When people with some science background first read the explanations of CO2 warming the earth due to its “greenhouse” properties, they usually start objecting on various grounds. Actual greenhouses do something entirely different, or that breaks the laws of thermodynamics, and so on. It takes a considerable amount of investigation (for me anyway) before one gets to the conclusion that the claims made are fundamentally correct. It isn’t the science that is flawed (per se) it is the explanation.
The conclusions on the other hand are often flawed and sometime horrendously so. Conclusions however, cannot be evaluated against just CO2 and how it behaves when interacting with long wave radiation. They must be evaluated against the climate system in its entirety, and we are increasingly seeing evidence that many of the catastrophic claims are alarmism based on a massive over estimation of any number of factors that govern climate as a whole.
Yet the discussion always returns to CO2 and what it does. The claims being fundamentally correct, but the basic explanations being flawed, there have been many attempts to arrive at an explanation simple enough for almost anyone to grasp, while at the same time being accurate enough to be useful as part of the larger discussion. Not that easy to do, I and many others have failed and failed often.
Recently, Ira Glickstein came up with a physical analogy and posted it on WUWT where it generated a considerable amount of discussion, both pro and con.
I think it is one of the best explanations I’ve ever come across that explains what is happening in the atmosphere at a basic concept level from the perspective of CO2 as a “greenhouse gas”. But as can be seen from the comments, the model isn’t sufficient for other purposes such as understanding how CO2 behaves in a real atmosphere comprised of many other gasses. It also generates additional questions. Once people get the basics in their heads, they ask more in depth questions. In the thread at WUWT, a commenter named “wayne” asked several questions and then concluded with “so, its not warming, its more like delayed cooling”.
My immediate thought was YES! Not exactly, but close. How could I depict the issue that wayne was trying to understand at the most basic level possible while retaining a reasonable level of accuracy? Something I’d done during a white board discussion came to mind and this is the result. This is my first crack at it, and feel free to be critical or ask questions, I’m looking for feedback so I understand if the messages I’m trying to encapsulate are coming through properly or not. Then of course there is the possibility I may be wrong. Well, to be more accurate, I AM wrong. Models are by their very nature not reality. They might be close, but at some level they are wrong. Good modeling is about coming up with a model that is useful, and to the extent possible, as least wrong as can be.
So the explanation that follows is wrong on many levels. To demonstrate how CO2 results in a warmer earth, I’ve constructed an atmosphere of sparsely populated CO2 molecules and nothing else. I’m not trying to depict how CO2 interacts with long wave radiations in the atmosphere, I’m only trying to depict how CO2 would interact with long wave radiation if there were no other factors. To do that, I’ve depicted it in a manner that is far out of proportion from reality. For example, I show in the slides a photon being emitted by earth surface (LW radiation) as possibly escaping to space unimpeded. In theory, that is possible. In practice, highly unlikely.
The chances of any given photon not being absorbed and re-emitted thousands, perhaps millions or billions of times on the way out to space it nearly zero. Further, when a photon is absorbed and re-emitted, it can be emitted in any possible direction. It may zig zag up, down and sideways many times before it reaches space. But at day’s end, the sideways moving photons cancel each other out, and the upward moving photons slightly exceed the downward moving photons. The model presented is not to in any way to quantify the end result. The model is only presented to provide an understanding of what the process is, why it in fact does conform to the laws of thermodynamics, that it does not “invent” new energy as many claim, and how the result is a warmer earth surface (in the absence of any other factor, which is again, not realistic)
To quickly review the theory, the sun emits Short Wave (SW) radiation that for the most part goes right through our atmosphere as if it did not exist. That is because the molecules that make up our atmosphere cannot absorb such high energy photons. The SW radiation as a result heats up the earth, which in turn results in the earth radiating energy back up. But the earth is much cooler than the sun, so it radiates much lower energy photons, and we call these Long Wave (LW) radiation.
LW photons can in fact be absorbed by CO2 molecules, which increases the energy level of that molecule. But energy always tries to even itself out. If the molecule absorbs a photon and as a result is at a higher energy state (warmer) than the other molecules around it, that photon will be gotten rid of. That could happen by transferring it during a collision with another molecule, or simply emitting it so that it zips off at the speed of light in some random direction.
The model below presumes that a certain amount of energy from SW is entering the system at all times. The number of photons of SW doesn’t matter for this part of the discussion. What matters is that the amount of energy they carry is equal to the amount of energy of six LW photons being generated by the earth surface. The model assumes also that even if the CO2 “atmosphere” and the earth surface change in any way due to the “greenhouse” effects of CO2, that the amount of SW absorbed still won’t change. Again, not realistic, but the model still serves its purpose in terms of illustrating why CO2 in theory warms the earth.
And that, believe it or not, is what all the fuss is about. The question asked on WUWT by commenter “wayne” is a reasonable way to think about it. As a consequence of additional CO2 in the atmosphere, a new equilibrium is established in which the photons escaping to space still escape, but some number of them may be “delayed” on the way out, either existing for an extra moment in time absorbed in a CO2 molecule, or perhaps recycled back down to earth where the result is a warmer earth surface that generates photons at a higher rate to compensate.
But as for the laws of thermodynamics? Intact.
As for actual magnitude and actual behaviour in an actual chaotic atmosphere comprised of many different kinds of molecules and subject to many different kinds of processes?
Sorry. I have no model for that, and my answer is I really don’t know. That is similar to a lot of climate scientists mind you. The difference is that they do have a model, but really don’t know.