Arguments
Explaining how the water vapor greenhouse effect works
What the science says...
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Increased CO2 makes more water vapor, a greenhouse gas which amplifies warming |
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Climate Myth...
Water vapor is the most powerful greenhouse gas
“Water vapour is the most important greenhouse gas. This is part of the difficulty with the public and the media in understanding that 95% of greenhouse gases are water vapour. The public understand it, in that if you get a fall evening or spring evening and the sky is clear the heat will escape and the temperature will drop and you get frost. If there is a cloud cover, the heat is trapped by water vapour as a greenhouse gas and the temperature stays quite warm. If you go to In Salah in southern Algeria, they recorded at one point a daytime or noon high of 52 degrees Celsius – by midnight that night it was -3.6 degree Celsius. […] That was caused because there is no, or very little, water vapour in the atmosphere and it is a demonstration of water vapour as the most important greenhouse gas.” (Tim Ball)
At a glance
If you hang a load of wet washing on the line on a warm, sunny day and come back later, you can expect it to be dryer. What has happened? The water has changed its form from a liquid to a gas. It has left your jeans and T-shirts for the air surrounding them. The term for this gas is water vapour.
Water vapour is a common if minor part of the atmosphere. Unlike CO2 though, the amount varies an awful lot from one part of the globe to another and through time. Let's introduce two related terms here: 'non-condensable' and 'condensable'. They set out a critical difference between the two greenhouse gases, CO2 and water vapour.
Carbon dioxide boils at -78.5o C, thankfully an uncommon temperature on Earth. That means it's always present in the air as a gas. Water is in comparison multitalented: it can exist as vapour, liquid and solid. Condensed liquid water forms the tiny droplets that make up clouds at low and mid-levels. At height, where it is colder, the place of liquid droplets is taken by tiny ice-crystals. If either droplets or crystals clump together enough, then rain, snow or hail fall back to the surface. This process is constantly going on all around the planet all of the time. That's because, unlike CO2, water vapour is condensable.
CO2 is non-condensable and that means its concentration is remarkably similar throughout the atmosphere. It has a regular seasonal wobble thanks to photosynthetic plants - and it has an upward slope caused by our emissions, but it doesn't take part in weather as such.
Although water vapour is a greenhouse gas, its influence on temperature varies all the time, because it's always coming and going. That's why deserts get very hot by day thanks to the Sun's heat with a bit of help from the greenhouse effect but can go sub-zero at night. Deserts are dry places, so the water vapour contribution to the greenhouse effect is minimal. Because clear nights are common in dry desert areas, the ground can radiate heat freely to the atmosphere and cool quickly after dark.
On the other hand, the warming oceans are a colossal source of water vapour. You may have heard the term, 'atmospheric river' on the news. Moist air blows in off the ocean like a high altitude conveyor-belt, meets the land and rises over the hills. It's colder at height so the air cools as it rises.
Now for the important bit: for every degree Celsius increase in air temperature, that air can carry another 7% of water vapour. This arrangement works both ways so if air is cooled it sheds moisture as rain. Atmospheric rivers make the news when such moisture-conveyors remain in place for long enough to dump flooding rainfalls. The floods spread down river systems, causing variable havoc on their way back into the sea.
Atmospheric rivers are a good if damaging illustration of how quickly water is cycled in and out of our atmosphere. Carbon dioxide on the other hand just stays up there, inhibiting the flow of heat energy from Earth's surface to space. The more CO2, the stronger that effect.
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Further details
When those who deny human-caused global warming use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as potent a greenhouse gas as water vapour, which there's already a lot of, adding a little more CO2 couldn't be that bad, they insist.
What this argument misses is the critical fact that water vapour in air creates what scientists call a 'positive feedback loop'. That means it amplifies temperature increases, making them significantly larger than they would be otherwise.
How does this work? The amount of water vapour in the atmosphere has a direct relation to the temperature in any given region and the availability of water for evaporation. Heard the weather-saying, "it's too cold to snow"? There's more than a grain of truth in that; very cold air has a low capacity for moisture.
But if you increase the temperature of the air, more water is able to evaporate, becoming vapour. There's a formula for this, the figure being 7% more moisture capacity for every degree Celsius of warming. All you then need is a source of water for evaporation and they are widespread - the oceans, for example.
So when something else causes a temperature increase, such as extra CO2 emissions from fossil fuel burning, more water can evaporate. Then, since water vapour is a greenhouse gas, this additional moisture causes the temperature to go up even further. That's the positive feedback loop.
