Water vapour and warming potential

[Pragma]

I have been trying to get a number for the relative warming potential of water vapour and it seems elusive.

More than that, there seems to be some possible deception going on.

I have been told that H2O is a more powerful GHG but, IIRC, in Wikipedia, under GHGs, they state that H2O is not included because it is not persistent.

I think this is a rubbish statement. Methane is also not  persistent, but on a longer timeframe. Same for CO2.

Here is my reasoning.

At a given average earth temperature, there will be x amount of water vapour keeping the earth warm. The next day it may be some totally different water vapour doing the same job.

If the level of water vapour increases 7%/deg C, and we have raised the earth's temperature by one degree C, then on average, we now have 7% more water vapour in the air at any time, and whatever effect the water vapour had, it is now 7% more, regardless of how persistent it is.

I would like to know what the CO2 equivalent of the increased water vapour is, as it is cited as a large component in a runaway warming scenario.

Am I missing something that cancels this out, or is it a pesky little detail that no one wants to talk about? Why is methane and NO used to calculate a total CO2 equivalent, but water vapour is not?

[Richard Rathbone]

I have been trying to get a number for the relative warming potential of water vapour and it seems elusive.

More than that, there seems to be some possible deception going on.

I have been told that H2O is a more powerful GHG but, IIRC, in Wikipedia, under GHGs, they state that H2O is not included because it is not persistent.

I think this is a rubbish statement. Methane is also not  persistent, but on a longer timeframe. Same for CO2.

Here is my reasoning.

At a given average earth temperature, there will be x amount of water vapour keeping the earth warm. The next day it may be some totally different water vapour doing the same job.

If the level of water vapour increases 7%/deg C, and we have raised the earth's temperature by one degree C, then on average, we now have 7% more water vapour in the air at any time, and whatever effect the water vapour had, it is now 7% more, regardless of how persistent it is.

I would like to know what the CO2 equivalent of the increased water vapour is, as it is cited as a large component in a runaway warming scenario.

Am I missing something that cancels this out, or is it a pesky little detail that no one wants to talk about? Why is methane and NO used to calculate a total CO2 equivalent, but water vapour is not?

What you are missing is that the increased water vapour is an effect of CO2 (or whatever else caused that temperature increase) and not of agricultural irrigation (or some other way in which humans increase evaporation). Consequently it counts as part of the GWP of CO2 (or whatever else caused the temperature increase).

Water does stay in the atmosphere for a few days, but not the decades to centuries that CO2 does.

https://en.wikipedia.org/wiki/Greenhouse_gas#Role_of_water_vapor

[gerontocrat]

Yes water is being recycled all the time. But that is not the point. The increased water vapour in the atmosphere will remain as long as there is AGW, and since CO2 once up there stays up there AGW is built in for a long, long time.

Basic physics tells us AGW by increasing the warmth of the air has increased the ability of the air to retain water. I think I have seen a figure of on average 7% more water in the atmosphere at any time due to AGW. This vapour is a greenhouse gas.

The increase in water vapour is therefore a positive feedback to AGW caused by increasing CO2 ppm. And until CO2 ppm is reduced the effect will remain. And while CO2 ppm goes up the amount of water vapour in the atmosphere will go up. Obviously, if the magic silver bullet of technology gets rid of CO2 and global temperatures go down, water vapour will quickly go down accelerating that downward trend in global temperatures. I will be long dead before that day.

It is like a tap being turned on more and more increasing the flow more and more that will never stop increasing until temperatures stabilise, and will never get less unless global temperatures drop.

Try this article from skepticascience.com explaining what it is and how this is manipulated by deniers to claim CO2 ain't the big problem. https://skepticalscience.com/water-vapor-greenhouse-gas.htm

Extract

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.

[Pragma]

y is methane and NO used to calculate a total CO2 equivalent, but water vapour is not?

