball directly across it without going into at least one hole, if you increase the density of gopher holes more, it still lowers the chance that the ball will make it to the other side. *edit - This article[1] I just found goes more into the math, it helped clarify it a bit for me. [1] https://www.geoexpro.com/articles/2022/04/recent-advances-in-climate-change-research-part-xiv-the-doom-of-a-photon-on-a-random-walk https://www.geoexpro.com/articles/2022/04/recent-advances-in...

s1artibartfast 4y ago
>I'm curious where you're getting the figure that the relevant frequencies are 100% absorbed, I didn't think it was that high, but I'm far from an expert on this. Last time I researched this I saw various estimates that IR in the CO2 absorptions bands only makes it 10-25 meters before it 99% of it is absorbed by CO2. Your link gives and average distance of 2.6m for IR in the CO2 absorption band. >Assume that [C]=400 ppm. The mean free path becomes 2.646m so that N=3024. The probability of returning to the Earth becomes p(1)=0.9997. A photon with frequency or wavenumber near the centre of the absorption band is virtually certain to make the return to Earth. If the CO2 concentration is doubled, the probability doesn’t change much; it becomes p(1)=0.9998. Any CO2 increase has minor effect on the photons in the center of the absorption band. This part of the band is ‘saturated’. The website and other papers I have read seem to argue that it is not the majority of photons in the center of the emission and absorption band, but a much smaller number of off band photons in the "wings" that matter for the greenhouse effect. When I have tried to read about how this works, I get bogged down by the complexity of the science, and a lot of different arguments from the climate skeptic community which I don't have the foundational understanding to reject. They say that the shoulder theory is bunk because it ignores interactions between CO2 and other molecules. Instead of photons bouncing from CO2 to CO2, they say that the CO2 passes the energy off to nitrogen and oxygen. I don't really give too much weight to the climate skeptic criticism, but I do have to acknowledge that greenhouse effect is a lot more complicated than taught in grade school and there is still a lot of active research using extremely simplified models trying to model how it works from first principles with a lot of exclusions and assumptions. like the paper cited in the link you shared from 2012. Thank you for sharing the link, I will take some time to try to read and digest it.
- NumberWangMan 4y ago
  > Assume that [C]=400 ppm. The mean free path becomes 2.646m so that N=3024. The probability of returning to the Earth becomes p(1)=0.9997. A photon with frequency or wavenumber near the centre of the absorption band is virtually certain to make the return to Earth. If the CO2 concentration is doubled, the probability doesn’t change much; it becomes p(1)=0.9998. Any CO2 increase has minor effect on the photons in the center of the absorption band. This part of the band is ‘saturated’. Ok, so as I'm thinking about this, and forgetting about the edges of the absorption bands -- in steady state, all the heat of the sun must be radiated out into space, otherwise the earth would be hot enough to have killed off all life millions of years ago. When you think about it, an IR photon that returns to earth is not really done, it doesn't get "stuck" to the earth. The surface of the earth either reflects it back up, or absorbs it and its energy, and then re-emits it some time later. Even with 0.9997 probability of returning to earth, if you try enough times, you'll eventually escape, and photons get a lot of tries, really fast. That 0.0003 probability matters, and if you change it to 0.0002, it seems to me like it's a really, really big deal. To take it the other way -- if any given IR photon had, say, a 50% chance of escaping the earth, my intuition says that it would let the earths temperature drop extremely drastically at night, like the moon or mercury. The sheer speed at which photons get "tries" to escape means that the greenhouse gas blanket has to absorb and send back almost all photons in order for the earth not to become icy every night.

s1artibartfast 4y ago

>I'm curious where you're getting the figure that the relevant frequencies are 100% absorbed, I didn't think it was that high, but I'm far from an expert on this. Last time I researched this I saw various estimates that IR in the CO2 absorptions bands only makes it 10-25 meters before it 99% of it is absorbed by CO2. Your link gives and average distance of 2.6m for IR in the CO2 absorption band. >Assume that [C]=400 ppm. The mean free path becomes 2.646m so that N=3024. The probability of returning to the Earth becomes p(1)=0.9997. A photon with frequency or wavenumber near the centre of the absorption band is virtually certain to make the return to Earth. If the CO2 concentration is doubled, the probability doesn’t change much; it becomes p(1)=0.9998. Any CO2 increase has minor effect on the photons in the center of the absorption band. This part of the band is ‘saturated’. The website and other papers I have read seem to argue that it is not the majority of photons in the center of the emission and absorption band, but a much smaller number of off band photons in the "wings" that matter for the greenhouse effect. When I have tried to read about how this works, I get bogged down by the complexity of the science, and a lot of different arguments from the climate skeptic community which I don't have the foundational understanding to reject. They say that the shoulder theory is bunk because it ignores interactions between CO2 and other molecules. Instead of photons bouncing from CO2 to CO2, they say that the CO2 passes the energy off to nitrogen and oxygen. I don't really give too much weight to the climate skeptic criticism, but I do have to acknowledge that greenhouse effect is a lot more complicated than taught in grade school and there is still a lot of active research using extremely simplified models trying to model how it works from first principles with a lot of exclusions and assumptions. like the paper cited in the link you shared from 2012. Thank you for sharing the link, I will take some time to try to read and digest it.

NumberWangMan 4y ago
> Assume that [C]=400 ppm. The mean free path becomes 2.646m so that N=3024. The probability of returning to the Earth becomes p(1)=0.9997. A photon with frequency or wavenumber near the centre of the absorption band is virtually certain to make the return to Earth. If the CO2 concentration is doubled, the probability doesn’t change much; it becomes p(1)=0.9998. Any CO2 increase has minor effect on the photons in the center of the absorption band. This part of the band is ‘saturated’. Ok, so as I'm thinking about this, and forgetting about the edges of the absorption bands -- in steady state, all the heat of the sun must be radiated out into space, otherwise the earth would be hot enough to have killed off all life millions of years ago. When you think about it, an IR photon that returns to earth is not really done, it doesn't get "stuck" to the earth. The surface of the earth either reflects it back up, or absorbs it and its energy, and then re-emits it some time later. Even with 0.9997 probability of returning to earth, if you try enough times, you'll eventually escape, and photons get a lot of tries, really fast. That 0.0003 probability matters, and if you change it to 0.0002, it seems to me like it's a really, really big deal. To take it the other way -- if any given IR photon had, say, a 50% chance of escaping the earth, my intuition says that it would let the earths temperature drop extremely drastically at night, like the moon or mercury. The sheer speed at which photons get "tries" to escape means that the greenhouse gas blanket has to absorb and send back almost all photons in order for the earth not to become icy every night.