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Explaining how the water vapor greenhouse effect works

What the science says...

Select a level... Basic Intermediate

Increased CO2 makes more water vapor, a greenhouse gas which amplifies warming

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.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

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

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Comments 151 to 175 out of 375:

  1. With a good grasp of the algorithms involved to calculate a forcing, we could probably just analyze that.

    And if someone was actually going to carry out the computational experiment, perhaps limit the number of steps to avoid approximation error propagation from interferring.

  2. Jose_X:

    1)  The total forcing of current CO2 concentrations relative to zero concentrations is, by best estimate, 38 W/m^2.  This is different from the all sky contribution of CO2 to the total greenhouse effect because of the overlap with the contribution from H2O and clouds.  This is different from the all sky contribution of CO2 to the total greenhouse effect because of the overlap with the contribution from H2O and clouds.  To determine the forcing, we must imagine an initial situation with, effectively, no water vapour and no clouds because of the very cold temperatures (-19 C mean global surface temperature).  Because there is minimal water vapour and clouds, there is also minimal overlap in absorption frequencies with CO2, so the forcing of adding the CO2 correlates best with single factor addition contribution from table 1.

    Clearly calculating this value relies on using a Global Circulation Model (GCM), and the use of different GCM's will give slightly different results.  Because we are reliant on GCMs, this represents our best theoretical prediction, for (of course) we cannot actually conduct the experiment.

    Of interest, given the formula ΔT = λ RF (change in global mean temperature equals the climate sensitivity parameter times radiative forcing, see IPCC AR4)  The change in temperture of 33 C as a result of the greenhouse effect with a forcing of 38 W/m^2 gives λ = 0.87 C/(W/m^2), equivalent to a climate sensitivity of  3.2 C per doubling of CO2.  This is, of course, a very crude estimate, but it would be surprising of the Charney climate sensitivity was not in that ball park.  It should be noted that if we reduce the forcing (to allow for some overlaps), the estimated climate sensitivity increases.

    2)  The only way to determine the radiative forcing for a given value of CO2 for values significantly different from current values is by means of model runs on GCMs.  You would run two concurrent experiments.  In the first experiment you run the GCM with zero CO2 until it reaches equilibrium.  You then increase the CO2 concentration to 40 ppmv, holding all feedbacks constant and determine the difference in TOA flux, thus determining RF(0->40) (Radiative Forcing for a change from 0 to 40 ppmv).  You then increase the CO2 concentration to 80 ppmv and determine RF(0->80), and so on.  In the second experiment you would do the same thing, but after determining the RF for each increment, you would allow the feedbacks to vary, and run the GCM until equilibrium was obtained before introducing the next increment.  By doing this you would obtain RF(0->40), RF(40->80), etc.

    As I understand your question, you are asking whether  RF(0->40) + RF(40->80) + RF(80->120) + ... + RF(360->400) = RF(0->400).

    I think, based on the points I made in (1) above, they would not.  Specifically, RF((0->400) =~= 38 W/m^2, but because of the overlaps with water vapour, the sum of the smaller steps would be closer to the net current contribution of CO2 to the total greenhouse effect (30 W/m^2).  Specifically, it would equal the sum of the radiative forcings for each step minus the contribution to the overlap from each step but the last.  The smaller the steps you used, the closer it would approximate to the current CO2 contribution.  If you used very few steps, however, the result would not be very different from using just one step.

    What follows from this? Virtually nothing.  I would need to amend my comments above, but:

    a) The partition of current effect in cases of overlap is largely a matter of convention, and adoption of a different convention would resolve the discrepancy; and most importantly

    b) Radiative Forcing is not a concept in a basic physical theory, but rather a concept used in calculating the approximate consequences of the complex interactions of basic physical theories.  Consequently what is required of it is that it be sufficiently useful in its range of operations - which of course, it is.

    Finally, it is possible (as if this needs mentioning) that my reasoning above is faulty.  Consequently I would be very interested in any criticism of my comments.  

