Five Reasons Why Water Vapor Feedback Might Not Be Positive
By Dr. Roy Spencer
Since it has been a while since I have addressed water vapor feedback, and I am now getting more questions about it, I thought this would be a good time to revisit the issue and my opinions on the subject.
Positive water vapor feedback is probably the most “certain” and important of the feedbacks in the climate system in the minds of mainstream climate researchers. Weak warming caused by more carbon dioxide will lead to more water vapor in the atmosphere, which will then amplify the weak warming through water vapor’s role as the atmosphere’s primary greenhouse gas.
Positive water vapor feedback makes sense intuitively. Warmer air masses, on average, contain more water vapor. Warmer air is associated with greater surface evaporation rates, which is the ultimate source of almost all atmospheric water vapor.
And since water vapor is the atmosphere’s main greenhouse gas, most scientists have reasonably inferred that climate warming will be enhanced by increasing water vapor amounts. After all, water vapor feedback is positive in all of the IPCC climate models, too.
But when one looks at the details objectively, it is not so obvious that water vapor feedback in the context of long-term climate change is positive. Remember, it’s not the difference between warmer tropical air masses and cooler high-latitude air masses that will determine water vapor feedback…its how those air masses will each change over time in response to more carbon dioxide. Anything that alters precipitation processes during that process can cause either positive or negative water vapor feedback.
Here are some of those details.
1) Evaporation versus Precipitation
The average amount of water vapor in the atmosphere represents a balance between two competing processes: (1) surface evaporation (the source), and (2) precipitation (the sink). While we know that evaporation increases with temperature, we don’t know very much about how the efficiency of precipitation systems changes with temperature.
The latter process is much more complex than surface evaporation (see Renno et al., 1994), and it is not at all clear that climate models behave realistically in this regard. In fact, the models just “punt” on this issue because our understanding of precipitation systems is just not good enough to put something explicit into the models.
Even cloud resolving models, which can grow individual clouds, have gross approximations and assumptions regarding the precipitation formation process.
2) Negative Water vapor Feedback Can Occur Even with a Water Vapor Increase
Most atmospheric water vapor resides in the lowest levels, in the ‘turbulent boundary layer’, while the water vapor content of the free troposphere is more closely tied to precipitation processes. But because the outgoing longwave radiation is so much more sensitive to small changes in upper-layer humidity especially at low humidities (e.g. see Spencer & Braswell, 1997), it is possible to have a net increase in total integrated water vapor, but negative water vapor feedback from a small decrease in free-tropospheric humidity. See #4 (below) for observational support for this possibility.
3) Cause Versus Effect
Just because we find that unusually warm years have more water vapor in both the boundary layer and free troposphere does not mean that the warming caused the moistening.
There are a variety of processes (e.g. tropospheric wind shear causing changes in precipitation efficiency) which can in turn alter the balance between evaporation and precipitation, which will then cause warming or cooling as a RESULT OF the humidity change – rather than the other way around.
This cause-versus-effect issue has been almost totally ignored in feedback studies, and is analogous to the situation when estimating cloud feedbacks, the subject of our most recent paper.
Similar to our cloud feedback paper, evidence of causation in the opposite direction is the de-correlation between temperature and humidity in the real world versus in climate models (e.g. Sun et al., 2001).
4) Evidence from Radiosondes
There is some evidence that free tropospheric vapor has decreased in recent decades (e.g. the Paltridge et al., 2009 analysis of the NCEP Reanalysis dataset) despite this being a period of surface warming and humidifying in the boundary layer. Miskolczi (2010) used the radiosonde data which provide the main input to the NCEP reanalysis to show that the resulting cooling effect of a decrease in vapor has approximately counterbalanced the warming influence of increasing CO2 over the same period of time, leading to a fairly constant infrared opacity (greenhouse effect).
Of course, water vapor measurements from radiosondes are notoriously unreliable, but one would think that if there was a spurious drying from a humidity sensor problem that it would show up at all altitudes, not just in the free troposphere. The fact that it switches sign right where the turbulent boundary layer pushes up against the free troposphere (around 850 mb, or 5,000 ft.) seems like too much of a coincidence.
5) The Missing “Hot Spot”
Most people don’t realize that the missing tropospheric “hot spot” in satellite temperature trends is potentially related to water vapor feedback. One of the most robust feedback relationships across the IPCC climate models is that those models with the strongest positive water vapor feedback have the strongest negative lapse rate feedback (which is what the “hot spot” would represent). So, the lack of this negative lapse rate feedback signature in the satellite temperature trends could be an indirect indication of little (or even negative) water vapor feedback in nature.
