How not to discuss the Water Vapor feedback

I want to do a post soon on feedbacks, but just a quicky here

There is a lot of discussion about climate feedbacks in climate science, notably the role of water vapor. In short, the total amount of atmospheric water vapor should go up in a warmer climate under the assumption of approximately fixed relative humidity, at an increase of ~7% per degree Celsius warming, as per the Clausius-Clapeyron relationship. Water vapor is the strongest greenhouse gas, and so increases in water vapor will amplify any temperature changes from any initial forcing (e.g., CO2). However, many times the “water vapor feedback” being discussed on the internet is not the water vapor feedback at all. For example, in a recent web blog by Roger Pielke Sr., entitled Third Follow Up To Climate Metric Reality Check #3 – Evidence For A Lack Of Water Vapor Feedback On The Regional Scale or here, Dr. Pielke discusses how the WV feedback may not be showing up on the regional level and thereby questioning our understanding of how the climate reacts to temperature increase.

First things first, it is well known that as temperature and water vapor concentration go up together in a warmer climate, the relative humidity of the atmosphere will stay roughly constant. This is an emergent property in models, not a built-in assumption. Just for some meteorology review, at an equilibrium state the flux of water molecules of liquid –> Vapor = vapor –> liquid. That is, evaporation and condensation are equal. At equilibrium, the air is saturated (this condition is called the saturation vapor pressure). When you turn up the temperature molecules move faster making them more difficult to condensate. Water Vapor is removed by condensation or by diffusion into a neighboring drier air parcel. Specific humidity is basically the mass of water vapor over the mass of air, or ratio of water vapor to water vapor + dry air. An increase in global mean specific humidity may reflect warming under conditions of constant relative humidity, or an increase in relative humidity with constant temperature, or some combination. Relative humidity is the amount of water vapor in the air compared to the amount the air can hold (if it were saturated). If the vapor pressure rises above the saturation vapor pressure, than vapor will condense (and eventually precipitate out), bringing the relative humidity back to unity. This is expressed as a percentage. As temperature and water vapor go up together, the specific humidity will increase, while relative humidity does not change much. This is confirmed by real world observations today (e.g., here and here). It is often remarked that “in a warmer climate, evaporation will go up, water vapor will go up, and clouds will go up.” This idea is wrong on several grounds. First of all, evaporation gives the rate of moisture flux through the atmosphere, which is not the same as the amount of water left behind in the atmosphere; in that sense, the factors governing the atmospheric relative humidity distribution are not exactly the same as those which govern the rate at which water fluxes through the system. This depends on many things such as wind speed, so it is not even a given that a warmer climate will have more evaporation. Secondly, it is relative humidity which determines the formation of clouds, and if that changes little because of competition of increased temperature vs. increased water vapor, then it is not intuitive that we would expect cloud cover to go up.

The point about relative humidity remaining constant applies mainly to global scale averages. It is well known that individual regions can become moister or drier as the climate changes, and so the lack of trends over one area is not a black swan in a “all swans all right” hypothesis: nothing is falsified, the world is not coming to an end. Another much more important point is that unlike CO2, and other well-mixed greenhouse gases, water vapor has a strong vertical gradient in its concentration. Because water vapor condenses and it has a short residence time which does not allow it to become well-mixed, it diminishes very rapidly with altitude. Air cools as it goes upwards because of adiabatic expansion, where the saturation vapor pressure diminishes more rapidly than the vapor pressure of H2O.

The centers of water vapor spectral lines are fully saturated under atmospheric conditions, so this means that photons emitted from the lower troposphere can only escape to space if they are emitted from the wings of spectral lines. This is where the upper tropospheric absorption is sufficiently weak, and so is emission. Emission from the upper troposphere occurs closer to the centers of the spectral lines where the emission is stronger than at the wings. But don’t be fooled, because it is primarily in the upper portions of the atmosphere, where the heat balance is determined, that is of primary radiative significance. In fact, as specific humidity changes in a warming climate, around 90% of the radiative response is dominated by the mid to upper atmosphere (above 800 millibars), with more effect in tropical latitudes. So, in discussing water, one must distinguish between water for such purposes as computing precipitation and latent heat transport,and water for the purpose of determining its radiative impacts. When discussing precipitation, runoff, and other such changes in the hydrologic cycle it is the lower level water vapor which is dominant. Dr. Pielke discusses preciptable water, but it is the upper levels which are radiatively significant.

Surface radiative fluxes are most sensitive to specific humidity variations in the lower troposphere. Lower-tropospheric moisture is key to the surface energy balance and coupled ocean-atmosphere system. However, the surface energy budget and the top-of-atmosphere budget are very different concepts, with the latter being of radiative significance to climate change.