  3. Some interesting discussions in this thread, thanks. Could someone please clarify one small issue for me? Early in this multi-year thread, mention was made of the short residence time of water vapour in the atmosphere. It was also suggested that, given a specific and stable temperature-pressure combination, the atmosphere responded "almost instantly" to the addition or subtraction of water.

    Tom Dayton (#69) wrote...What does primarily limit the amount of water vapor in the (Earth's) atmosphere is the atmosphere's temperature. At a given temperature, adding more water vapor "nearly instantly" forces water vapor to drop out of the atmosphere. "Nearly instantly" in this context means "so fast that there is no time for significant atmospheric heating from the extra water vapor."
    What's needed to increase water vapor for more than 10 days is an increase in atmospheric temperature.

    I would like to understand how quickly the atmosphere responds to a sudden influx of water vapour - say, from an anthropogenic source - does it really depend on the mean residence time of water vapour, and is 10 days a reasonable approximation of water vapour MRT? (It's probably not important in terms of AGW theory - 10 days is as good as instant anyway - but I am curious anyway.) My understanding of mean residence time (MRT) is the mean time that each water molecule stays in the atmosphere. (Speaking in terms of half-lives would seem more natural to me). Intuitively, I would have thought the atmosphere responded faster than the residence time suggests - couldn't the added water displace other water to reach equilibrium again even faster than the MRT? But my intuitions might be way off, so I'd be happy to be educated by those who have studied this.

    To pick a simplistic example (just to illustrate what I am asking, not because it is anything like the climate): take a dam that is always full, right up to the overflow wall, because of an inflowing stream. If I add a bucket of heavy (deuterium-rich) water, it might mix into the much larger volume of the dam such that the added molecules have a long MRT, measured in months. There would be an earlier distributive/mixing phase, from diffusion and convection, followed by an exponential decline dependent on the outflow from the dam. But the dam could discharge the extra volume (not the actual heavy water molecules from the bucket) very quickly, perhaps in minutes.

    So, is the atmosphere's return to its pressure-temperature-determined water content rapid, like the dam losing the added volume of water, or is it more like the dam eventually losing the radio-labelled molecules themselves? And does the answer change for different parts of the atmosphere?

    Thanks in advance.

  4. Leto, good question.

    Essentially, you answered the question yourself already, through your example.

    The atmosphere is a fluid, much like the water behind the dam. So it behaves like the dammed water, aka mathematically there is no difference here. Scale, however, is important.

    Your example illustrates two aspects:

    1. Residence time: The dam is in equilibrium, discharging as much water as it takes in. Residence times are e-folding times (time to 1/e) and represent the solution to their defining equation, namely tau =  abundance / removal rate. That is long in the dam case, but relatively short in the atmosphere case. In the latter case, tau varies strongly by location, for example latitude (high cloud density and rain (=removal) rates in the tropics!)

    2. Local vs. global viewpoint: You cannot instantaneously mix a local emission into a larger volume. As everyone knows from looking at car exhaust ("instant" injection), high water vapor amounts condense very quickly  when the surrounding air cannot support that much water vapor, such as during cold winter days. Only if your heavy water mixed uniformly and instantaneously into the large dammed water volume (which it does not) would its residence time be equal the dammed water residence time.

    Increasing atmospheric temperature is equivalent to slowly raising the dam wall height in your example, allowing the lake behind it to hold more water over time. Aka, during that time, the dammed water (the atmophere) is not in equilibrium.

    Locally/regionally, a stronger greenhouse effect does indeed occur where atmospheric water vapor is in higher concentration (air-temp. during humid nights drops slower than during dryer nights; think of how cold a desert can get at night). One needs to integrate over these effects to get the global picture. There is not one scale (tau) fits all, there are many scales that matter.

  5. Tom Curtis 152:
    Concerning item 2:

    The two experiments might answer part of what I was asking, but I am interested in at least one variation in order to try to factor out the water/CO2 overlap so to answer the main question I had.

