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
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So the science isn’t settled then? sigh
I wonder if we could get Dr. Spencer to comment on the latent heat effect of the water cycle. As water evaporates from the surface, it carries latent heat with it, cooling the surface. Convective transport of this water vapor to higher altitudes effectively moves that energy around the greenhouse effect. When that vapor condenses, it not only removes the vapor, but emits that stored latent heat at a higher altitude, bypassing much of the greenhouse effect. If higher temperatures drive more evaporation and precipitation, then they also drive greater convective transport of latent heat. At least, that’s what appears to me.
I think I’ve never heard so loud
The quiet message in a cloud.
And now I hear so loud so clair
Resounding clarion in the air.
==================
My question reflects personal experience and recent comments on a blog:
more moisture in summer air still produces uncomfortable perceptions of heat;
more moisture while sailing led to severe hypothermia one summer in Seattle;
clothes air-dry indoors, here in San Jose Costa Rica, even during the wet season.
¿Would discussions of water vapor effects be more understandable to non-scientists were the focus on energy? Joules? Calories?
How come this missing “hot spot” isn’t being shouted from the rooftops, regarding AGW?
Here, surely, is one testable hypothesis of AGW. It failed this test, so the entire theory must be discarded and reworked.
Or has direct observation been retired as a scientific method and replaced with statistical extrapolation?
Rows and floes of angel hair
And ice cream castles in the air
And feather canyons everywhere
I’ve looked at clouds that way
But now they only block the sun
They rain and they snow on everyone
So many things I would have done
But clouds got in my way
I’ve looked at clouds from both sides now
From up and down, and still somehow
It’s cloud illusions I recall
I really don’t know clouds at all
Ok, if there is a decline in free tropospheric water vapor, could that mean an increase in stuff is bringing it down in the form of rain? Dust, ash, ions, pollen and spores, etc.
What John Galt said. Also more clouds leads to more sunlight being reflected straight back out into space.
Roy, I’m not sure if you read these comments but regarding your cloud feedback data, I’d suggest contacting Stewart Daw at ORNL to see if he can help with the data. He is very good at analyzing data from non linear systems. Your data reminds me of stuff he looked at about twenty years go.
John Galt – Bypasses the greenhouse effect? Let’s think about this. Latent heat effects aren’t always easy to figure out. So the ocean gets hotter, evaporates a bit more, the vapor travels up the atmosphere, collects a bit more radiation along the way and then heats up the upper atmosphere as it condenses. The upper atmosphere is now hotter and with a higher water mole fraction, absorbs more radiation…. No, that’s the case for global warming. I’m not sure why it would bypass the greenhouse effect What it may do though is reduce the temperature difference between the upper and lower atmosphere, make it more homogeneous. More liquid water falling would also make the atmosphere more homogeneous. More water vapor would lower the altitude where the water vapor starts to condense. Interesting. There’s something here that may explain the missing tropospheric “hot spot” but I’m not quite sure what.
Neil – It failed it a long time ago. It’s one of the big problems with climate models. I’m not sure how they’ve addressed it. Lindzen would talk about this frequently.
Bizarre schemes to build fleets of robot ships to put salt spray into the air and seed clouds to increase the Earth’s albedo are somehow regarded as viable proposals to combat global warming. Yet the increasing cloud cover caused directly by warming itself is not regarded as a cause of significant negative feedback. Consistency check anyone?
I wonder if Roy Spencer might be interested in the rather good correlation I found between specific humidity at the tropopause and solar activity:
http://tallbloke.files.wordpress.com/2010/08/shumidity-ssn96.png
Discussion here:
http://tallbloke.wordpress.com/2010/08/08/interesting-correlation-sunspots-vs-specific-humidity/
If solar activity is playig a role in controlling the level of humidity in the upper atmosphere at the boundary between the troposphere and the stratosphere, it would seem to me that the strong levels of solar activity in the latter half of the C20th may have had a much bigger part to play in ‘global warming’ than a calculation of the change in TSI at the top of the atmosphere would account for in simple energy terms such as those frequently cited by Leif Svalgaard.
