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.
=================================================
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
Discover more from Watts Up With That?
Subscribe to get the latest posts sent to your email.
Dr . Spencer is asking the most important question of all and thinking about the possible answers(s). He, and others asking similar questions, know that the “science is (not) settled”. With an open mind, we look forward to further work in this crucial area!
Is there a published global summary of annual surface precipitation over the years, like there is an effort to have a global temperature summary? If so, could this help solve some questions raised above? Ditto relative humidity?
Ferenc M. Miskolczi’s paper Greenhouse Effect in Semi-Transparent Planetary Atmospheres published in the Quarterly Journal of the Hungarian Meteorological Service in 2007 explores the effects of humidity.
http://met.hu/doc/idojaras/vol111001_01.pdf
“Abstract—In this work the theoretical relationship between the clear-sky outgoing infrared radiation and the surface upward radiative flux is explored by using a realistic finite semi-transparent atmospheric model. We show that the fundamental relationship between the optical depth and source function contains real boundary condition
parameters. We also show that the radiative equilibrium is controlled by a special atmospheric transfer function and requires the continuity of the temperature at the ground surface. The long standing misinterpretation of the classic semi-infinite Eddington solution has been resolved. Compared to the semi-infinite model the finite
semi-transparent model predicts much smaller ground surface temperature and a larger surface air temperature. The new equation proves that the classic solution significantly overestimates the sensitivity of greenhouse forcing to optical depth perturbations. In
Earth-type atmospheres sustained planetary greenhouse effect with a stable ground surface temperature can only exist at a particular planetary average flux optical depth of 1.841 . Simulation results show that the Earth maintains a controlled greenhouse effect with a global average optical depth kept close to this critical value. The broadband
radiative transfer in the clear Martian atmosphere follows different principle resulting in different analytical relationships among the fluxes. Applying the virial theorem to the radiative balance equation we present a coherent picture of the planetary greenhouse effect.”
SPPI published a paper last December by another Hungarian scientist, Miklos Zagoni, discussing Miskolczi’s work, which was based on actual observations and measurements. You might recall that Miskolzci was a senior scientist at NASA, which refused to publish his work. The SPPI paper can be found here –
http://scienceandpublicpolicy.org/originals/co2_cannnot_cause.html?Itemid=0
Dinostratus says:
September 14, 2010 at 11:55 pm
Transport of energy from surface to altitude by latent heat of evaporation not only bypasses the greenhouse effect it reverses it. Once the heat is released at altitude via condensation the greenhouse gases below it now serve to insulate the surface from the heat of the cloud so instead of the GHGs keeping the surface warmer in this case it is keeping it cooler.
Ceri Phipps says:
It doesn’t make intuitive sense for those used to thinking about control systems – positive feedback usually leads to some kind of runaway system.
However, in the case of the climate there are also some non-linear negative feedbacks – surface radiation increases in proportion to the forth power of (absolute) temperature. If you analyze the system mathematically, a higher range of potential positive feedback leads to a thermal runaway. The lower range of positive feedback leads to amplified warming (compared with no feedback), and of course, negative feedback leads to reduced warming.
This doesn’t mean that water feedback is a positive feedback. But the idea isn’t mathematical or scientific madness.
Below is a comment that I recently posted on Roy Spencer’s blog.
RE: The Positive Water Feedback Hypothesis
At 14 deg C and 1 atm .pressure, 1 cu. meter of air has 12.1 g of water vapor for 100% humidity. If the temperature of the air is increased to 15 deg C, 1 cu. meter will now have 12.8 g of water vapor for 100% humidity , a small increase of only 0.7 g or 6.7% of water vapor. However, 100% humidity only occurs if it is raining or snowing or if there is dense fog. So how does the enormous amounts of surface water enter the atmosphere?
The wind is the force that transports surface water into the atmosphere and is far more important than simple evaporation of water in still air. When wind blows over a body of water, the surface will cool but water will still be transported into the air. Due to their momentum the much heavier nitrogen and oxygen molecules and argon atoms just blast the lighter water molecules out the surface water into the air. The lake effect is due to strong winds blowing water vapor from warm surface water onto the usually colder land.
Changes in air pressure are also more important than a slight increase in air temperature as is shown on an aneriod barometer. An air pressure drop of a few inches (ca 60 mm) of mercury will often cause rain or snow. If pressure increases, the air becomes dry. The heat of vaporization of liquid is depends mostly on external pressure. The low air pressure in tropical a cyclone causes enormous quantities of water to “flash evaporate” into the air as it moves into warm coastal waters.
Clouds are liquid water in the air and depending on local temperature, pressure and humidity, they can readily release water vapor into the air or drop excess moisture as rain, snow or ice pellets. On average cloud cover in the atmosphere is about 65%.