    First, I am not sure why to get RF(0->400) in experiment 1 (where you didn't run feedbacks), you went through a sequence in steps of 40? The key to the main question I had might be there. Since the question of CO2 contribution was a vehicle to address the partion equivalence question (ah, the truth comes out), I think I would now like to "run" a new pair of experiments, each of which avoids running feedbacks.

    Experiment 1:
    1- allow the GCM to reach equilibrium with 0 CO2
    2- step to 400 CO2 and measure the RF.
    Experiment 2:
    1- same as step 1 for Experiment 1
    2- step to 40 CO2 and measure RF
    3- allow equilibrium to be reached
    4- repeat steps 2 and 3 but increasing the CO2 concentration each round by 40.
    5- the repetition ends when at step 2 we have stepped to 400 CO2 and measured RF.

    Question: Does the sum of all 10 RF measurements made at step 2 in Experiment 2 equal the single RF measured from Experiment 1?

    Later I intend to look at 2 more experiments where each run feedbacks, but I'm not ready to present that example and associated questions. I will be interested in the imbalances at TOA as the feedbacks are in effect (transients towards equilibrium). One question now: Is it meaningful to ask for the RF value of say H2O (a feedback of CO2)?

  6. Jose_X - "Question: Does the sum of all 10 RF measurements made at step 2 in Experiment 2 equal the single RF measured from Experiment 1?"

    Yes, rather by definition. The radiative forcing of 400ppm CO2 will be the same no matter the path; the increments at whatever slicing have to add up to the same number. 

    How could the forcing at 400ppm possibly not equal the forcing at 400ppm? With forcings at different intermediate concentrations (as the first one or two of your small steps may still be in the linear range, not logarithmic, depending on step size) falling somewhere between the 0ppm and the 400ppm levels??? I cannot conceive of the math working out any other way...

    If you want to play around with these numbers, you might try out one of the online MODTRAN packages. Keep in mind that the 3.7 W/m2 forcing change per doubling of CO2 (curve fit in the log range) was calculated from multiple numeric runs at different latitudes (Myhre 1998). 

    In regards to the CO2/water overlap, again, there is no substitute for actually doing the math. Which in this case means line-by-line numeric codes using a multiple layer model; I believe MODTRAN-style calculations converge for any particular conditions at about 20 layers or so, with additional segmentation not greatly affecting the numbers. There is no simple analytic formulation - the radiative effects depend on GHG concentration (water vapor falling off faster than CO2), altitude, and temperature at each level, and as in ordinary differential equations, a numeric approach (similar in concept to Runge-Katta) is the most appropriate. 

  7. KR 156 >> How could the forcing at 400ppm possibly not equal the forcing at 400ppm?

    I'll quote Tom Curtis 152:
    > b) Radiative Forcing is not a concept in a basic physical theory, but rather a concept used in calculating the approximate consequences of the complex interactions of basic physical theories.  Consequently what is required of it is that it be sufficiently useful in its range of operations - which of course, it is.

    One might ask in Calculus, how could the derivative of x^3 + x^2 not possibly be the same as the derivative of x^3 plus the derivative of x^2? Well, the derivative operator was designed, among other things, to be linear. But we can design many algorithms/operators that don't have that feature. In fact, it's not really clear that an algorithm has such a property until it is "proven" in a rigorous analytical sense.

    The odd thing to me about RF is that it disappers after equilibrium is reached. By looking at equilibrium radiation at TOA or on the surface, you can't tell. In fact, there are many independent variables that go into deriving RF and if any of those are left out of the analysis, you really can't recapture that value. And improvements in our understanding in the future might even lead to different algorithms that would derive different RF. Each time we engage in a new algorithm, arguably, we should try to prove that certain mathematical properties exist. I don't think it is obvious that a complex algorithm dependent on numerous factors would automatically be well-behaved in any particular sense of the word.

    OK, let's assume we are going from a given starting concentration of CO2 to another where the RF value "at" each path point can be modeled by roughly the same logarithmic function (dependent on a reference point). We can take multiple paths there.