How much does water vapour amplify warming? Studies show that water vapour feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C upward temperature change caused by CO2, the water vapour will cause the temperature to go up another 1°C. When other demonstrable feedback loops are included, and there are quite a few of them, the total warming from a 1°C change caused by CO2 is as much as 3°C.
The other factor to consider is that water evaporates from the land and sea and falls as rain, hail or snow all the time, with run-off or meltwater returning to the sea. Thus the amount of water vapour held in the atmosphere varies greatly in just hours and days. It's constantly cycling in and out through the prevailing weather in any given location. So even though water vapour is the dominant greenhouse gas in terms of quantity, it has what we call a short 'atmospheric residence time' due to that constant cycling in and out.
On the other hand, CO2 doesn't take an active part in the weather. It does hitch a lift on it by being slowly removed from the air as weak solutions of carbonic acid in rainwater. These solutions are key weathering agents, affecting rocks on geological time-scales. Weathering is a key part of the slow carbon cycle, with the emphasis on slow: CO2 thus stays in our atmosphere for years and even centuries. It has a long atmospheric residence time. Even a small additional amount of CO2 thus has a greater long-term effect - and in our case that additional amount is far from small.
To summarize: what deniers are ignoring when they say that water vapour is the dominant greenhouse gas, is that the water vapour feedback loop actually amplifies temperature changes caused by CO2.
When skeptics use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as powerful as water vapor, which there's already a lot of, adding a little more CO2 couldn't be that bad, right? What this argument misses is the fact that water vapor creates what scientists call a 'positive feedback loop' in the atmosphere — making any temperature changes larger than they would be otherwise.
How does this work? The amount of water vapor in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback.
How much does water vapor amplify CO2 warming? Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C. When other feedback loops are included, the total warming from a potential 1°C change caused by CO2 is, in reality, as much as 3°C.
The other factor to consider is that water is evaporated from the land and sea and falls as rain or snow all the time. Thus the amount held in the atmosphere as water vapour varies greatly in just hours and days as result of the prevailing weather in any location. So even though water vapour is the greatest greenhouse gas, it is relatively short-lived. On the other hand, CO2 is removed from the air by natural geological-scale processes and these take a long time to work. Consequently CO2 stays in our atmosphere for years and even centuries. A small additional amount has a much more long-term effect.
So skeptics are right in saying that water vapor is the dominant greenhouse gas. What they don't mention is that the water vapor feedback loop actually makes temperature changes caused by CO2 even bigger.
Last updated on 23 July 2023 by John Mason. View Archives
Pluto @300 , there is no need to take exception to the word "governed" in the phrase <being governed by the Clausius-Clapeyron relation> .
In the English language, governed has quite a number of meanings. In this particular case, the word "constrained" might arguably be a better choice. And as I am sure you are aware, the rate of evaporation depends on more than simply the temperature of the ocean and air.
[ Your comment on the saturated/unsaturated state of atmospheric water . . . is of course so obvious that it needs no reply. ]
But on a planetary scale, and in relation to the GHG effect of vaporous water, we needn't be concerned whether a water phase change rate is measured by a few minutes or a few days.
Also, considering that (at current terrestrial temperatures and partial saturation levels) the atmospheric water vapor GHG effect comprises something over 50% of the total GHG effect — then that leaves very little room for "gross over-estimation of the water vapor greenhouse effect" (unquote).
All in all, the situation is fairly straightforward.
Eclectic @301
Your comment on the saturated/unsaturated state of atmospheric water . . . is of course so obvious that it needs no reply.
No, I'm afraid it is not so obvious to me. Exactly which comment are you talking about and what is it about this comment with which you agree or disagree?
Pluto @302 :
Your comment was that the atmosphere is not always fully saturated with H2O. And of course, I must agree with you there — as, I am sure, do all scientists & meteorologists, including James Frank (the OP).
Eclectic @303
Thanks for getting back with me. I appreciate your answering my rather dumb question, but I do want to be certain we are both on the "same page" with this GH effect issue. So, if you don't mind, I would like to ask another question. Suppose you brought a bucket of water into a room with unsaturated air (ie. less that 100 percent relative humidity). What would happen to that water (assuming, of course, you didn't spill it)? My answer is that the water in the bucket would evaporate until either the humidity does reach 100 percent or the bucket runs dry while the air, water vapor, and liquid water remain at the same temperature. I would, however, like your thoughts on the question. Thanks
[TD] Water vapor levels vary locally and globally, because of numerous factors including not just temperature, but liquid water sources for evaporation, ground & water temperature, wind, air pressure, amount of condensation nuclei, and more. Scientists have known about those things for many, many decades. Check any textbook for meteorology, atmospheric physics, climatology, or many other fields. None of those things eliminates the dependence on temperature. That's why your local weather person reports both relative and absolute humidities. Read the Intermediate tabbed pane of this post and then read the studies cited, regarding measurements of humidity changes.