UPDATE – Dr. Pielke Sr. posted a comment on his web blog on this post at http://climatesci.org/2008/01/26/963/ .  I thank him for his patience.

13 responses to “How not to discuss the Water Vapor feedback

  1. Pingback: Climate Science: Roger Pielke Sr. Research Group News

  2. Chris – I posted a response to your weblog on my website Climate Science
    http://climatesci.org/2008/01/26/963/

    Response– Dr. Pielke, I am glad you stopped by, and thanks for the response in your blog. I hope I did not intend to state that the lower atmosphere/surface energy balance is unimportant for everything, or even that regional variation is not worth studying. As I know you agree, regional changes are extremely complex, have larger variability than the global scale, and may not even be indicative of what the globe is doing (e.g. the “antarctica is cooling, growing” argument). I’m sure you are familiar with an older realclimate post by Dr. Pierrehumbert on the Philipona et al. paper on WV feedback over Europe, and the misleading nature that the paper (and corresponding media coverage, like here) could have had. I believe that your piece could have had the same misleading nature (regardless of the intention), because you were not talking about the water vapor feedback that is expected to double climate sensitivity as CO2 goes up.

    The radiative importance of the upper level WV should be made distinct from the preciptable water as it may relate to the surface energy budget, as that is meant to eliminate the surface-air gradient through sensible and latent components, and this sometimes needs to be made explicit even in technical papers (for example, the Soden-Held paper “Robust Responses of the Hydrologic cycle to Global Warming), so that people are not confused. The fractional change in water vapor mass (not the absolute change) is what governs the WV strength as a feedback mechanism, as its absorptivity changes with the logarithm of its concentration. The upper atmosphere is largely the dominant area where “water vapor feedback occurs.”

    The concluding remarks in the PNAS paper also say “Detection and attribution studies have now moved beyond ‘‘temperature only’’ analyses and show physical consistency between observed and simulated temperature, moisture, and circulation changes. This internal consistency underscores the reality of human effects on climate.” I completely agree that observations must be made for GCM confirmation, and your points on the last few years are well taken. You would have more first hand knowledge of internal variability over the last few years, though the “no trend” line you are looking at is extremely small when compared to the trend of the total atmospheric moisture change observed by Santer et al., and certainly longer and improved observations will be made, and will be necessary to address uncertanties regarding changes in water vapor as a response to climate change.

    Looking at the larger trend out to 2004, by Brian Soden and colleagues, they conclude GCMs reproduced the moisture observations very well, including reproduction of the observed radiances requires a moistening of the upper troposphere, as well as the assumption of an approximately fixed relative humidity.

    Soden, B. J., D. L. Jackson, V. Ramaswamy, M. D. Schwarzkopf, and X. Huang, 2005: The radiative signature of upper tropospheric moistening. Science, 310(5749), 841-844

    Thanks again,
    Chris

  3. Chris – As a broader follow up, please discuss on your weblog the findings and recommendations of

    National Research Council, 2005: Radiative forcing of climate change: Expanding the concept and addressing uncertainties. Committee on Radiative Forcing Effects on Climate Change, Climate Research Committee, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C., 208 pp. http://www.nap.edu/openbook/0309095069/html/

    This report has not been widely discussed, and your weblog is an ideal site to do this.

    Best Regards
    Roger

  4. Hi Chris,

    it is very nice to have a scientist in here for a real discussion. You also have a nice piece, but like andrew I cannot take sides. I noticed this last line in Dr. Pielke’s web blog

    “It is true that the upper levels are radiatively important. However, so are the lower levels, as Chris acknowledged where he wrote “[s]urface radiative fluxes are most sensitive to specific humidity variations in the lower troposphere”.

    You also wrote in your last line

    “however, the surface energy budget and the top-of-atmosphere budget are very different concepts, with the latter being of radiative significance to climate change.”

    I do not know much about these differences, or how either one relates to climate change. Could you clarify in laymen’s terms??

    Thanks
    paul

    Response- Paul, aside from Dr. Pielke we’re all students, so the more people that add to the educational experience, the more people (including myself) learn about this rather important topic.

    There are three ways to change the radiative balance of the planet (and that effects temperature): change the incoming solar radiation, change the outgoing infrared (longwave) radiation, or change the planetary albedo (which is a measure of reflectance, i.e., the solar radiation that is directly reflected right back to space which is distinct from the outgoing longwave radiation). A radiative forcing perturbs the radiative balance of the planet. According to the IPCC TAR, a radiative forcing is “the change in the net vertical irradiance (expressed in Watts per square metre: Wm-2) at the tropopause due to an internal change or a change in the external forcing of the climate system, such as, for example, a change in the concentration of carbon dioxide or the output of the Sun.”