    Question: is it obviously true that a*ln(b*(x_1/x_0)) + a*ln(b*(x_2/x_1)) = a*ln(b*(x_2/x_0)) for all x_1 and x_2? At best we should perform the algebra first to be sure (or to show instead that the path does matter). Here I believe the path doesn't matter.

    What if the approximating functions used along the partitioning path were entirely different from each other?

    Also, we can even look at forcings by different gases and ask, what if the gases are added in different orders and quantities?

    If the approximation method used to address any of these questions gives a result that the partition chosen does matters, one can't argue that if we simply had used the true and best method (codes) then it all would have worked out because it would adhere to reality, etc, etc. Every algorithm/calculation is an approximation of reality to some degree. Why should today's current best procedure necessarily be the best we will ever get so to allow that logic to work?

    OK, since I am writing before carefully reviewing the logic of this comment sufficiently, I too would certainly appreciate comments, complaints, etc. I heretoforth reserve the right to backtrack through an unlimited number of "undos".

    PS: "KR" and "RF" can get a little confusing. They each have an R and that looks like the other letter, a verticle line with two smaller lines connected each in at least a quasi horizontal position.

    KR, thanks for the modtran link. I'll see if I can make use of it.

  8. Jose_X @155:  In taking multiple steps in the first experiment, the atmosphere was never allowed to equilibriate.  As a result, the mean global surface temperature, water vapour content of the atmosphere, etc, was constant at 0 ppmv CO2 levels throughout the experiment.  You could, it you want run multiple experiments, where in each experiment you allow the atmosphere to equilibriate at 0 ppmv CO2, then add slugs of 40 ppmv CO2, 80 ppmv CO2, 120 ppmv CO2, etc, but you would get the same result.  That result is the RF(0->40), RF(0->80), etc, which allows you to see the incremental difference in RF not just for the step from 0 to 400, but for all the intermediate steps as well.

    In fact, thinking about it, it would be best to run 10 experiments.  One in which you set CO2 to 0 ppmv, and allow it to equilibriate, then incrementally increase to 400 ppmv without allowing equilibriation between each step.  One in which you set CO2 to 40 ppmv, then incrimentally increase without allowing equilibriation, and so on.  This series of experiments would allow you to calculate the RF(0-40), (0-80), ... . ((0-400); (40-80), (40-120), ..., (40-400), ... ,(360-400).  

    Doing so, I suspect you would find the difference between RF(120-400) and the sum of the differences RF(120-160), RF(160-200), etc would be small.  That is, most of the H2O/clouds/CO2 overlap would arise in the first few increments because the first few increments of CO2 have the largest effect on temperature and hence on CO2 content.  If, however, we pushed the experiments out to 2000 ppmv, the difference introduced by each incremental step would start rising again as the increase in vapour pressure of water with increase in temperture rises rapidly above 40 C (ie, typical tropical tempertures with very high CO2).

    Finally, your suggested experiment is no different than mine, except that it does not obtain intermediate values for the RF relative to 0 ppmv.     Consequently, by my analysis, it would also show the RF(0-400) to be greater than the sum of the incremental radiative forcings.

  9. KR @156, I think you aren't sufficiently considering the fact that the radiative forcing between two concentrations is the difference in TOA radiative flux at equilibrium for one concentration and the TOA radiative flux for the other concentration with all other values (ignoring the stratosphere) retaining the equilibrium values for the other concentration.

    In fact, one relevant experiment for assessing a related issue has been done.  In Schmidt et al, 2010 they compared the effect of adding a slug of IR active compounds (and clouds) to a pristine atmosphere (N2 and O2 only), and the effect of removing the same size slug from an atmosphere with the composition found in the 1980s.  Because CO2 has virtually no overlap with any factor other than water vapour and clouds, that experiment effectively determines the RF of adding 340 ppmv to an atmosphere with no CO2; and the RF of removing 340 ppmv from an atmosphere with 340 ppmv of CO2.  The result for adding the CO2, ie RF(0→340), is 38 W/m^2.  The result for removing the CO2, ie, RF(340→0), is 22 W/m^2.  The difference is because, in the very cold climate with 0 ppmv CO2 after equilibriation, there is virtually no water vapour and no clouds in the atmosphere (not quite true but close enough for exposition).  In the case of addition, that means there is no overlap, and the full effect of the addition can be experienced.  In the case of removal, the full load of water vapour and clouds are retained in the atmosphere because this is a no feedback situation.  So, not only does the RF of a large slug not equal the RF of the sum of a series of small slugs of the same size, but the RF varies depending on whether you are adding, or removing the slug.

