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.