Diogenes wrote:tomclarke wrote:Diogenes wrote:
You seem as if you are right on the edge of grasping my point.
You grant that H20 diffusion increases with heat. You grant that H20 is a far better absorber of IR and other radiation, ala the "greenhouse effect." You grant that these two things together constitute a positive feedback effect which according to the theory ought to result in an ever increasing temperature.
OK - this is your misunderstanding. Positive feedbacks do not result in ever-increasing temperature unless they are so large as to make the system unstable.
How do you figure that positive feedbacks do not result in ever increasing temperature? My understanding is that a positive feedback will cause a temperature increase until it is offset by some effect which will halt or neutralize it.
Stability, or lack of it is based on overall feedback.
"A positive feedback" does not mena the system as a whole has positive feedback. In this case the feedbacks on the earth surface temperature include a factor of:
-5.5 W/Km^2.
In other words, for every degree K rise in temperature, the power lost from black body radiation increases by 5.5W.
Suppose you have a
positive feedback from H2O of 2W/Km^2 and a
forcing of 1W/m^2 from CO2.
Without the H2O effect the 1W/m^2 would combine with the -5.5W/Km^2 to give 0.18C temp rise.
With the H2O effect the overall feedback is still negative, but only -3.2W/Km^2.
So the temp rise is now 0.3C, and increase (or amplification) of slightly more than 50%.
tomclarke wrote:
Basically there is a stabilising negative feedback from the black body radiation out, which increases as temperature increases.
If other effects are held stable, sure. An Increase in temperature would result in increased radiation, but I think you are oversimplifying by modeling the system as a black body with a fixed radiation emission. I think the system also varies in volume due to the atmosphere increasing in size with temperature.
I am vastly simplifying the system. But it would be strange indeed of energy lost from the earth to space did not go up with the earth temperature, whatever extra effects atmosphere adds. And indeed the climate modellers who have looked in detail at the effect of atmosphere find that the result is not that different from black body radiation, however the temperature is a bit lower than the surface temp.
tomclarke wrote:
As long as total other feedback is less positive than this is negative the system will be stable, but with forcing inputs effectively amplified. I'll give you the math if you need it, it is simple.
Math is a great thing, but when you don't put in all the variables, your equation will be incomplete. That is what I think is happening with your's, and other's models. For example, water both absorbs and emits radiation in the spectrum as outlined by the chart I posted up thread, but I believe this chart is for molecular water. I do not believe this chart accurately represents the reflection characteristics of water in droplet form. (in air) I believe water in droplet form will reflect all spectra (to some extent) which possesses a wavelength shorter than double the diameter of the water droplet size.
Indeed my approximation is very incomplete. That is why atmosphee radiation balance have much more complex models. Droplets in air => cloud and they include this. You are right that things are more complex with clouds, and whether the effect is heating or colling depends on the height of the cloud.
If you read the detailed work (now quite old) from climate modellers on the effects of cluds you will get the full messy equations.
I believe this characteristic makes water far more reflective than is it's ordinary absorption spectrum. Does your model take this phenomena into account?
My model does not, but people who do this can accurately model clouds. the difficult issue is to determine how temperature affects cloud cover. But for typical cloud cover the basic relationships hols, it is just a slight modification.
Show me your math. I think some of your assumptions regarding forcings and feedbacks are not accurate.
I'm not sure what you are asking for. The example above shows that a positive feedback can exist while the system as a whole stays stable. As for whether the overall feedbacks are negative well it is pretty clear that they are because we get small chnages in earth temperature not jumps. But to prove this you ned detaild quantitative values for all feedbacks, positive and negative.
I think I've answered your original reason for disbelieving the climate science - that you could not see how the system could remain stable with positive feedback from H2O?
But if you want me to write stuf down more formally as a heat balance equation I could?
tomclarke wrote:
Where I don't think you are following is in the area of what's stopping the positive feedback from creating a runaway greenhouse effect. Water vapor flips from a positive feedback effect, and it BECOMES a negative feedback effect. Rather than continuously increasing the temperature via absorption, it actually reduces temperature via reflection.
All these effects, feedbacks and forcings, are for small peturbations approximately linear.
How can you say that the reflectivity of water is a SMALL perturbation? Is there a larger effect, other than perhaps the Sun itself? I would like to see you produce a list of effects ordered from the largest in magnitude down to barely significant.
You are misunderstanding. The issue is that extra CO2 in atmosphere makes a small change to what would be the case without the (extra) CO2. This small change is a peturbation in the heat balance, to track it you can treat the whole system as linear, because the changes are small. (1K is small compared with 250K). The
overall effects of H2O can be much larger than the change as the result of increased CO2.
tomclarke wrote:
As above, a stable surface temperature results from forcings + feedbacks as long as the total feedbacks (including BB radiation - the big one) are negative. Adding a positive H2O feedback to the mix then amplifies the effect of forcings without leading to instability.
