Diogenes wrote:tomclarke wrote:
I do in fact fully understand your point. I've tried to explain why it is a misconception several times. In fact it is a
popular AGW myth and people other than me have spent time trying to explain as well.
short answer:
H2O equilibrates with a few weeks - amount in air depends only on temperature.
CO2 does not equilibrate: amount in air is fixed.
Long answer from
my fave semi-skeptical AGW blog
This answer that you gave demonstrates what I can only perceive as a logical disconnect that is simply incomprehensible to me. You acknowledge the pieces necessary to understand this, but you can't seem to grasp what this information is telling you.
The diffusion of water into the air is pretty much a continuous process, and it is irrelevant that a particular molecule of H20 only remained in the air for a few weeks or so. It's brethren are always in the air, and in that regard, it might as well be considered to be permanently in the air, in the same manner as CO2. For all intents and purposes, it effectively is.
Do you understand what equilibrium means? In this context it means that the water vapour in the air will reach a level determined by temperature. If more than this, it will form surface water to reduce level, if less, surface water will evaporate to increase level, this process happens within a few weeks to keep water vapour level at that determined by temperature.
Therefore humans adding water vapour to air is irrelevant, the amout we add is insignificant compared with the this equilibrium.
Whereas CO2 in the ir stays in the air. It can't go anywhere else.
So - the first part is do you agree with this essential difference between water vapour and CO2? They are both greenhouse gasses, but adding CO2 changes things, adding water vapour has no effect (the extra water just condenses, and does not accumulate over years).
Apart from that, you even acknowledge in your own words above that it is temperature dependent, seemingly unaware that this is the most important characteristic. It INCREASES with Temperature, so not only is it effectively lingering in the air in the same manner as CO2, it's presence INCREASES every time the temperature increases.
It is in the air, yes. It is a very significant GHG, yes. It contributes a positive feedback, yes.
That is what I've been telling you throughput this thread. I've also been saying this does not lead to runaway because the BB radiation provides a larger negative feedback.
According to this chart, the specific heat of water is 1.93 kJ/kg K. (~0.40% over full atmosphere, typically 1%-4% at surface)(At constant pressure, which the Atmosphere mostly is.)
CO2 comes in at 0.844 kJ/kg K,(0.039445%) and Oxygen is 0.919, (20.946%) while Nitrogen is 1.04. (78.084%) Why are we even discussing Carbon Dioxide?
Water Vapor is a better greenhouse gas than is Carbon Dioxide, both in spectral absorption and in specific heat capacity. It also produces the largest effect of all the greenhouse gases.
Specific heat capacity is not relevant here. The radiation budget trumps the effect of vapourisation. Yes H2O is about 2X better than CO2 as a GHG (think I've got factor right).
The issue is that H2O as above cannot be artficially increased by any significant amount. So although it is the most important GHG gas it has no anthropogenic component and need be considered only as a feedback. Of course the fixed amount of H2O is very important as well as the change with temperature - without the large greenhouse effect from this the earth would freeze.
CO2 can be increased, and we are doing that. The CO2 we add stays in the atmosphere and contributes to a large GH effect than would be true at lower levels of CO2. The overall greenhouse effect (H2O+CO2 + some other minor stuff) makes earth livable, maybe +33C or something. That gives us the baseline temperature. The AGW question is how does the temperature change from that baseline on addition of CO2.
I've looked at your link, and I've read many of the comments there.
But clearly not understood the basic theory, why CO2 and H2O do not work in the same way, although both are GHGs.
Oddly enough, many of the commenter seem to comprehend what I am talking about better than do you. They also point out a lot of things which I thought were self evident, but apparently not to everyone.
As I said, misconceptions about basics are common. You are in good (bad?) company. But it is not very complex, so I hope you can understand the difference between H2O as feedback and CO2 as forcing, given the different ways I have explained it.
Let me ask: after reading this, suppose 10Mton water is emitted into atmosphere by humans in just one day. Does it change atmospheric composition, and hence the radiation balance, after a few weeks?
Now 10Mton CO2 emitted into atmosphere. Same question?
Now after 20 years what is the atmospheric change from continuous H2O emission each day as above? (Answer, 14*10=140Mton)
What CO2? Answer 20*356*10 = 71000Mton
Thus CO2 emmissions make a big difference, because they accumulate. H2O emmissions make negligible difference because they do not accumulate.
(Such as the presence of Carbon Dioxide not being in constant distribution throughout the atmosphere.)
CO2 is different locally close to CO2 sources. But this has no GHG effect because too thin a layer is affected. In atmosphere CO2 mixes very well, with differences maybe a few % if that. Of course CO2 concentration changes with air pressure, and also there is a very small CO2 gradient with height since the MW of air is not identical to that of CO2.
All these things have been considered and added to accurate models. This is basic work that was done maybe 50 years ago.
So: why do you think people have got this wrong? And does the fact that textbooks over the last 50 years all agree, and don't see what you think is obvious, make you want to re-examine your own ideas?
Best wishes, Tom