Consequently it counts as part of the GWP of CO2 (or whatever else caused the temperature increase).

https://en.wikipedia.org/wiki/Greenhouse_gas#Role_of_water_vapor
[/quote]

The article you quoted corresponds to what I wrote, but it makes no mention of the water vapour effect being included in the forcing function or warming potential of CO2 levels. Do you have any other references that confirm what you say?

Personally, it sounds "off" because the water vapour does not appear contemporaneously with the CO2, in sort of the reverse of how methane GHG potency is stated, dependent on time. Unlike water vapour, the GHG is clearly stated, and that is my point.

Cheers

[Pragma]

Gerontocrat,

I understand the basics and the mechanisms. What you say confirms what I wrote.

The quote you provided dances around what I am looking for, and I could intuit some rough values from it, but I don't understand why hard numbers are so hard to find.

It is easy to find an equation that takes two different CO2 concentration levels and can then give you, in watts/m², the difference in the radiative forcing.

It is: ln(C1/C2)*5.35. (IIRC)

I see no reason why the same could not be done for water vapour.

Cheers

[Tor Bejnar]

H2O vapor concentration is actually mostly controlled by its relation to temperature and not CO2 concentration.  (But, yes, temperature rises with increased atmospheric CO2 concentration.)

From Skeptical Science linked above:

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 when the temperature goes down (such as in clouds), the water vapor precipitates out.  Therefore, 'watching' H2O vapor go up and down will be generally unhelpful in discerning the climatic changes due to increases in the more stable greenhouse gases - but definitely included in the calculations of 'the blanket thickness' that makes places hotter than they used to be.

If there is not an H2O source, e.g., mid-continent deserts, hot air can remain relatively dry.

[Pragma]

H2O vapor concentration is actually mostly controlled by its relation to temperature and not CO2 concentration.  (But, yes, temperature rises with increased atmospheric CO2 concentration.)

From Skeptical Science linked above:

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 when the temperature goes down, the water vapor precipitates out.

If there is not an H2O source, e.g., mid-continent deserts, the air can remain relatively dry.

Thanks Tor, but that is still skirting the issue. The above are individual mechanisms. I would like to be able to find out specific numbers for the big picture.

As I said. we have increased earth's temperature ~ 1 deg C, which has, on average, increased the water vapour in the atmosphere ~7%. A previous quote states that the 7% increase will directly result in a further 1 deg C rise. As we have increased the CO₂ level by 51%, is it valid to say the radiative forcing function of water vapour is ~ 7x CO₂? (51/7)

To put it another way, after the 1 deg C rise, do we now have a CO₂ equivalent concentration of 555 ppm?

I see the quote, "water vapour is a much more powerful greenhouse gas" everywhere, but no one seems to want to put a number to it.

[Rich]

Water vapor is indeed the most powerful GHG on a cumulative basis. The problem is that unlike other GHG's it is not a "well mixed" gas and quantifying it's earth concentration is difficult.

If you set up shop in Mauna Loa, the CO2 or methane levels are going to be relatively constant on a day to day basis. If it's 10C or 40C outside, the level of well mixed GHG's like CO2 are not going to vary.

The level of water vapor is going to vary dramatically on a daily basis. So what concentration of CO2 are we going to use in our radiative forcing calculation?

Climate folks consider water vapor GHG effect to be a positive feedback included in ECS. The latest models are turning up much higher preliminary results for ECS. It will be interesting to see what falls out.

[gerontocrat]

I see the quote, "water vapour is a much more powerful greenhouse gas" everywhere, but no one seems to want to put a number to it.

I assume the +ve feedback of increased water vapour is included in the assessed effect of increases in CO2e.

If you then put a figure for the value of the water vapour +ve feedback, is there not a real risk of double counting?

Here's my draft tweet.
"They said that CO2 was gonna push up temps by 2 degrees. But Wikipedia says for every 1 degree up from CO2, water vapour adds one degree. So those dumb scientists are wrong. Temperatures are going up 4 degrees. "

Closely followed by Voldermot and his followers telling everybody that climate science is a heap of faeces.