    The greenhouse effect is responsible for about 150 W/m-2 of longwave absorption, a number that would be zero without greenhouse gases, and under conditions of 2x CO2 that will go up to about 170 W/m-2. That accounts for CO2 + feedbacks, where water vapor is very important. That water vapor in the upper troposphere accounts for the very vast majority of that water vapor effect, and so that effects he planet’s heat balance.

    The surface energy budget, in contrast, plays a secondary role and more or less acts to get rid of the temperature gradient between the surface and atmosphere. Radiative heating of the ground is only one part in the surface energy budget, and fluid dynamics also plays a very large part: convection, turbulence, wind, evaporation, etc. But the planet itself can only get rid of energy by radiation. Generally, when you talk about the water vapor feedback which amplifies global warming, you’re referring to the radiative significance (i.e., LW absorption), and this is dominated in the higher, cooler layers. This is discussed in more detail in my 3 part series on “basic radiative models…”

    Chris

  5. Chris, very good discussion, I hope your site starts to get more attention. I can’t really “take sides” here and some of this just escapes me.

    My understanding though is that doubling CO2 gives 3 degrees C of warming, and then you have to add feedbacks, which gives between 1.5 and 4.5 C extra warming. Is most of that “extra push” water vapor, because I know decling ice cover gives some, but that would be far from “doubling climate sensitivity” (that would be 6 degrees, right)?

    The water vapor issue is interesting though, because I had someone tell me that since water vapor dominates the greenhouse effect, CO2 can’t be causing much warming…is there a good response to this? I do understand the idea of forcing and feedback, but…

    Response- No, the temperature response is 1.2 C per doubling of CO2, without any feedbacks included (except the response of radiated thermal flux from Planck). Feedbacks give an overall climate response (CO2 + feedbacks) of 2 to 4.5 C, with a regular value generally cited ~3 C.

    Feedback but not a forcing is a good answer. Your correspondent has apparently not taken into account that there are small segments of the thermal infrared spectrum that water vapor misses, where CO2 is very strong, and in regions very important for where bodies at Earthlike temperatures emit radiation. The CO2 is actually quite important,as it is about 2/3 WV to 1/3 CO2, but since Water Vapor concentration is so temperature dependent, if you suddenly removed all the CO2, you’d also lose must of the WV effect as well, and add in the ice (albedo) effect, and you would easily ice over.

  6. Thanks for that clarification. For more, what do you mean when you say a “doubling of CO2.” From what value to what? I see this a lot

    Response- It actually doesn’t matter since the radiative forcing relationship is logarithmic (i.e., 200 to 400 is the same as 400 to 800 ppmv). The “first” doubling since pre-industrial time would be about 2x (280 ppmv) = 560, and we’re a bit over 380 now

    chris

  7. Chris – for further comparison for the relative roles of water vapor and CO2, please see the weblogs http://climatesci.org/2007/08/24/further-analysis-of-radiatve-forcing-by-norm-woods/ and http://climatesci.org/2006/05/05/co2h2o/

    Using a column radiative transfer model, Norm Woods (who works for Graeme Stephens at Colorado State University) looked at changes in concentrations of water vapor and carbon dioxide for a typical tropical and typical higher latitude winter and summer sounding. These are not global average values but they provide a useful perspective for your readers. One conclusion is that a global average top of the atmosphere radiative forcing is actually of very limited value in terms of describing how humans are altering the climate system.

    As just one example, the spatial patterning of atmospheric radiative heating matters more; i.e. see our paper

    Matsui, T., and R.A. Pielke Sr., 2006: Measurement-based estimation of the spatial gradient of aerosol radiative forcing. Geophys. Res. Letts., 33, L11813, doi:10.1029/2006GL025974.
    http://climatesci.colorado.edu/publications/pdf/R-312.pdf

  8. Thank you Roger for clearing this up

    Response- I stand by everything I said, which is not an extremely complex objection; they do total precipitable water, which is dominated by the lower atmosphere, whereas it is the higher level water vapor that matters most for radiation. The very purpose of my post was not to show that the lower levels are unimportant, but that Pielke is not at all discussing the water vapor feedback that most people think of when they are talking about “water vapor feedback” which amplifies the CO2 forcing, to give you over 2 degrees of warming, and I have seen it cited at least once on the internet getting that aspect wrong. You don’t need to believe me, there are many good sources to learn about how the surface energy budget/preciptable water differ from the top-of-atmosphere energy budget which affects the planetary energy balance. I didn’t say “that was all that matters.” If you want to look at some references, Held and Soden (2000) “Water vapor feedback and global warming” or “Pierrehumbert RT, Brogniez H, and Roca R 2007: On the relative humidity of the atmosphere” or Dr. Pierrehumbert’s climate book here (Ch. 6) will be good starts.