Pluto @304 ,
I aspire to Christian ethics, but I do not aspire to being led through a catechism. If you yourself have a dispute with mainstream science, then please state the case you wish to make, as succinctly as possible, and with minimal rhetoric.
TD @304
Water vapor levels vary locally and globally, because of numerous factors including not just temperature,...
I agree. The trouble is that your statement is in direct conflict with the needs of the AGW community for water vapor to be a feedback (not a forcing) in the greenhouse effect, and that happens only when the vapor concentration is a function solely of temperature. This is what allows the much weaker GHGs (such as CO2) to dominate the greenhouse effect (in model and theory) even in the presence of water vapor. Now, in order to constrain the H2O vapor concentrations to solely a function of temperature, John Cook and numerous climate scientists use the Clausius-Clapeyron equation from which one gets the saturation concentration of H2O vapor for a known given temperature. At this point, the actual H20 vapor concentration values are replaced their saturation values, which results in a substantial over-estimation of the enhanced greenhouse effect. This is one place where I along with numerous non-climate scientists get very concerned about what's happening in the climate science field, and don't particularly trust their results.
[DB] Evidenceless assertions and the "physics" of your personal pocket universe snipped.
This user has recused himself from further participation here.
Greenhouse gases include water vapour ,carbon dioxide ,methane ,nitrous oxide and other gases.
National Oceanic and Atmospheric Administration (NOAA).”Deforestation is the second largest anthropogenic
source of carbon dioxide to the atmosphere ranging between 6 percent and 17 percent,”said Daley.Then water vapour is actually the most powerful greenhouse gas and has a strong effect on weather and climate.As the planet get warmer ,more water evaporates from the Earth’s surface and become water vapour in the atmosphere.
If there a place where is ocean then it will have more of water vapour than the desert or not? And if want to reduce these problem then the place which dry or desert will have more stable of temperature than the ocean which has higher evaporation of water in the atmosphere.Because these issue is the positive feedback loop.How human can figure out greenhouse gas effect? And it’s not just only 1 problem because it will effect in a chain for example climate change ,global warming and methane pollution.
Global Warming is harming the environment in several ways including desertification ,increased melting of snow and ice ,sea level rise , stronger storms and extreme events.These problem made by human activities affect to the environment so in every year water vapour will more increasing to reach greenhouse gas effect and other problems.It was the responsibility of human to take care our Earth’s.
What effect does the vapor emit at
a) burning of wet organic matter
b) composting
compared with the fact that the wet organic material is used as a raw material for the production of biogas and biofertilizers and the biogas is subsequently burned and thus converted into electricity and heat.
Ruzena @308 , I have not seen scientific figures for the amount of water vapor emitted from composting or from the burning of wet organic matter (or from dry organic matter, too). But the planet has over 300 million square kilometres of ocean to produce water vapor by evaporation — so presumably the amount of vapor from composting/burning, would be negligibly small in comparison.
There would be a difference in the timing of release (of vapor) from human-caused composting/burning versus the release by natural composting/burning from organic materials [which would have occurred eventually, producing H2O and CO2]. But over the course of a decade or two, the end result would be about the same. This is all part of the natural cycle of organic carbon [unlike the CO2 from fossil fuels].
You cannot really make any long-term change to the water vapour content in the atmosphere by injecting water vapour by any means. Water just condenses out. What the atmosphere will hold is function of air temperature (Clausius-Clapeyron relation) and the oceans provide the main source.
It is actually rather more consistent with rising global temperature. Evaporation from irrigation can only make local, short term change to water vapour in atmosphere. Change in tropospheric specific humidity over past 40 years is about 3.5%, consistant with 0.5C increase in temperature. See Fig 2.30 and accompanying text in AR5 WG1 for the list of peer-reviewed papers discussing this.
In explaining the positive feedback loop, you make the statement
If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. This statement, of course, assumes that there is a source of liquid H2O in the immediate vicinity of the temperature increase. Otherwise, there would be no water available for evaporation which would break the feedback loop you describe. Therefore, it seems that this positive feedback loop can occur only at the surfaces of water sources (such as streams, lakes, or oceans) or if it is raining, and is not something that happens globally.