    This does not mean that the equilibrium temperature will differ for a given concentration of CO2 depending on whether you arrive at that concentration by increasing or decreasing CO2.  To the extent that the difference in RF between the two methods is a consequence of the overlap with water vapour and clouds, as the vapour pressure of water in the atmosphere adjusts to a reduced (or increased) temperature, the extent of overlap will equalize.  So, λ also differs between the two cases such that λ'RF(0→340) = λ"RF(340→0).

    Again, this later property is not necessarilly true, and is not true for some values of CO2 concentration and for Earth System Climate Sensitivity, with a bifurcation between snowball earth and non-snowball earth states resulting in λ'RF(a→b) ≠ λ"RF(b→a) for some CO2 concentrations, a and b.

    Finally, and as you point out, the simplified formular does apply within error, and has been shown to apply for a large range of CO2 concentrations close to the present value (ie, from about 150 ppmv to several thousand ppmv at least), and in that range, to a close approximation, it does not matter whether you increase or decrease, or change the concentration in a single slug or by increments, the answer will be the same.

  10. Tom Curtis, Jose_X - If I am referring to running the same experiment in steps (one or many), and you are referring to something else entirely (with/without feedback, under different conditions, for example), then my apologies, that's apples and oranges. Different questions entirely, and comparing the two is not particularly relevant. 

    What I was discussing is that a numeric analysis of 400ppm will be the same as another analysis of 400ppm, all other things held constant (including the presence/absence of feedbacks and whether or not sufficient time for equilibrium is allowed), regardless of other calculations, and that interim values for GHG levels will and must fall somewhere between the 0ppm and 400ppm numbers. Not retaining feedback levels for one forcing level over to another, which is invalid, but running each step of the experiment under the same conditions as the final 400ppm evaluation. To be really clear, I'm speaking of total forcings, not deltas, as the deltas will be dependent on the temperature at the time of the delta - if you are looking at varying temporal evolution without equilibrium, all bets are off. 

    But I'll freely admit that I may not be fully following the conversation - I'm still quite unclear on what Jose_X wishes to investigate, what issues he's seeking insight into. 

  11. gws #154, thanks.

  12. KR 160:
    >> To be really clear, I'm speaking of total forcings, not deltas, as the deltas will be dependent on the temperature at the time of the delta - if you are looking at varying temporal evolution without equilibrium, all bets are off.

    To clarify a bit on what you mean by "deltas", would you say that the following is a description of deltas that are off the table if each "slug" was carried out to equilibrium in the runs and if both the slugs and the overlapping large jump avoided feedbacks?

    > So, not only does the RF of a large slug not equal the RF of the sum of a series of small slugs of the same size, but the RF varies depending on whether you are adding, or removing the slug.

    My question was about the nature of RF. Specifically, I am interested in knowing if doing a 2x CO2 and when that is reached doing another 2x CO2 from that new equilibrium point and then another .. if those three added together would give the same value as if we do a single 8x CO2 calculation (or perhaps for some other ghg or other ratio). If the answer is that the values would differ nontrivially, then I have to wonder about the meaning of even a single RF used in a model (though I'm not worried if the model approximations are linear and reflect reality within a limited domain we would work in) and about whether what we get from the ideal situation of doing a 2x in one shot to calculate RF is meaningful to a planet that is adding CO2 in very much smaller increments (smaller relative to the ability of the planet to keep up, if that is true). A primary goal of mine is to understand the model decently.