If water vapour feed back were strongly positive, the Earth’s temperature would have spiralled out of control during the Roman Climatic Optimum and the Mediaeval Warm Period. This clearly did not happen. Vapour /condensate heat pipes in computer heat sinks work. Climate models assuming positive water vapour feedback do not.
“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.”
You and everyone else, Roy. This is the key issue yet we seem no nearer to solving it than we were ten years ago. Meanwhile huge sums are being wasted on yet more anecdotal observation to support a meme based on a few tenths of a degree rise in temperature (cause unknown) over a period of time that gets ever shorter as the odd wheel falls off. And the economic hinterland behind it grows more massive by the day.
This to me defines “Climate Science” – the business of reinforcing anecdotal evidence to support conjecture, supposition and an agenda while avoiding pursuit of the truth.
Surely it is not that difficult to roll out a programme to sort this issue out based on sound empirical evidence. In a rational world, is that not what the IPCC and all the rest ought to be doing? How much would it cost to improve water vapour measurements from the radiosondes you mention compared to launching yet more satellites to confirm water freezes and thaws seasonally by varying amounts? If you want to play with satellites how about a matrix of them to replace the aging ERBE and measure the earth’s radiation budget directly and accurately over the entire globe? The key question is why this hasn’t already been done as a matter of urgency.
Why, when it’s supposedly the survival of the human race that’s at stake? Hang the cost. There’s only so much spleen grease and post normal clap trap one can swallow before the obvious kicks in I’m afraid, no need for conspiracy theories.
Dinostratus says:
September 14, 2010 at 11:55 pm (Edit)
Roy, I’m not sure if you read these comments
I think Roy is busy enough doing science running his own blog. Why not drop by and post your comment on the original thread?
http://www.drroyspencer.com/2010/09/five-reasons-why-water-vapor-feedback-might-not-be-positive/
More moisture in the air, more clouds, rain, storms, which seek to remove the latent head and ensure stability. If these systems didnt exit, the earths atmosphere would have gone nuts a long time ago. Its no coincidence that the moonson permanently sits over the area of the earth recieiving the most energy.
You cant have more evaporation, cloud cover and rainfall whilst also having more warmth, it just doesnt work!
Thank you Dr. Spencer for your thoughtful and stimulating article. I read Paltridge et al. paper according to your suggestion, and understood your idea that precipitation may induce low specific humidity in upper-air.
One question on your statement. You explain the lacking of warming at the tropical upper troposphere as indirect evidence of the negative feedback, but there is an interesting recent article: R. J. Allen and S. C. Sherwood, “Warming maximum in the tropical upper troposphere deduced from thermal winds,” Nature Geoscience VOL 1 JUNE 2008, 399-403. Allen and Sherwood claim that reanalyzed temperature deduced from wind fields rather coincides with the warming at the upper troposphere which is predicted by models.
Of course, the warming at the troposphere upper air may be different matter from the climate sensitivity, but I wonder whether or not their result is consistent with your explanation on the effect of the precipitation.
Step by step we are approaching a refutation of the Global Warming Myth.
The two major battlefields are Feedback and Sensitivity.
Prove that negative feedback operates, and the hotter the world gets the more it cools, and the IPCC claims of positive feedback and tipping points are demolished.
Prove that there are forcings more significant than greenhouse gases, such as solar, such as aerosols, such as some as-yet unidentified forcing, and the IPCC’s other iconic graph – the bar chart on p.136 showing GHGs and trivial also-rans – is discredited. It’s too early to say “it’s the Sun, stupid”, but the astrophysicists are boarding the troopship.
Latent heat is most important. Evaporation needs latent heat so will cool the surface from which the water is being removed. Once in the atmosphere this heat is given up during the condensing into cloud. It is cloud that has both positive and negative feedbacks depending on situation and acts as a regulatory control on climate. Water vapour will carry heat around the atmosphere, as latent heat. once this heat is released, during condensation, it is free to warm the surrounding atoms in the atmosphere or escape to space.