[Sciguy]

Here's another explanation of the water vapor feedback (note that this article is from 2008, so the exact numbers have changes a little):

https://www.yaleclimateconnections.org/2008/02/common-climate-misconceptions-the-water-vapor-feedback-2/

Unlike water vapor, carbon dioxide, methane, and nitrous oxide are long-lived greenhouse gases. Carbon dioxide remains in the atmosphere for about 100 years (though this is somewhat of a simplification, as some is removed quickly, some stays for around a century, and some remains almost indefinitely). Methane stays in the atmosphere for a dozen years on average before decomposing into carbon dioxide and water vapor. Nitrous oxide remains around for over a century.

These long-lived greenhouse gases produce sustained warming, which drives the water vapor feedback. If concentrations of greenhouse gases are reduced, the planet will cool and the water vapor feedback will work the opposite way: lower temperatures lead to lower atmospheric water vapor concentrations, further cooling the Earth. The short residence time and relatively constant magnitude of evaporation as a function of temperature mean that water vapor will always follow, not lead, changes in long-lived greenhouse gases.

Climate scientists can quantify the effect of the water vapor feedback on the climate system, as shown by frequently modeled effects of doubling CO2. In the absence of a water vapor feedback, doubled CO2 would increase global temperatures by around 1 to 1.2 degrees C (1.8 to 2.2 degrees F). However, the additional water vapor in the atmosphere triggered by this initial warming will result in roughly 1.6 degrees C (2.9 degrees F) more warming, and positive feedbacks caused by changes in cloud formation add around 0.7 degrees C more (1.3 degrees F). This cloud feedback varies significantly between models, ranging from 0.3 to 1.1 degrees C (0.5 to 2 degrees F). See the IPCC AR4 WG1 chapter 8.6.3 (pdf) for a more detailed discussion on uncertainties regarding cloud forcings.

Climate scientists can also verify the effects of the water vapor feedback by examining the response of water vapor to a global decrease in temperatures after a major volcanic eruption. After Mount Pinatubo erupted in 1992, for example, water vapor concentrations decreased within the range predicted by the model and amplified the cooling by 60 percent more than would have occurred as a result of the sulphate aerosol emissions alone.

Claims that water vapor is the “dominant” driver of recently observed climate change are spurious at best. While uncertainties in the magnitude of water vapor feedbacks are one of the key areas concerning climate change, none of this research casts any doubt on the role of carbon dioxide and other anthropogenic greenhouse gases as the initial forcings behind our current climate perturbation.

[Pragma]

Yes Rich, all of what you say is true and it's not an easy problem on a local level, and yes, it is going to vary dramatically on a daily basis but only on a local or regional area, IMHO. I'm not sure, but I would posit that the total amount in the air at any time is pretty constant.

Similarly, the CO₂ levels are not uniform and the concentration in the southern hemisphere has a lag of about a year (IIRC).

Likewise, temperatures vary wildly across the planet.

That said, "we" are still able to come up with a CO₂ concentration number and a temperature for the entire globe.

There is no stated, that I can find, global water vapour level, but there has been a 7% increase, so it has been quantified relatively, and I would like to know what is the relative increase is, in terms of CO₂ concentration equivalent.

That seems to be a very elusive number, even as an estimate. If I were to put on my tin foil hat, I would say that there is a reluctance to quantify it as the denier crowd would then claim that the CO₂ levels are irrelevant.

I believe some are already doing that anyway, and in someways they are right.

Personally, I think we have tweaked the dragon's tail and all we can do is hang on and see where he takes us.

[Rich]

 If you Google greenhouse gas and look at the Wikipedia entry, you'll find that someone has made an effort to quantify the elusive # you seek.

[Sciguy]

Gerontocrat,

I understand the basics and the mechanisms. What you say confirms what I wrote.