  13. Jose_X - Short answer, yes. The sum of equilibrium forcings and temperature changes for 3*2x CO2 will equal 1x8x CO2. Or for any other subdivision. 

    What is being changed is to total emissivity of the atmosphere, which by the Stephan-Boltzman law and the amount of incoming solar energy sets the climate temperature. 

    The only possible differences would be if 2^3*concentration did not equal 8*concentration (mathematic nonsense), or if the temporal evolution of feedbacks differed with increment size (at equilibrium, there should be no difference), or passing some hysteresis point (say, driving into an Icehouse Earth state that requires a huge amount of forcing change to switch out of - which would require a forcing overshoot and reversal). So no, there should be no differences whatsoever in equilibrium total forcing, in equilibrium temperature, dependent on the path to that increase. 

  14. KR and Tom Curtis, I would guess we are probably in agreement that at least within some mostly linear range and to first approximation that the path of deltas/slugs doesn't matter for CO2 and probably also not for most other greenhouse gases. We also probably agree that, in contrast, moving across certain temperatures (eg, hysteresis points) will cause RF calculations to depend on the details of the path taken.

    I have a better understanding of this question and will probably consider it more later on rather than sooner, although feel free to keep tempting me back into the conversation if you have something else to add (fyi and if I appear not to return to this thread, my email is hozelda at the yahoo com). Thanks.

  15. I was listening to Radio National (Australia) yesterday and heard Pielke Jr claim that water vapour actually has a negative forcing, meaning a doubling in CO2 would cause a global temperature change of 0.2oC. I was astounded, not surprisingly! Hands up all those scientists who think CO2 has a negative forcing? Anyone? (Cue sound of crickets ...)

  16. Doug - it's a bogus claim, but there is a tiny kernel of truth in there. Increasing the water vapour content of the atmosphere  would indeed block more sunlight from reaching the Earth's surface. It isn't sufficient to counteract the warming impact from increased water vapour though. 

  17. Rob Painting @ 166, the extraordinary claim that the net response to a doubling of CO2 would be a temperature increase of 0.2OC requires more than "a tiny kernel of truth", I would have thought. I have emailed Radio National to see if they have a transcript of the interview and will post here again, if I get a response.

  18. Absolutely Doug. But it gives insight into how some contrarians operate.  

  19. The theory is that adding CO2 to the atmosphere will further retard the passage of IR radiation through the atmosphere that will cause warming.  The warming in turn will raise the capacity of the atmosphere tho hold H2O which in turn will cause H2O to be added to the atmosphere which further retards the passage of IR radiation which will cause more warming.  So the H2O additional warming effect is modeled as a positive feedback to adding CO2 to the atmosphere.  This is what appears to happen in the lower atmosphere.  But in the upper atmosphere the opposite occurs.  The retarding of CO2 that warms the lower atmosphere acts to cool the upper atmosphere.  It is in the upper atmosphere where IR radiation is radiated to space.  Assuming a constant solar radiance and constant value of earth albedo, for the earth to gain energy the black body appearance of the earth has to drop which means temperatures in the upper atmosphere decrease.  Decrease in temperature in the upper atmosphere causes H20 capacity in the upper atmosphere to decrease which causes H2O levels to decrease.  H2O is a green house gas.  H2O decreasing will have the opposite effect of increasing CO2 in the atmosphere.  Decreasing H2O will allow more IR radiation to leak through cooling the lower atmosphere and warming the upper atmosphere.  As the upper atmosphere warms back up again the net flow of energy into the earth is decreased.  So in the upper atmosphere H2O acts as a negative feedback to added CO2.  Negative feedback systems are inherently stable.   

  20. Further to my comment above, Radio National have come back to me. I was wrong to attribute the comment to Pielke  Jnr: it was actually Lindzen. From the transcript:

    "Richard Lindzen: My own particular research on this topic has dealt with water vapour, for a very simple reason; water vapour itself could account for 98.5% of the present greenhouse effect. It's the giant greenhouse gas, and in fact no model would give more than about 1° warming for a doubling of CO2 unless it had water vapour amplify it. And so we've been looking at how they deal with water vapour.