Clouds also contain atmospheric gases and can transport these, in particular CO2, from one local to an other local where these gases can be released into the air or be deposited on the surface in rain drops.
Over land transpiration from plants contributes to the local humidity as does respiration from all plants and animals which includes soil organisms such as bacteria, fungi, worms and insects.
I don’t recall reading that climate models take the above into account.
“Warmer air is associated with greater surface evaporation rates….”
Yes, but other effects have such as wind speed have large impacts on evaporation rate. Wind can dry the surface (that is evaporate water) on cold days/nights quicker than warmer days/nights with no wind. The GHG IR effect is of little relevance where wind dominates.
Spector says:
“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. ”
This is something that is not often referred to, as the energy transfer diagrams never seem to include the effect of IR active gases on the absorption/re-radiation of IR receivedfrom the sun. Nor do they really consider what happens at night, when GHG’s are for the most part atmospheric coolers.
Edward Bancroft:
I don’t know about energy transfer diagrams but the climate models all take into account the radiatively-absorbing gases on absorption of solar radiation.
In fact, even the best known one, by Kiehl and Trenberth shows 1/3 of the solar radiation being absorbed by the atmosphere.
At night? Well, I don’t know what diagrams you are referring to, but in the case of Kiehl and Trenberth for example, this is a globally, annually averaged diagram, so you won’t see day and night. But once again, this is climate basics so it is considered in every climate model.
We don’t know if water vpour is increasing as predicted.
We don’t know if increased water vapour is producing or will produce a net warming as predicted.
Since it is expected to be responsible for close to 2.0C of the warming by 2100, it is a little strange that we still don’t have any evidence to say one way or the other.
The range of GMST over the Earth’s history has been ~ 12-15C; today’s GMST of ~ 15C is at the lower end of the scale; some of the periods in the past featured very high levels of CO2 and extensive or no glaciation. If combined feedbacks were positive the Earth would now either be a snowball or like Venus, with Venus being the more likely scenario because of the growing sun over this time. CO2 cannot be the agent of change because it does not have the causal correlation with temperature; water, however does; and water must be a moderator of temperature trend given the relatively low range of GMST.
The science should be about water but unfortunately there is not a buck in it; at least to the same extent as CO2.
”
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.”
________________________________________________
If one takes a look at the months with the greatest +ve deviations from normals in unusually warm years, higher solar wind speeds will be evident.
The major rainy periods this summer so far, 2nd and 4th week of July, 2nd half of August, and the past few days, had lower solar wind speeds, providing the drop in temperature required to increase volume of summer rainfall.
http://www.lmsal.com/solarsoft/latest_events/
The idea that water vapor acts as feedback to magnify the effects of CO2 is like saying a flee can control the direction of a horse. It is much more likely that water vapor and condensed moisture (clouds) is taking CO2 for a ride and controlling it’s global distribution in the process. http://www.kidswincom.net/CO2OLR.pdf.
From scienceofdoom:
“In fact, even the best known one, by Kiehl and Trenberth shows 1/3 of the solar radiation being absorbed by the atmosphere.
At night? Well, I don’t know what diagrams you are referring to, but in the case of Kiehl and Trenberth for example, this is a globally, annually averaged diagram, so you won’t see day and night. But once again, this is climate basics so it is considered in every climate model.”
My take on this: The K&T diagram shows the 1/3 absorption, but does not say in which energy bands it takes place. If it is shorter wavelength, there is no GHG/IR re-radiation. It does show emitted radiation, but the diagram implies that the source of this radiation is from the surface/clouds, not the re-radiation of the incoming energy in the upper atmosphere.
The day/night effects are lumped together, but it would be instructive to see them separated so that the compensating IR heating/cooling effects can be clearly shown.
As a style criticicism, the K&T diagram also attempts two analyses on the same image. Namely, on the left there is an energy analysis in direct W/m2. On the right it also uses W/m2, but as part of a black body cloud/surface interchange, with the apparently anomalously large numbers actually being correctly expressed artefcats of the blackbody dynamics.
It is good to see the topic move to the area that makes the most sense, only by understanding the role that water liquid/vapour equilibrium plays in moderating our climate will we be better able to predict the outcome from variable energy inputs into the system.
John Galt
I agree with you that water vapour provides for effective movement of heat energy through the atmosphere. And, huans have used it continuosly since the invention of the steam engine. A quick browse of the steam tables for the latent and sensible energy transferred from a source at an initial temperature and pressure to a destination at final temperature and pressure will show just how effective water vapour is in this regard.
Ulric Lyons says:
September 15, 2010 at 5:48 am
The major rainy periods this summer so far, 2nd and 4th week of July, 2nd half of August, and the past few days, had lower solar wind speeds, providing the drop in temperature required to increase volume of summer rainfall.
http://www.lmsal.com/solarsoft/latest_events/
The second half of August wasn’t all that rainy where I live.