Conclusion
While it seems rather obvious intuitively that a warmer world will have more atmospheric water vapor, and thus positive water vapor feedback, I’ve just listed the first 5 reasons that come to my mind why this might not be the case.
I am not saying that’s what I necessarily believe. I will admit to having waffled on this issue over the years, but that’s because there is evidence on both sides of the debate.
At a minimum, I believe the water vapor feedback issue is more complicated than most mainstream researchers think it is.
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Be sure to check out Dr. Spencer’s book:
The Great Global Warming Blunder: How Mother Nature Fooled the World’s Top Climate Scientists
Highly recommended – Anthony
“”” Bob_FJ says:
September 15, 2010 at 11:24 pm
Surely everyone has heard of evaporative cooling?
* Why is it said that a healthy dog has a cold (moist) nose?
* Why did the young Jacqueline Bisset become famous for her wet T shirt scenes in the movie “The Deep”, of 1997?
http://www.mademan.com/chickipedia/jacqueline-bisset/
SORRY, but to be more serious:
The K & T “Earth’s Energy Budget” diagram, (in some quarters referred to as the “Trenberth Cartoon), in the IPCC’s AR4 report of 2007, claims that by far the greatest proportion of energy leaving the Earth’s surface is via evapo-transpiration. (~46% of all the four basic processes).
So, if there is warming, particularly of the ocean surfaces, should there not be a nominal increase in evaporation? (cooling). Given that the greatest proportion of cooling is suggested to be evapo-transpiration? Surely a small increase in this would be significant.
Yes; I agree, it is complicated, but why is there not more attention paid to this basic point in Physics?
BTW, there is an elegantly simple explanation for evaporative cooling in quantum theory: Any fluid has a mix of molecules of varying energy levels. (speeds). Thus the hotter molecules are more able to escape and leave a higher proportion of lower energy (colder-slower) molecules behind. “”””
Well I wouldn’t exactly call it quantum theory; just ordinary classical statistical Mechanics. For an “ideal” gas there is the Maxwell-Boltzmann distribution of velocities; and in liquids you have a similar distribution; but as you say; the higher energy molecules are the ones that have a greater probability of escaping from the surface into the atmosphere; which leaves a lower average molecular energy behind which is thus a cooling of the surface.
But this is peanuts compared to the Latent heat of Evaporation that leaves the liquid upon evaporation. That energy really is the energy that is required to separate the moleculse so they can act independently of each other except for the occasional collisions; whereas in the liquid phase they are somewhat bound to each other by attractive forces.
In the ideal gas the RMS velocity of the molecules is proportional to the square root of the Temperature (Kelvins); and this velocity distribution is part of the reason for a Doppler broadening of the spectral absorption lines with Temperature; although it is a fairly small effect at atmospehric Temperatures.
George E. Smith,
It is interesting that you would use the example of radio receivers as an example, as I have spent most of my 30 years as an RF engineer. I thoroughly understand systems with gain that are dominated by negative feedback, i.e. amplify and are stable, regardless of the particular feedbacks within the system.
The key to my statement was “dominated by positive feedbacks”. To state it more clearly, Beta greater than 1, which is what I hear climate scientists saying all the time (in words). The point was made by Joel Shore that “what they really mean is…” and then explains that they use the same terminology as engineering control theory, but change the meaning of the words. I don’t think I agree with Joel’s statement. I think the language is used specifically to cause people to come to certain conclusions. The very phrase “tipping point” defines an unstable system dominated by positive feedback.
I stand by my statement that systems dominated by positive feedback are unstable. Certainly, positive feedback has been routinely used for force modification (power assist brakes, aircraft control systems, etc), but a careful analysis will find that the system is stable over the control range, i.e. dominated by negative feedback, regardless of the amplification of the input control signal.
Gerry
Geoge E Smith
Sept 16, 7:10pm
George, I find the arguments along that line to be rather effect at getting warmistas to quickly change the subject to avoid it.
The latent heat absorbed from evaporation carries far more energy than a few degrees of additional temperature and it is carried aloft by lower density moist air where it is released as it must be given off for condensation high up in the air, above most of the water vapor lower down – which is the dominant ghg. The condensation forms into water droplets or ice, becoming radiators of a continuum rather than a spectrum at the cloud tops.