Black Thunder, while true that you need a source of water, with 72% of planet covered by ocean, that usually isnt a problem. Furthermore, weather moves airmasses around so under-saturated air from a desert rapidly takes up water when it passes over an ocean. Observations show that relative humidity has remained the same (Clausius-Claperyon relation holds) and globally TPW pretty much matches the 7% per 1C rise. This is discussed in chapter 2 (2.5 ) of the AR5.
scaddenp — It doesn't matter how much of the earth is covered with water. If that liquid H2O is not where the warming is occurring so that it can evaporate there, then the feedback loop is broken. Also, the Clausius-Claperyon is valid only if the system (earth in this case) is in thermal equilibrium. Such is not the case if air masses are moving around carrying water vapor from the oceans to the deserts. Furthermore, deserts generally are at higher altitudes so that moist airmasses tend to lose H2O vapor to condensation before arriving, which is why the area is a desert. The bottom line is that if the water vapor concentration is less than saturation, the feedback loop is broken.
Globally averages and over a period of few days, the relation holds. Dry air moving over sea rapidly absorbs the deficit moisture. As I pointed out, the observations directly support this interpretation, they do not support yours. The water vapour increase in the troposphere as it warms is consistant with the temperature rise. You need to provide observational support for any other interpretation which seems lacking from numerous different analyses of available data source summarized in the IPCC reports.
Further reading - as to how climate models actually handle water vapour.
https://scienceofdoom.com/2017/11/05/water-vapour-feedback-is-simply-written-into-climate-models-as-parameters/
and for gory detail. Held and Soden 2000 (dated but good start).
Globally averages and over a period of few days, the relation holds.
The Clausius Claperyon (CC) relation was derived on the basis of an isolated system containing gaseous and condensed phases of a certain substance in thermal equilibrium with each other. This means that the system is characterized by a single temperature and a single vapor pressure (or equivalently vapor concentration), and hence cannot be applied globally since the earth contains wide variations in temperature and humidity. It would be mathematically invalid and meaningless to assign global averages of these values into the CC relation. Therefore, this relation does not hold globally.
Dry air moving over sea rapidly absorbs the deficit moisture. As I pointed out, the observations directly support this interpretation, they do not support yours.
I never claimed that dry air over the sea doesn't absorb moisture or that the water vapor increase doesn't cause warming. But what you are describing here is a water vapor forcing, not a feedback.
You need to provide observational support for any other interpretation which seems lacking from numerous different analyses of available data source summarized in the IPCC reports.
The observation I have made is that the current climate science arguments for the water vapor feedback are physically and mathematically incorrect, and therefore the existence of such a feedback is highly unlikely. The implication of this is that whatever global warming we may be experiencing is not being driven by any CO2 greenhouse effect.
BlackThunder , whatever argument you are trying to put forward, promptly loses itself in a welter of poorly-thought-out semantic quibbles & excessively "binary" thinking on your part (in a way not unlike the good poster Pluto @ #306 and prior posts). Please step back and look at the overall picture.
Eclectic@318
I don't quite know what you are talking about here, but since you mentioned Pluto@306, I would very much like to know just what he said and why moderator TD took it down (except for the "I agree" part). Somehow, you seem to know something about this posting, so why don't you fill me in?
[DB] Sock puppet of serial spammer cosmoswarrior/Pluto/et al removed.
Well, well. Looks our dear old friend cosmowarrior back pushing the same half-baked garbage again with yet another sock puppet. Seriously, do you think repetition of nonsense and demonstrations of your problems with logic is somehow going to change the logic of science if only can repeat enough times? Bye bye.
After reviewing the comments posted by JeffDylan@267 regarding the rebuttals written by James Frank and John Cook to the concept of water vapor being the controlling greenhouse gas, I fail to see how JeffDylan misunderstood the explanation as indicated by moderator TD. In his rebuttal, Frank states (in the fourth paragraph)
"So even though water vapour is the greatest greenhouse gas, it is relatively short-lived."
and JeffDylan states (in the second paragraph of @267)
"... H2O vapor may be the strongest greenhouse gas, but it is much more "short-lived" in the atmosphere that [than] CO2."
It seems to me that these statements by James Frank and JeffDylan are exactly the same. So what is the misunderstanding?