The quote you provided dances around what I am looking for, and I could intuit some rough values from it, but I don't understand why hard numbers are so hard to find.

It is easy to find an equation that takes two different CO2 concentration levels and can then give you, in watts/m², the difference in the radiative forcing.

It is: ln(C1/C2)*5.35. (IIRC)

I see no reason why the same could not be done for water vapour.

Cheers

It's difficult to calculate the direct forcings for water vapor for a couple of reasons.  Water vapor leads to clouds which can both trap heat and reflect incoming sunlight and also some of the greenhouse gases overlap in the wavelengths of energy that they absorb.  They can sometimes act as additional positive feedback or sometimes work as additional negative feedback.  So it's difficult to come up with an exact formula for water vapor.  However, the relationship between water vapor and temperature can be calculated using the Clausius-Clapeyron relationship.  Climate science includes the water vapor feedback in the formulas for the greenhouse gas forcings.

The formulas for the greenhouse gas warming forcings were updated in 2016 to include additional feedbacks, primarily for methane.  Here's a link to the paper:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL071930

Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing

M. Etminan, G. Myhre , E. J. Highwood , K. P. Shine

Abstract
 

New calculations of the radiative forcing (RF) are presented for the three main well‐mixed greenhouse gases, methane, nitrous oxide, and carbon dioxide. Methane's RF is particularly impacted because of the inclusion of the shortwave forcing; the 1750–2011 RF is about 25% higher (increasing from 0.48 W m−2 to 0.61 W m−2) compared to the value in the Intergovernmental Panel on Climate Change (IPCC) 2013 assessment; the 100 year global warming potential is 14% higher than the IPCC value. We present new simplified expressions to calculate RF. Unlike previous expressions used by IPCC, the new ones include the overlap between CO2 and N2O; for N2O forcing, the CO2 overlap can be as important as the CH4 overlap. The 1750–2011 CO2 RF is within 1% of IPCC's value but is about 10% higher when CO2 amounts reach 2000 ppm, a value projected to be possible under the extended RCP8.5 scenario.

The most striking feature of the results is the enhancement of the methane RF by about 17–27% compared to the old expressions; this is beyond the nominal uncertainty estimates (about 10%) given in successive IPCC reports for WMGHG forcings. Two mechanisms are responsible–it is primarily due to the role of methane's shortwave bands, which were not included in MHSS98, with a secondary effect of an update to the water vapor continuum strength relative to that used in MHSS98.

Methane has a strong band at 3.3 µm (Figure 1a), which lies within a region of relatively strong water vapor absorption (Figure 1c), leading to a negative net forcing. The weaker bands at 1.6 and 2.3 µm lie toward the center of the windows in the water vapor spectrum. They cause a positive forcing which more than compensates for the 3.3 µm negative forcing. In summary, the magnitude and sign of the solar RF depend on band strength, gas concentration, and overlap with water vapor.

In trying to reconcile the present (longwave) results with those in MHSS98, we identified a further influence on the forcing due to CH4 (and, to a lesser extent, the N2O). MHSS98 used the Clough‐Kneizys‐Davies (CKD) water vapor continuum version 0 in the OLBL calculations. In the region of the CH4 and N2O bands that are most responsible for the longwave forcing (around 1300 cm−1), the foreign continuum was weakened by about a factor of 3.5 in subsequent versions of CKD [e.g., Mlawer et al., 1998] and successor versions (the Mlawer‐Tobin‐CKD) [Mlawer et al., 2012]. This reduces the effect of water vapor overlap with CH4, increasing its RF. Changes in the self‐continuum in this spectral region were much smaller during these updates. To isolate the effect of the continuum changes, the instantaneous cloudy‐sky longwave forcings were examined when CKD version 0 is updated to version 2.4.1. The update caused the CH4 forcing for a 1800 to 3500 ppb change to increase by 4.2%. For comparison, the N2O forcing increased by about 1.7% for a 323 to 525 ppb change; for CO2, RF changed by less than 0.2%.