Nice work, Roy.
Given the change in CO2 over the last 100 yrs & the expected water vapor feedback in the models, wouldn’t we have expected significantly more warming than we actually have observed? And if that is the case, this would strongly argue that the water vapor feedback is significantly less than is in the models. Which of course would also imply that the forecast temp increase in the future is significantly overdone & that pretty much blows up the catastrophic AGW case, doesn’t it? Water vapor feedback is a critically import issue.
Let’s add a little more to Harold Pierce Jr’s post. Water is lighter than oxygen and nitrogen. Thus, moist air is less dense than dry air. We focus on heat making air rise. Yes, heat helps water evaporate, but the wet air is lighter anyway. That helps lift it to higher cooler altitudes. When moist air cools enough, the air is saturated and clouds form.
Wet air is less dense and must rise through drier air, all else being equal. Thus, we have a strong mechanism for transporting heat. In this case, the heat of evaporation / condensation. It works like a refrigerator. It will operate until the temperature at the surface drops sufficiently, or there is not enough water to make clouds. Seasonal hurricanes are the perfect illustration.
With 70% of the Earth covered by oceans, I think we have enough ‘refrigeration’ capacity to stop worrying about climate hysteria. 250 million years ago, the center of Pangaea was an enormous desert, lacking water to cool it. Obviously, the Earth’s oceans didn’t dry up. The areas of Pangaea that did receive rainfall were very fertile. We have the fossil record to prove that. And we are here.
If there were a way for a positive feedback mechanism to kill us, it would have done so already over the almost 4 billion years life has existed on Earth. Stop worrying about it. Oh, that’s right, some of us need the grant money, so keep worrying, and keep paying.
As per usual I’m prolly missing something, but water vapor isn’t a coolant.
Water is a coolant, just try it in a desert, or hell spill your water on your crappy ski pants high up in the mountains at minus 20 but I guess a big commercial freezer works too. When in the jungle in the summer time everyone present want those instant 15 minute showers, cool things down a bit… however afterwards it get warmer due to the water vapor raising from the ground, cloths drying, and what not. In the desert it really is the same you just doesn’t really notice the difference when the splash in the face that cooled things down for a micro second is then vapor in the atmosphere, the vaporization process is felt as cooling though.
Its obvious there is a negative feedback!
I wonder if the whole climate science isn`t starting by passing over the river to fill the bucket with water. When clouds form it’s because of heat, and they protect the earth from further heating by radiating back the incoming solar radiation. The daily observations from the tropics confirm this every morning and every afternoon.
Of cource there are “time lags” and smaller aerosolic influenses as well as the oceans heating circles in the thermostatic effects. But in the end the negative feedback from water vapour dominates and we land around the historical top records as “worst” (best?) case.
From the historical data we have, its interesting to observe that there seems to bee a “soft roof” for how high the temperature on earth can get (with the same solar output) this is an indication that the hotter i gets the harder it gets to get even hotter.
My simple answer is that the total amount of water on earth is the allways present allways reliable functioning thermostat protecting us from owerheating.Its when you try to calculate and understand why and how the details in the thermostat works that you may be lost on your way. Its much easier to find and understand factors why it can get so much cooler.
IMO Cohenite hits the nail on the head. As an engineer I have studied feedback systems and positive feedback results in system failure. We know this for a fact. If CO2/H2O forcing was positive we would have become Venus a long time ago when CO2 levels were many times what they are now. How can scientists continue to say “we have no real ideal how water vapor, by far the strongest greenhouse gas, responds to increasing temperatures but we’re pretty sure it’s catastrophic” despite the evidence all around them. At what point do reasonable people call bullcrap?
Fred H. Haynie says: “The idea that water vapor acts as feedback to magnify the effects of CO2 is like saying a flee can control the direction of a horse. It is much more likely that water vapor and condensed moisture (clouds) is taking CO2 for a ride and controlling it’s global distribution in the process…”
Does rain scrub CO² from the atmosphere?
jorgekafkazar said on Spencer on water vapor feedback
September 15, 2010 at 10:03 am
Yes, clouds do absorb CO2. Relatively pure rain will have a pH that is roughly in equilibrium with the atmospheric concentration (between 5 and 5.5). Some CO2 will be returned to the ocean surface in rain. Some will be transported up in towering clouds where the water freezes and releases CO2 to the upper atmosphere. This process of evaporation/condensation/freezing will occur many times as an air mass containing clouds goes from the equator to the poles. http://www.kidswincom.net/climate.pdf.
Humidity shorcircuits condensers….
How do any of us negative feedback ourselves when too hot? With water. Water refreshes. Do some alien “peers” think differently? Probably,…. if cold-blooded.