The supposed added energy in the system due to more co2 is mostly hitting the water surface and not penetrating any distance below the surface skin like visible light is only going to raise the energy of the surface skin because all the heat flow mechanisms are working against it heating downward – at least for almost all the surface area. Ever try to boil a pot of water by heating the top? You’ve got a nice heating mechanism to provide the latent energy right where it is needed. Or should I say you’ve got a very nice mechanism to suck out the added IR and carry it out from the surface area. The formation of clouds and blocking of incoming sunlight will have even more of an effect to reduce warming. That leaves the warmistas with the problem of how to explain that more h2o vapor and a more active water vapor cycle will result in substantially less cloud formation that will allow more sunlight energy in.
The ‘hot spot’ is a signature of the atmosphere warming from any cause, not just CO2.
If the sun warmed the atmosphere, you should get a ‘hot spot’, too. Same with aliens shooting lasers into the atmosphere ;-).
So, an alleged missing hot spot is not the death knell to AGW. At most it suggests that we don’t have a good handle on how heat is transported through the atmosphere.
Now, if the stratosphere showed no cooling over a climatically significant period (25 years would do), then that would be a big problem for AGW. A cooling stratosphere IS a signature of AGW. If the sun was heating the atmosphere, the stratosphere would heat, too.
Over the long term, the stratosphere has been cooling.
“”” cba says:
September 17, 2010 at 4:48 am
Geoge E Smith
Sept 16, 7:10pm
George, I find the arguments along that line to be rather effect at getting warmistas to quickly change the subject to avoid it.
The latent heat absorbed from evaporation carries far more energy than a few degrees of additional temperature and it is carried aloft by lower density moist air where it is released as it must be given off for condensation high up in the air, above most of the water vapor lower down – which is the dominant ghg. The condensation forms into water droplets or ice, becoming radiators of a continuum rather than a spectrum at the cloud tops. “””
cba, it frustrates the hell out of me; that the evaporative cooling principle seems to go right over so many people’s heads.
I don’t keep a table of the heat capacity of water vapor in my head; but the way I fake it, is to remember that at the normal boiling point (100 deg C) the Latent heat of evaporation (at astp) is (or used to be) 539 cal/gm. If the temperature of evaporation is lower than that; say 30 deg C, then I simply add on one extra calorie for every degree below 100. I don’t know whether that is because of the Principle of cussedness, or what but it just seems to be the right thing to do, since that many calories is what it would take to get the 30 deg water to 1oo and then another 539 to boil it.
And that is just so much more energy than the question of the high velocity tail of the Maxwell-Boltzmann statistical spread issue that Gerry raised. His point is valid; it’s just not material because of the latent heat.
Then the other issue is that direct solar spectrum 6,000K BB spectrum energy from the sun, penetrates deeply (coupla hundred metres) into the ocean; so it doesn’t immediately inspire a whole lot of evaporation.
On the other hand the LWIR emitted by the atmosphere or clouds, that reaches the surface (ocean), is absorbed in the top 10 microns of the ocean surface. At > 2.0 microns wavelength the absorption coefficient of water is about 1000 cm^-1, so the 1/e attenuation depth is 10 microns; so ok 50 microns for 99% absorption. At 3.0 microns the coefficient goes to a wild 8,000 cm^-1, but beyond that it settles into the 1-2,000 range.
So whether that atmospheric radiation was due to CO2 or H2O or CH4 GHG heating; or whether it was caused by direct H2O vapor absorption of incoming solar energy in the 0.75-3 micrin range; that LWIR is the primary instyigator of surface evaporation. How many times have I read here, that the warmer atmosphere encourages more evaporation ? No it doesn’t; it is the warmer water that encourages evaporation; the H2O molecule doesn’t even know the atmosphere is there, warm or not, until it breaks free of the surface and eventually clobbers into some N2/O2/Ar critter.
Yes it is nice to have warmer air to hold more water; but the water is going to rise any way to where the air is colder; it is even nicer to have some wind to carry off that H2O before it gets a chance to return to the surface.
I’m not a chemist; but even I know that physical/chemical reaction rates can be limited by the ability to remove the reaction products from the reaction site. whether you are laying down epitaxial Gallium Arsenide or boiling water; you better deal with the effluent at the interface, if you want the reaction to continue.
That fundamental distinction between the “Forcing” due to solar spectrum energy hitting the water, and that due to LWIR thermal radiation doing the same thing, is fundamental to understanding those interractions.
Too many so-called climate scientists seem to think that one “forcing (w/m^2) ” is as good as another.
No! they behave quite differently.
