This relates to water vapor "feedback" due to CO2's alleged "radiative forcing." See below for further comments from Richard Linzen, Brian Valentine, Lord Monkton, Fred Singer, David Deming & Ben Herman
From A Simple Method to Measure the Dew Point Temperature
by R. L. Snyder, Biometeorology Specialist
Gaseous water molecules consist of one oxygen and two hydrogen atoms. When in a liquid state, the water molecules form strong hydrogen bonds between the molecules. In order to evaporate water, heat must be supplied to break the hydrogen bonds and allow individual molecules to break off as a gas.
This is what happens when water is boiled. The heat supplied by your stove breaks hydrogen bonds and the liquid water is rapidly evaporated. Therefore, heat must be supplied to break hydrogen bonds and evaporate water. If air is the source of the heat (called sensible heat), then the air temperature will drop and the energy is stored as chemical energy in the water molecules (called latent heat). When water vapor condenses into liquid water, the hydrogen bonds form again and release latent heat, which increases the sensible heat and causes the air temperature to rise.
This exchange between latent and sensible heat is one of the most important factors controlling our climate and environment. Much of the heat transfer on a global scale results from water evaporating at the surface near the equator and condensing into clouds and moving poleward to redistribute the energy. The main point is that sensible heat is removed from the air and the temperature drops when evaporation is occurring and latent heat is converted to sensible heat and the temperature rises when condensation occurs.
All the authoritative bullshitting and muddle-headed equivocation in the world cannot change the fact that when water vapor is generated it COOLS the air. So much for its "positive feedback." And so much for long-accepted greenhouse theorems that have no basis in reality.
For your records, here’s a synopsis of the thread that inspired my outburst. It's a record of the bullshitting and equivocation I'm referring to.
Synopsis: Andrew Dessler had posted his views about the greenhouse effect
and respondents on Marc’s list commented about one angle in particular, Dessler’s claim that water vapor provides a strong positive feedback to CO2 forcing, thus exacerbating global warming.
Ben Herman wrote: Andy should look at Roy Spencer's latest paper if he is searching for the negative feedback that he doubts exists. It can be accessed from the icecap.us site.
David Deming wrote: This, indeed is the question. What will be the water vapor feedback? Will it be negative or positive, and if so, how much? Dessler claims that his conclusion of a significant positive feedback is "derived from fundamental physics combined with observations." Of course, predicting a negative feedback can also be derived from "fundamental physics."
The critical point is that Dessler lacks the necessary observation(s). He needs to put increased CO2 in the atmosphere of an earth-like planet and observe what will happen in a complex climate system after a hundred years. Lacking this test, everything else is speculation.
I don't know much about atmospheric physics, but it seems to me that until the impossible experiment is done, all we have is speculation. That is one reason I'm a skeptic. Another reason is that I have seen how ideology is driving the science. I believe many of the true believers are quite sincerely self deluded. They see everything that reinforces their existing beliefs, and are oblivious to anything that might create cognitive dissonance.
How did we ever get this far?
Alan Siddons wrote: What will be the water vapor feedback? Negative. Water vapor has quite a high specific heat, meaning that it demands more thermal energy than many other substances to raise its temperature. As a result, introducing water vapor into an otherwise dry heating scenario will result in cooling. Simply put, it sucks the heat in. Unfortunately, water’s reputation for storing heat has gotten confused with an ability to induce higher temperatures. But heat and temperature are not synonymous. Water’s so-called "latent heat" is absorbed energy that isn’t expressed as temperature. That extra margin is tucked away, always held in reserve.
Fred Singer: Caution: This argument is not correct.
Brian Valentine: Depends. Water droplets can be stabilised by dust over the water vapour pressure that is increased by the (positive) curvature of the drop over the vapour pressure of water in bulk; at a fixed temperature and total atmospheric pressure. As the external pressure drops, there must be some pressure that the droplets must evaporate; any phase change is accompanied by the absorption of the latent heat of vaporisation. That heat has to come from someplace (i.e., the surroundings …)
Christopher Monckton: Actually there is both (reliable) theoretical and (less reliable) empirical evidence for a net-positive feedback from water vapor. The theoretical reason is in the Clausius-Clapeyron relation, by which as the space occupied by the atmosphere warms its carrying capacity for water vapor increases near-exponentially (subject only to a trivial dependence upon temperature in the denominator). The empirical evidence is in observations that purport to show that the concentration of water vapor in the atmosphere has indeed increased by a proportion consistent with the Clausius-Clapeyron value mandated by the observed warming of the past 30 years. However, because water vapor is not a well-mixed GHG like CO2, measuring its concentration is far from easy, and I suspect that the results had been tuned to fit the equation: there has been a lot of that about in the climate field. A further complication, as Roy Spencer has rightly pointed out, is that, particularly in the tropical cloud-belt, warmer weather increases thunderhead formation and consequently planetary albedo, causing cooling not only by evaporation (which Wentz et al., 2007, point out has been underestimated prodigiously by the IPCC) but also by a measurable increase in planetary albedo where it matters most - in the tropics. So, although I think it is settled science that the water vapor feedback simpliciter is strongly net-positive, numerous other mechanisms, including increased evaporative cooling and tropical albedo, exercise a homoeostatic effect - one of many such effects that have succeeded in keeping Earth's temperature within 3% above or below the billion-year mean, notwithstanding the major phase transitions that caused the ice ages and the interglacials (the last four of which were all considerably warmer than the present).
Brian Valentine: Nil. I think Lord Christopher and Alan are both correct, but their interpretations are dependent on the conditions present.
Nevertheless I have never seen evidence that any such effect could be discerned amidst what must the background of white noise; there would simply have to be some things observed in the geological strata during a warming period that followed some cooling, which are not observed. Just isn’t there.
Alan Siddons: Just to pin a few things down, specific heat is the amount of heat required to increase the temperature of a substance’s mass by one degree. Sources vary somewhat, but here’s one:
Specific Heat of Dry Air at Constant Pressure: 1004 J K-1 kg-1
Specific Heat of Water Vapor at Constant Pressure: 1952 J K-1 kg-1
That’s nearly twice the heat required. Other sources put the figure higher.
Imagine a substance whose specific heat is infinite. No matter how much thermal energy you pushed into this stuff, its temperature would never rise. Water is almost like that, the difference being that its specific heat is merely high. A high specific heat equals resistance to a temperature increase. The energy input goes into hiding.
In the earth’s particular circumstances, this makes water a grand moderator, providing a negative response to the anomaly in question. If a watery milieu receives more heat, liquid water will suck it in and also evaporate more, thus lending the air a higher specific heat, i.e., cooling it. If that environment then grows colder, precipitation occurs, releasing the heat that kept the water vapor aloft. CO2 doesn’t behave at all like this.
To repeat: water’s high specific heat means that it resists an increase in temperature. The "latent heat" that water tucks away and makes unavailable doesn’t register on thermometers; they only measure "sensible heat." I think that this confusion about latent heat has kept climatology chasing its own tail. Water’s ability to "trap" an influx of heat is not at all the same as raising the temperature, in fact it’s just the opposite.
Richard Lindzen: For what it's worth, the Clausius-Clapeyron relation is only relevant for a saturated atmosphere -- which our atmosphere is not. The spatial heterogeneity of rh is indeed crucial since the impact of changes in humidity are vastly different for dry and moist regions. Getting the right area for each is impossible without getting clouds right -- which the models don't. Finally, the water vapor feedback is the most important and robust feedback in models. I am by no means sure that this is the case in nature. I discussed most of this years ago in the attached paper. Although there are details in that paper that I might reconsider, by and large it still holds up.