As I have stated earlier, I personally suspect that role played by earthshine emitting/absorbing (greenhouse) gases at the tropopause level may be being underestimated as a mechanism for removing convected heat from the atmosphere. I would think that the most important spectra may be those lines that have a 50 percent chance of getting out from that level at the typical temperature of 220 K as I believe these should have the highest relative emissivity.
The important parameter for measuring how effective convection is as a thermal regulator of surface temperatures would be a determination of how the effective surface temperature, as controlled by the adiabatic lapse rate from the tropopause temperature, would change as a function of the total energy being convected upward. This would account both for tropopause altitude changes and temperature changes with increasing upward convected thermal energy. I do not know if anyone has ever attempted to estimate or determine this.
Mike D. at 11:36 should have credited his words. I believe their creator was Joni Mitchell.
Now that I have seen latent energy brought up: should the quantity of energy needed to melt ice and evaporate water be somehow included in the calculations of percent increase in temperature? In other words, if ice goes from -1C to water at 1C it has increased in temperature only two degrees Kelvin, but its energy absorption is a rather large multiple of this. I feel a bit like the spectator who runs out on the field when I ask this, but it is something that I have been wondering for some time.
Again, positive feedback, if it exists in the climate system, would have caused the Earth to fry, or freeze, billions of years ago.
Dinostratus,
I’ll try to respond. First I think it’s the air over the ocean, not the ocean itself, that’s warming, minor point. I think you’re not considering the magnitude difference between specific heat and latent heat. The specific heat of water is 1 cal/g-K. The heat of vaporization is 539 cal/g. That 539 cal then, rather than trying to radiate from the surface, through the rather dense, ghg laden atmosphere, moves by convection, which is unaffected by the greenhouse effect. At some higher altitude, above much of the greenhouse effect, the water vapor precipitates, releasing this 539 cal. This energy is now able to radiate upward with much less greenhouse effect. This is what I mean by going around the greenhouse effect. It works much like a heat pipe, which is much more effective than radiation. A small additional effect is that the water falls back to the surface at a lower temperature than it left at, adding a few more cal’s of cooling.
During the time the water vapor is in the atmosphere, it’s a positive radative feedback. The evaporation/precipitation cycle is a negative convective feedback. The longer the cycle is, the greater the radiative effect will be wrt the convective effect. As the evaporative effect becomes greater, due to surface warming, the cycle will shorten, precipitation will increase. This should move the positive/negative balance toward negative.
I haven’t tried to calculate how much delta warming a given mass of water contributes per day vs. the latent heat it moves. I suppose I should, but I expect Dr. Spencer already has that answer. I do know, just from peeling the lid off of something from the microwave, that the latent heat of water is nothing to trifle with.
I posted this comment at Roy Spencer’s blog, still in moderation:
——–comment——
I think it’s very complicated and I can’t comment on “most researchers” whereas you would probably know a lot of them – plus their papers may well be more cautious than their general views.
But the papers I’ve read don’t appear to deal with it simplistically and do cover these same questions. Of course some of the papers I’ve read are “Spencer & Braswell 1997″, Lindzen, but also Soden & Held, Sun & Oort, Bony & Duvel, Ramanathan, many others.
In Clouds and Water Vapor Part Two I cited Held and Soden (2000):
Held and Soden have their point of view but in theirs and most of the papers I don’t see a simplistic approach.
—end of comment—
Water vapor feedback is a complex subject. You can see the approach of the great Ramanathan in Clouds and Water Vapor – Part One. For those who don’t simplistic answers, it’s worth taking a look. Many interesting comments as well.
Water vapour can not be a positive feedback as there is no limit to available water and consequently if it were a positive feedback the atmosphere would be saturated with respect to water vapour which it is not.
If the atmosphere warms, the amount of water vapour it can hold increases, therefore if there were a positive feedback it would not matter what caused the warming, it would kick of a positive feedback until the atmosphere became saturated with water vapour. This should, therefore, have happened in the last El Nino event, which it clearly didn’t.
The idea that water vapour can be a positive feedback makes no scientific sense. It is blatantly obvious that there is a large negative feedback that keeps it in balance. Clouds and rain seem the most obvious.