But it is my contention that Water flips from being a positive feedback effect, into a NEGATIVE feedback effect, because it forms clouds high in the sky that reflect a significant portion of radiation before it gets an opportunity to warm the atmosphere and the ground beneath the clouds.
Diffusion increases with temperature, and until clouds form, the diffusion and temperature produce a positive feedback effect which eventually becomes offset by the NEGATIVE effect created by denser cloud cover. At some point, the constant diffusion of water into the air results in a balance between heat absorption and radiation reflection. The system attains equilibrium because the opposing forces of positive and negative feedback cancel each other out.
Water is the dominate effect.
It is arguable that change in cload formation could make the overall effect negative. But you need evidence. The evidence (go read the literature) is against you on this. Certainly whether clouds form at high or low altitudes is complex and as one type is positive the other negative whether cloud cover overall makes a positive or negative extra fedback is not going to be simple. However it can be possible to bound the overall effect (because we know what is max change in cloud cover over globe at differnet temps) even if all details are not known.
I'm afraid this is a "go read the literature" for more details.
tomclarke wrote:
For larger changes the system will indeed have many nonlinear effects which come into play. But this is a second-order effect.
Negative feedback can achieve a pseudo stability even with non-linear systems. They may tend to oscillate more, but they will oscillate around an attractor.
For small peturbations all systems look linear
tomclarke wrote:
My argument is that the system is self regulating, and the dominate component of this regulation is the ability of water vapor to reflect radiation away from the planet. It's reserve ability to accomplish this is so great as to make all other factors trivial and irrelevant. No plausible amount of CO2 or Methane can possibly overcome water vapor's ability to compensate for it.
I can do the maths to show you. Quantitatively the overall greenhouse effect is roughly 50% - the surface radiation is approx 390W/m^2 - the BB radiation to space is approx 237W/m^2. Of this 153W/m^2 greenhouse effect H2O contributes roughly 95 and CO2 roughly 50.
I have no doubt that you can produce an equation that yields a result of some sort. I have doubts that your equations accurately represents the system which you are trying to model with them.
What number are you using for surface area? Did you consider that the radiating area of the atmosphere changes with temperature, and that it is not a constant?
For BB radiation from earth I'm looking at other people's work. No doubt there is a correction for atmospheric expansion. But if so this cannor affect the overall negative sign of the BB radiation feedback.
Where do you get your numbers for H2O, and where do you get your numbers for C02?
I was giving exmplary numbers, I got them from an internet page somewhere.
I think water in droplet form emits more than it does in molecular form because it creates trillions of parabolic reflectors which are structures that ought to reflect (to some degree) every wavelength within a range of double the droplet size.
This paper appears to support that contention.
I think what you mean is that clouds are more opaque (for given thickness) than air. Also whereas air has absorption (and therefore re-emmission) that peaks in infra-red, clouds absorb visible light equally. So they would be less good at providing GH effect. But actualy this has been calculatd, and it epends on the type (and height) of the clouds.
This stuff is basic and has ben worked out in detail by many people. Unless they are all in conspiracy together I would trust the generally accepted ideas.
tomclarke wrote:
These are total figures. We then consider how the figures
change if for example CO2 doubles and therefore forcing is 1.5W/m^2, and methane increases by more than 2 for a forcing of 0,5W/m^2 (this is the current increase in GH effect over pre-industrial times- that 50 becomes 52). That will increase temperature and as a result the H2O GH contribution will also increase (say by roughly 2W/m^2 for example). The total forcing of 4W/m^2 is then balanced by BB radiation feedback of -5.5W/Km^2 to give total surface temp increase of roughly 0.8K instead of the 0.25K expected from CO2 forcing alone.
The negative feedback capabilities of water vapor are simply too large to be seriously impacted by any other gas. Water vapor has too much leverage over any other effect. They (other gases) are as to a mosquito trying to hold back an automobile.
You can see from the maths and figures that the mosquito does have a significant effect. If you diagree with the ballpark figures here we can go through the emmission characteristics at different frequencies and CO2/H2O absorption, and work out from first principles for a single atmospheric layer, then integrate over many layers. I'm prepared to do this at a very coarse level of approximation.
Best wishes, Tom
Now that sounds like a much better way to start. I would like to see how you obtain your figures for H20 and C02 radiation, and why you think the figure for H20 will be a sort of constant. I think it will increase and decrease in relation with the quantity of H20 in the air.
Umm, I think I left out /K. So the overall feedback for H2O is a difference in heat loss that varies linearly with temperature (for small chnages in temp). As you say, this is because greater temp => more H2O vapour => more GH effect => less heat loss.
Overall this is modelled (in linear approx) as x W/Km^2
where x detemined the size and sign of x determines whether positive or negative.