The new formulas for the forcings are included in a table that doesn't copy and paste well into this blog.  Here is the new formula for CO2 forcing with a brief explanation.

For CO2, the logarithmic form used in MHSS98 (RF = α ln(C/Co), where Co and C are the initial and final CO2 concentrations) is retained but the nature of α term is changed; instead of being a constant, it is now a function of the CO2 and N2O concentrations. Note that the absolute value of CO2 concentration change |C‐Co| is adopted in the α term, to ensure that the forcing is symmetric for increases or decreases in CO2. That is, RF(C, Co) = − RF(Co, C). The C‐Co terms only become important for large changes in CO2; as shown below, for historical forcings, ignoring these terms and using the simple α ln(C/Co) form, and a midrange N2O concentration, yields a value of α within 1% of the value of 5.35 W m−2 given in MHSS98.

[Pragma]

I assume the +ve feedback of increased water vapour is included in the assessed effect of increases in CO2e.

That's not an unreasonable assumption, but dangerous because the mechanisms aren't simultaneous. I would like to see a paper or statement that explicitly states that.

If you then put a figure for the value of the water vapour +ve feedback, is there not a real risk of double counting?

Yes, I see your point, but with a time delay, you have described the ugly truth of positive feedback in a nutshell. 

As an aside, there is reference to the "runaway effect" and "other mechanisms" will stabilize things at a new level. "other mechanisms" as an explanation, does not give me the warm fuzzies.

Closely followed by Voldermot and his followers telling everybody that climate science is a heap of faeces.

LOL! Deniers and others that want to put simple answers on complicated questions throw nuance and subtlety right out the window.

To be fair, I understand their frustration because I have spent my life trying to explain to suits why the laws of physics won't let them have their way.

[Pragma]

It's difficult to calculate the direct forcings for water vapor for a couple of reasons.  Water vapor leads to clouds which can both trap heat and reflect incoming sunlight and also some of the greenhouse gases overlap in the wavelengths of energy that they absorb.  They can sometimes act as additional positive feedback or sometimes work as additional negative feedback.  So it's difficult to come up with an exact formula for water vapor.  However, the relationship between water vapor and temperature can be calculated using the Clausius-Clapeyron relationship.  Climate science includes the water vapor feedback in the formulas for the greenhouse gas forcings.

The formulas for the greenhouse gas warming forcings were updated in 2016 to include additional feedbacks, primarily for methane.  Here's a link to the paper:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL071930

Ken, there is some excellent and pertinent info there. Not only does it discuss the water vapour issue but I can update a lot of what I presently have regarding conventional GHGs. Thank you for taking the time.

I was thinking somewhat simplistically, as obviously, clouds are water vapour and various types of clouds have wildly differing characteristics. It is little wonder no one is willing to put hard numbers to it, although I am sure they want to get there.

I knew I could easily get the spectroscopic absorption numbers, but that was just too simple and I was looking for how it behaves in the wild.

The short answer is "It's complicated" and for my purposes, unnecessarily so. From the information everyone has provided, I think a reasonable multiplier for CO₂e is 7 for now, with the caveat that if cloud patterns change, all bets are off.

Cheers to all

[Sciguy]

Here's an example of how the water vapor feedback is included in the development of the Earth Systems Models that are used to project the impacts of climate change.  This is very detailed, so be forewarned.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017MS001209

The GFDL Global Atmosphere and Land Model AM4.0/LM4.0: 2. Model Description, Sensitivity Studies, and Tuning Strategies

Abstract
 

In Part 2 of this two‐part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part 1. Part 2 provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.