    As far as I am aware, there are no widely accepted set of metrics for quantifying regional non-radiative forcing, and I agree that much work is needed to understand and quantify them at the regional and global scales. Moreover, it is well taken that the TOA forcing may not be fully useful (as Pielke brings up with aerosols, which apply more than LLGHG’s), and that land effects cannot be ignored as they relate to turbulent exchanges, etc. But the planet itself only loses energy from radiation, and when discussing the “WV feedback” it is the upper atmospheric moistening that matters mostly for this.

    I believe that Roger Pielke should have been much more clear on this point, regardless of what the data for his purposes showed. He was not, and I think this should be clarified

    chris

  9. Chris,

    I think you have been a bit too light on Roger’s set of points. I fully agree with him in that non radiative processes are especially important, but this introduction quote on Pielke’s blog was quite clear on what he tought was being referred to, or at least how a “laymen” would read it:

    “An essential component of the IPCC perspective of global warming is that atmospheric water vapor must increase in order to amplify the radiative warming effect of carbon dioxide. Without this amplification, the global warming that would be due to just carbon dioxide would be quite modest.”

    Although total column vapor may be a “useful metric,” most people who read these types of sites would not get the clarification that you did here, and Roger should have known that.

    I beleive that the radiative effects of water vapor are one of those things that can be put in the “certain” category, but further understanding of the hydrological cycle and surface changes are also very important.

  10. Chris – You are certainly correct that the global average upper level water vapor change is very important with respect to the water vapor radiative feedback. However, the column total precipitable water remains a valuable climate metric for assessing this feedback since the global average upper level water vapor cannot increase without also increasing the precipitable water, by any mechanism that I am aware of.

    The multi-decadal global climate models certainly predict such an increase in total atmospheric water, yet observations are suggesting that this has not occured, at least for the locations and region that we have looked at.

    Precipitable water vapor is an integral quantity with more data measurements available than for the upper levels alone. Since upper level water vapor trends should closely correlate with precipitable water this metric should be assessed in this context. My weblogs on this subject have identified an issue with the skill of the model predictions of this metric.

  11. While the authors focus on the linear trends from 1992 to 2005 which they plot in each of the panels in Figure 2, the trends since about 2002 have been flat, or in the case of the 0-60N values, have even been negative!

    (from Third Follow Up …)

    Just curious – is it possible to identify a trend over 3 to 4 years worth of data with any significance? Eyeballing the charts in the paper Pielke cites suggests otherwise.

    If the upper troposphere is warming faster than the surface (at least at the equator), is there an implication for cloud cover? Would we be likely to see an increase in higher level clouds and a decrease in lower level clouds?

  12. As a mere mortal aerospace engineer, I have some questions for my acedemic friends…

    1…I haven’t seen any discussion yet of separation in the atmosphere by molecular masses, i.e., the tendency of light water molecules to rise and the heavier-than-air CO-2 molecules to remain near the surface…

    2…It seems to me the primary heat transfer in the atmospherre is by water molecule state changes, not by convection or radiation thru the atmosphere…the water molecules ferry their latent heat upward where it is released via state change, the energy of which is spectrally spread to the quantum wavelengths attributable to the water molecule state change, and the subsequent photons find their way to outer space by direct radiation (mostly)…

    3…It seems to me the concentration of greenhouse gases in the high atmosphere would be negligible from an energy trapping perspective…and especially for emitted wavelengths other than Infra-red…

    4…it seems to me the near-to-surface CO-2 concentrations would have a nearly negligible effect on this heat ferrying aspect of the water cycle…

    5…what am I missing here…???

  13. Your discussion of the upper atmospheric radiative dominance makes sense to my engineeeroing brain..
    I then would submit the following conjecture….

    If, in fact, essentially the earth heat input is presumed constant, and the by-far dominant heat moving mechanism is latent heat transfer and the upper atmosphere radiative effect, I would expect very little, and possibly negligible effects in the overall water flow in the atmosphere, and no global warming at all due to CO-2 and associated lower-atmospheric water vapor effects…

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