The statement made by TD@267 that the residence times (or atmospheric lifetimes) of individual molecules are irrelevant is correct but in conflict with the rebuttals posted by both Frank and Cook and numerous peer-reviewed modern climate science articles. Many such articles can be found by doing a keyword search on "condensable greenhouse gases" as suggested by Eclectic@290. [Please note that I am supplying evidence for my claims from credible sources.] From these statements and articles, it appears that in modern "climate science", the radiative forcing strength of a greenhouse gas (GHG) is highly dependent upon the atmospheric lifetimes of the GHG modecules whereas in fundamental physics, the greenhouse effect associated with any GHG is totally independent of atmospheric lifetimes of its molecules. In other words, according to basic first-principles physics, the greenhouse effect of any gas depends only on its concentration, IR spectrum, and to some extent its location; and is the same whether the individual molecules recently emerged from a condensed state via CO2 greenhouse heating, or if they had been in the atmosphere since the formation of the earth. The terms forcing and feedback do not appear anywhere in the laws of physics governing the greenhouse effect.
From the alleged dependence of the radiative forcing strength of a GHG on molecular lifetimes, climate science arrived at the general "principle" that no condensable GHG could be a radiative forcing, no matter how strong of a GHG it is. This then becomes the justification for disregarding the possibility of a water vapor radiative forcing and considering only the CO2 greenhouse heating as the radiative forcing which controls temperature. If there is a rise in temperature as a result of more CO2 being added to the atmosphere (causing more greenhouse heating), then more water (or ice) will be evaporated into the atmosphere as a result of this temperature change. At this point, the new climate science says that since this additional water vapor was introduced as a feedback to the rise in temperature, its contribution to the greenhouse heating is not disregarded on the basis of short molecular atmospheric lifetimes. Instead, this feedback H2O greenhouse heating is what's asserted to amplify the CO2 greenhouse effect. The laws of physics, however, do not distinguish between forcing and feedback GHG molecules in the atmosphere.
This result is what's called the CO2 "control knob" effect. Even though H2O vapor is the stronger GHG, CO2 is still believed to control the greenhouse effect. We have shown, however, that this is merely a prediction, or should I say artifact, of our new "climate science". Other artifacts include bazarre, highly anti-intuitive frozen world scenerios if all of the CO2 were removed from the atmosphere. Unfortunately, such predictions generally seem to be regarded as new discoveries rather than contradictions resulting from faulty assumptions.
In summary, we have shown a great departure of the newly discovered climate science from the fundamental laws of physics. Not only that, but this "science" isn't even consistent within itself. It would be total folly to base important environmental decisions on this version of "climate science".
[PS] Post currently embargoed pending sock-puppet investigation
Eclectic@279
I'm sure that albedo has its part in establishing temperatures, but that is not the issue here. The issue is in correctly calculating the greenhouse effect for water vapor. Now, James Frank and John Cook of this webpage along with Lacis et. al. (referenced by Dan Bailey@278) have claimed that water vapor and other condensable GHG cannot be a forcing of temperature since they too short-lived in the atmosphere to cause much greenhouse heating. TD@267, however, points out that GHG molecular atmospheric lifetimes are irrelavent in determining such heating. I won't argue that point at this time, but I think you would agree that we must at least be consistent. Therefore, if there is no temperature forcing due to the water vapor greenhouse effect, then there is no feedback to a rise in temperature either. There can only be one value for the H2O greenhouse heating, not two different values for forcing and feedback. Of course, with no H2O feedback, there will be no amplifying the CO2 greenhouse heating and no CO2 "control knob".
[TD] You misinterpreted Frank, Cook, and Lacis et al. by thinking they claimed that condensable GHGs cannot be forcings solely because individual molecules are too short-lived in the atmosphere. What actually is being "claimed" is that throwing into the atmosphere, more molecules of water vapor than the atmosphere can contain at the atmosphere's current temperature, results in that extra number of molecules (regardless of which individual molecules those are) quickly to fall out of the atmosphere without replacement of that fallen extra number, so that the total number of water vapor molecules in the atmosphere quickly returns to its original, lower value.
The original, lower value (equilibrium number of water vapor molecules) is maintained as water vapor molecules are thrown into the atmosphere by evaporating from the vast pools of liquid water that always are present around the world, followed by an equal number of water vapor molecules condensing out. If you throw extra water vapor molecules into the air, a molecule condensing out will not quickly be replaced by another molecule evaporating, because there is no room in the atmosphere at the atmosphere's current temperature. Therefore adding water vapor without first or simultaneously increasing the atmosphere's temperature by some other means, cannot force the temperature higher.