    And they don't have the physics that we know accounts for water vapour, they are having numerical errors all over the place. So here you have the major greenhouse gas, you're worrying about something that's in the 1% region, and you're getting the thing that is 98.5% totally wrong, 100% errors.

    We've been doing some studies on it, and I strongly feel that water vapour in fact is acting in the opposite direction from what the models suggest, and according to our calculations it should keep the warming for a doubling of CO2 down to about two-tenths of a degree. You couldn't tell that from natural variability."

    So, warming from a doubling of CO2 amounts to two tenths of a degree, if the good Professor does not have his facts bass ackwards. There, don't you feel safer now?

  21. There's a good article in the June 2013 issue of Physics Today: "Water in the Atmosphere," by Bjorn Stevens and Sandrine Bony, pages 29-34.


    Ok, I've been having a back and forth about CO2 and water vapor and the usual about how it's a much stronger green house gas..  I referenced SK and discriped how water vapor creates a positive feedback loop.  He then goes into areas of science that are pretty technical but I suspect it's somewhat convoluted.

    He claims to be a H2O vapor is that it is vastly more variable as a mol fraction in the atmosphere than CO2, and that the signal to noise ratio picking up the effects of a boost of CO2 from 0.25% to 0.40% is going to be a challenge given the background noise of H2O cycling from near 0% to over 5% wildly over vast areas of the earth such that it is hard to know what average is.
    That is just background physics from knowing how relative humidity works, and knowing quite a bit about radiation heat transfer in gasses from having taken graduate level engineering classes in that subject.

  23. If we have a positive feedback of amount of water vapor to the Earth's temperature, who stabilize tempereture for the last billion year?  Why our planet not frozen as Mars or wapored as Venus?  

    There is a theory of Biotic Regulation by Prof. Victor G. Gorshkov, where that stabilization entity is suggested to Forest -

    You can find a lot of information and arguments on that site.




    [TD] Positive feedback does not necessarily mean runaway warming, as is explained in a post here on Skeptical Science; after you read the Basic tabbed pane there, read the Intermediate and then the Advanced tabbed panes.  Another way to find that post is to enter "positive feedback" in the Search box at the top left of any page.  There are several influences on the Earth's temperature, some acting over long time frames and some over short time frames.

  24. yes, we can't see a positive feedback.  But the theory of greenhouse gases predict it.  So it's something wrong with theory itself or some other factors give more influence then predictions of greenhouse theory.   So my Q is - Which factors stabilize the climat in the relatively low temperature range for a long time frame? I can't find any powerfull enough factors by visiting a link "several influences on the Earth's temperature".

    That's why I'm asking here for critics or comments of Biotic Regulation Theory of Prof. Gorshkov 


    [TD]  bvt123, you either did not read or did not understand the first post I pointed you to, which explains that positive feedback can and does exist without it being runaway positive feedback.  As long as the gain is less than one, positive feedback limits itself, progressively reducing in each of its feedback cycles until it reaches zero feedback.  You are incorrect in saying "we can't see a positive feedback"; we do indeed see multiple positive feedbacks.

  25. of course I read that article, and of course I don't catch the idea.  There are nothing said why the gain is less than one. There are no arguments about positive feedback of water wapor and the gain for it.

    Nothing  said about negative feedback factors, but just proposed some not proved diagrams in very wide temperature range.  The average tempereture of Earth was stable  (d=10C) for the last millios years, and we need a clear answer of cause of such stability. Not a model, but some good theory.  


    [TD]  You seem to be assuming that positive feedback must run away unless there is some special, unusual, magical factor to make the gain less than one.  You are wrong.  As scaddenp now has explained to you, feedbacks are what they are.  Positive feedbacks of various phenomena in the wide universe are no more common than negative feedbacks.  To determine the signs and values of feedbacks we must measure.  Just one example is in the post on water vapor feedback.

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