And once that H2O escapes the surface; it just keeps on rising until it gets high enough and cold enough to revert to liquid/solid and relinquish that 609 cal per gr or even 689 if it becomes an ice crystal. And that energy is all transported to where ther is much less gHG interference with the thermal radiation that is generated at that altitude.
The evaporation/Convection thermal mechanisms are the biggest energy transporters (at these Temperatures); and that is why my computer CPU has a phase change convective heat pipe cooler. Eventually, Stefan-Boltzmann wins the game at higher Temperatures; but in the atmosphere it is the swamp cooler mechanism that works best.
I did post something over at Dr Spencer’s blog; but I guess I must not have explained myself too well; because mine seems to be the only post that he didn’t respond to. Well maybe what I wrote looks like too much of a bunch of rubbish.
I probably should have used the proper scientific (climate) terms so he could understand what I was saying.
ALL: Correction to my September 15, 2010 at 11:24 pm
The movie “The Deep”, which demonstrates one effect of evaporative cooling, was released in 1977, not 1997
George E. Smith Reur September 16, 2010 at 7:10 pm
I do not see evaporative cooling as a separate process to latent heat loss. In the case of evaporation of water, this phase change from liquid to vapour (gas) is very slow, (compared to that of vapourisation of boiling point), but my quick Google around gave the energy involved for evaporation at ~600 calories/gram. This is a measure of the cooling effect which also involves a change in temperature, as distinct from as in the boiling of water.
Well I [George] wouldn’t exactly call it quantum theory; just ordinary classical statistical Mechanics.
Perhaps I should have described the process as “Quantum Mechanics” instead of “Quantum Theory”, since the theory of escaping higher energy molecules is so well demonstrated. (Has it actually been observed?)
George Smith
yup, low down, it’s mostly convection (latent heat). radiative only works where you’ve got a temperature differential and where there’s lots of h2o vapor, the temperature differential is quite small over a path length. Another pair of factors going on is that as one rises in the atmosphere, the pressure drops and that reduces the effect of line broadening. As lines narrow, there’s less and less absorption happening.
as i recall, there’s some biology (stamp collector according to Rutherford) that decided to try to measure the T at the ocean surface and discovered it was lower at a ‘skin’ so that supposedly reduced the heat flow from down deep if the skin warmed up a bit. LOL. He totally failed to realize that the skin is at a lower temperature because it is incapable of maintaining a higher temperature because it’s already moving out all the power coming from below and from above.
I tried posting the following over at Roy Spencer’s blog, but it has been stuck in modertion for three days. (it seems that the spam filter did not like the links)
Christopher Game, thanks your reply of September 22, 2010 at 12:26 PM , Concerning the potential feedback resulting from evapotranspiration. (E-T)
The IPCC has estimated in 4AR that the forcing for anthro’ CO2 is about 1.7 W/m^2, and that the net for all anthropogenic forcings is about the same value.
http://en.wikipedia.org/wiki/File:Radiative-forcings.svg
Adding to my earlier comment, according to Trenberth, 2007 levels of E-T resulted in a forcing of -78 W/m^2. Thus, without disputing his numbers, if (E-T) were to increase by say 1% in order to achieve some arbitrary increased water vapour level, there would be a feedback of -0.78 W/m^2, which is rather significant by comparison.
Dare I add that Trenberth also gives “Thermals” as another -24 W/m^2, and that I think that there should also be an increase in thermals too.
Add clouds, and how now to 2xCO2 sensitivity, I would think.
If you need it, the Trenberth chart is here:
http://www.flickr.com/photos/26175880@N05/3065365160/
What do you think?
barry says:
September 17, 2010 at 10:05 am
Now, if the stratosphere showed no cooling over a climatically significant period (25 years would do), then that would be a big problem for AGW. A cooling stratosphere IS a signature of AGW. If the sun was heating the atmosphere, the stratosphere would heat, too.
Over the long term, the stratosphere has been cooling.
“Scientific Assessment of Ozone Depletion: 2010
New analyses of both satellite and radiosonde data give increased confidence in
changes in stratospheric temperatures between 1980 and 2009. The global-mean
lower stratosphere cooled by 1–2 K and the upper stratosphere cooled by 4–6 K between1980 and 1995. There have been no significant long-term trends in global-mean lower stratospheric temperatures since about 1995. The global-mean lower-stratospheric cooling did not occur linearly but was manifested as downward steps in temperature in the early 1980s and the early 1990s. The cooling of the lower stratosphere includes the tropics and is not limited to extratropical regions as previously thought.