In contrast, if the temperature of the atmosphere is increased by some other means, more water molecules will make the leap from liquid to gas, and when they condense out other water molecules quickly will take their place, thereby creating and maintaining a larger total number of water vapor molecules in the atmosphere at any given moment. So an increase in temperature by any other means causes an increase in total number of water vapor molecules--a feedback to whatever caused that initial increase of temperature.
Non-condensing GHGs such as CO2 have no such temperature limits on the total numbers of molecules that can exist in the atmosphere at a given time. So throwing more of them into the atmosphere results in them just staying there, for a really really really long time until other mechanisms remove them.
It appears that Cosmoswarrior is back again.
Moderators: good luck and keep up the good work.
I have recently been reading a lot of comments at RealClimate and their comment threads are severely clogged by two spammers. (their OP's continue to be terrific and very informative). Tamino is also discussing this problem.
I think that SkS's decision to remove the spammers is the best way to go.
[DB] Yes, it was yet another iterative and boring sock puppet of serial spammer Jeff Dylan/cosmoswarrior/et al. No further replies to it are necessary as its posting rights have been rescinded.
As will happen to all future iterations of such. Be forewarned, Jeff.
Hello guys! I really appreciate that you collect arguments here and discuss about them. Because that is how science should be.
Now to the topic: What I don't understand at the moment: How can water vapor be a reliable feedback loop for global warming?
I understand that if the temperature gets hotter, more water can be absorbed by the atmosphere. To be accurate, for every degree (1 °C) of warming (at an operating point of 15°C on earth in average), the air can absorb circa 6% more water (see below for a derivation of this value (*1)). This means: If the global temperature increases by 1°C, the atmosphere can (but not must) hold 6% more water molecules. But when at the same time relative humidity drops by approximately 6%, we have the same amount of water molecules in the air than before the warming.
Thus, to state this effect as a positive feedback loop, the total amount of water absorbed by air must be coupled only to temperature and no other effects (for example to random distributions due to other unknown or non-considered effects). But looking at meteorological stations, relative humidity changes quit a lot. As a rule of thumb, one can say that relative humidity is in the range of 80% (I looked up several meteorological stations, but this rule of thumb certainly will vary from station to station), but also when looking at the average relative humidity of one year at the same station, relative humidity fluctuates by approximately 10%-points (*2). This would compensate the impact of the increased saturation limit completely. So to me it seems that the feedback loop is just a mathematical model, which does not withstand empirical evidence by measurements of relative humidity.
But you may prove me wrong. Are there any studies about the (global) relative humidity, which state that it indeed stays constant in average? To me it seems to be a very harsh and unrealistic criterion, that the relative humidity must not fluctuate more than 6% (otherwise this fluctuation would outweigh the feedback effect).
So how can it be explained that the absolute amount of water in atmosphere is coupled only to temperature, and relative humidity stays constant over time and is not coupled to any other effects?
(*1): One can calculate the saturation limit of water in air using the formula Rho = Ew / (Rw * T), Rho being the saturation limit [kg/m^3], Rw the specific gas constant of water (461.52 J/(kg * K)) and Ew the pressure for water/steam equilibrium, which is also a function of T, the temperature in Kelvin: Ew = 10^(A-B/(T-C))*1000, A = 7.2326; B = 1750.286; C = 38.1. Linearizing around 15°C (equals 288.15 Kelvin) yields a slope of 6% saturation limit change per 1°C change. When linearizing around an operation point of 0°C or 30°C instead of 15°C, the slope does not change much (only by 1%-point).
Link 1
Link 2
(*2): For example I looked up the meteorological station of Hamburg, which has a nice graph of relative humidity over time. One can see that fluctuation is quite high, even averaged over one year. I also could not see a correlation to temperature at first glance. (Sorry, it's a German source, but you should be able to interpret the numbers nonetheless.)
Link 3
Link 4
[DB] Shortened links breaking page formatting.
Silbersulfid @324 :
When considering the greenhouse effect of water vapor, you need to be interested in the absolute water vapor densities in the upper troposphere — not at the ground-level meteorological stations. Relative humidity is irrelevant.
At low altitude, water vapor has a very high "optical density" [to the appropriate Infra-Red wavelengths]. At high altitude, the water vapor density becomes thin enough for IR radiation to escape from the Earth's atmosphere. The high altitude temperature determines the rate of loss of IR energy. (A similar mechanism operates for CO2, but at different wavelengths and altitude, natuerlich.)