3 Radiation

Both the longwave and shortwave components of the GFDL radiation code have been extensively benchmarked and updated for AM4. This represents the first significant update to the GFDL radiation code since AM2 (GFDL‐GAMDT, 2004), as the updates for AM3 (Donner et al., 2011) were generally minor. The longwave and shortwave codes can be considered as independent and will be dealt with separately in the following. These updates and their resulting impact upon climate will be detailed in an upcoming paper. Here we summarize the key changes.

The longwave code continues to be based upon the simplified exchange approximation (SEA; Fels & Schwarzkopf, 1975; Schwarzkopf & Fels, 1991; Schwarzkopf & Ramaswamy, 1999). For the AM4 version, we have retained the SEA approximation for H2O line calculations but have updated the spectral information used for other species. Random band coefficients for H2O, O3, CO2, N2O, CH4, and halocarbons were all updated from HITRAN 2000 (Rothman et al., 2003) to HITRAN 2012 (Rothman et al., 2013), while the water vapor continuum was updated from CKD 2.1 (Clough et al., 1989) to MT‐CKD 2.5 (Mlawer et al., 2012).
...

[Sciguy]

It's difficult to calculate the direct forcings for water vapor for a couple of reasons.  Water vapor leads to clouds which can both trap heat and reflect incoming sunlight and also some of the greenhouse gases overlap in the wavelengths of energy that they absorb.  They can sometimes act as additional positive feedback or sometimes work as additional negative feedback.  So it's difficult to come up with an exact formula for water vapor.  However, the relationship between water vapor and temperature can be calculated using the Clausius-Clapeyron relationship.  Climate science includes the water vapor feedback in the formulas for the greenhouse gas forcings.

The formulas for the greenhouse gas warming forcings were updated in 2016 to include additional feedbacks, primarily for methane.  Here's a link to the paper:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL071930

Ken, there is some excellent and pertinent info there. Not only does it discuss the water vapour issue but I can update a lot of what I presently have regarding conventional GHGs. Thank you for taking the time.

I was thinking somewhat simplistically, as obviously, clouds are water vapour and various types of clouds have wildly differing characteristics. It is little wonder no one is willing to put hard numbers to it, although I am sure they want to get there.

I knew I could easily get the spectroscopic absorption numbers, but that was just too simple and I was looking for how it behaves in the wild.

The short answer is "It's complicated" and for my purposes, unnecessarily so. From the information everyone has provided, I think a reasonable multiplier for CO₂e is 7 for now, with the caveat that if cloud patterns change, all bets are off.

Cheers to all

I'm not sure why you'd come up with a simplified CO2eq number for water vapor.  You seem to have missed the point that it's a feedback, not a forcing.  CO2eq numbers apply to the forcings.

[Pragma]

Thanks again, Ken. That will be weekend reading, or at least skimming, depending on how my week goes.

While I was making myself a fresh coffee, it dawned on me that my quest is not over, and may be a fools errand. The statement that there is a 7% increase for a 1 degree rise is valid, but it's difficult, if not impossible to correlate that with CO₂ levels, with the information I have.

The reason is that although the CO₂ levels are relatively instantaneous, the resultant temperature rise is anything but, and I have seen wildly differing numbers for the delay time. No one in the "consumer" literature would think of providing a time constant, even if it was available.

It all gets back to the thorny question of climate sensitivity, which, AFAIK, is still very uncertain.

[Pragma]

I'm not sure why you'd come up with a simplified CO2eq number for water vapor.  You seem to have missed the point that it's a feedback, not a forcing.  CO2eq numbers apply to the forcings.

Perhaps I am missing a key point, but is it not both a feedback and a forcing?

Similarly, CO₂ is a forcing but the additional resultant heat creates more CO2 (permafrost) and so is also a feedback.

If there is a difference between the two cases, then you're right. I have missed an important point.

[Sciguy]

I'm not sure why you'd come up with a simplified CO2eq number for water vapor.  You seem to have missed the point that it's a feedback, not a forcing.  CO2eq numbers apply to the forcings.

Perhaps I am missing a key point, but is it not both a feedback and a forcing?

Similarly, CO₂ is a forcing but the additional resultant heat creates more CO2 (permafrost) and so is also a feedback.

If there is a difference between the two cases, then you're right. I have missed an important point.

Here's a good overview of the differences between forcings and feedbacks:

https://www.climate.gov/maps-data/primer/climate-forcing

Another way to refer to climate forcings is to call them climate drivers. Natural climate drivers include changes in the sun’s energy output, regular changes in Earth’s orbital cycle, and large volcanic eruptions that put light-reflecting particles into the upper atmosphere. Human-caused, or anthropogenic climate drivers include emissions of heat-trapping gases (also known as greenhouse gases) and changes in land use that make land reflect more or less sunlight energy. Since 1750, human-caused climate drivers have been increasing, and their effect dominates all natural climate drivers.

Feedback: Amplifying Initial Forcings

Climate drivers can also trigger feedbacks that intensify or weaken the original forcing. For example, forcing from increased greenhouse gases also increases evaporation, which increases water vapor in the atmosphere and intensifies the forcing from greenhouse gases.

So taking CO2 as an example, we can show how much humans have emitted by burning fossil fuels and measure the changes in the concentration in the atmosphere.  That would be the forcing.  We can then estimate how much more CO2 is being emitted from natural sources due to the increase in temperatures caused by the melt of permafrost or increased biological activity from microbes, those are feedbacks.

What is your proposed forcing mechanism for water vapor?  I can't think of any.  (There might be more pools behind dams or irrigation canals that allow more water to evaporate, but that precipitates out of the atmosphere as rain or snow and winds up back in aquifers or lakes and oceans).  The effects of water vapor that I've read about are all feedbacks related to the increase in temperature due to other climate forcings.

[kassy]

Water vapour: feedback or forcing?

How do we know that the magnitude of this feedback is correctly simulated? A good test case is the response to the Pinatubo eruption. This caused cooling for up to 3 years after the eruption – plenty of time for water vapour to equilibriate to the cooler sea surface temperatures. Thus if models can simulate the observed decrease of water vapour at this time, it would be a good sign that they are basically correct. A good paper that demonstrated this was Soden et al (2002) (and the accompanying comment by Tony DelGenio). They found that using the observed volcanic aerosols as forcing the model produced very similar cooling to that observed. Moreover, the water vapour in the total column and in the upper troposphere decreased in line with satellite observations, and helped to increase the cooling by about 60% – in line with projections for increasing greenhouse gases.

http://www.realclimate.org/index.php/archives/2005/04/water-vapour-feedback-or-forcing/

[Pragma]

What is your proposed forcing mechanism for water vapor?  I can't think of any.  (There might be more pools behind dams or irrigation canals that allow more water to evaporate, but that precipitates out of the atmosphere as rain or snow and winds up back in aquifers or lakes and oceans).  The effects of water vapor that I've read about are all feedbacks related to the increase in temperature due to other climate forcings.

Well, in a highly simplified model, the atmosphere can only hold so much water vapour on average for a given average temperature, so based on that, there is no forcing, only feedback.

That said, there are many areas on the earth where the atmosphere is far from saturated, so my thinking was that humans have done so much to alter the water cycle, (Aral sea, Amazon, agricultural) that they have affected, and possibly (likely?) increase the circulating moisture. Yes, that moisture will fall out but we may be keeping "more balls in the air".

How much that affects the overall system, I can't even guess. We may just be shifting patterns around and I'm nitpicking, but we have a knack of screwing stuff up in unexpected and novel ways.

BTW, the article that kassy posted does a great job of discussing the whole thing (thanks kassy!) and it comes down fairly firmly on feedback only.

In this discussion I almost lost sight of my objective. Regardless of whether water vapour is forcing or feedback, it is a moot point. We are increasing the water vapour through our actions and I wanted to quantify what the effect of that would be.

I'm getting there...slowly