Global Warming, etc....what if
- Thread starter sly1
- Start date
how can you say "natural change" is accounted for when you cant even document reliably nature's "way" for even but a fraction of its entire existance.
Really no one knows anything about nature and this earth and how it works......we know a little bit about a extremely short period of existance that suggests but cannot conclude anything.
Take the last 200 years of research and you have but only 200 years out of BILLIONS........how can you conclude anything worth while with such a small observation period?
What are you gonna do? Take over China and make them stop? It will all stop when a shit load of people die. "Mother nature" does a "correction" when she needs to.
That's essentially what James Lovelock says in his " Gaia Hypothesis" and I don't doubt it. The earth will sustain itself whatever we do but we may not be here to witness the changes.
Why do you rule out negotiating with the Chinese ?
You are comparing the values in the lower atmosphere, for the anthro contribution, with the values impinging on the upper atmosphere, for the sun.milkweed said:
Two comments:
The very upper atmosphere is cooling, according to the satellites, last I checked - that agrees with the CO2 heat trapping predictions, and disagrees with predictions from solar forcing dominance.
And recent measurements reported a couple of months ago reaffirmed a "global dimming" effect first noticed several years ago, in which total solar flux at low altitudes is significantly less, over wide areas, than in decades past - proposed causes range from extra soot from rising third world economies to cumulative effects of jet contrails and heat trap increases in water vapor.
The Upper atmosphere can cool all it wants to but that doesnt change the W/m2 that the sun is blasting on it each day. I understand where your trying to go with this but the IPCC calculated this as W/m2. The satellite W/m2 is the basic agreement on the solar irradiance as translated into watts per square meter hitting the top of the atmosphere with approx 40% reaching the ground (varied by height and atmospheric conditions as noted). This is shown in my post for an average W/m2 of 342.
There is no way to avoid the results of the IPCC conclusion. Their calculation of ALL human caused greenhouse gasses is 1.6 W/m2 with a variable of -1. or +.8, leaving it to be possibly as low as .6 W/m2 up to 2.4 W/m2.
And here is the final impact numbers:
As a good comparison, the natural greenhouse effect that warms the Earth's surface by 35 K is, on average, ~150 watts per square meter.
http://www.esrl.noaa.gov/research/themes/forcing/
If you can find another source for what is the natural greenhouse effect I would take a look.
We had a check forger roll thru this area in the 1970s. He signed all his checks U. Benn Took
And 40% of 2 W/m^2 is .8 W/m^2.milkweed said:
That is the absolute maximum, not just the high end likely, solar flux contribution to lower atmosphere warming, from the data you have posted.
That is about a third (averaged night and day, about a sixth) of the likely high end calculated anthro contribution (the part within the 5% error bars) at the lower levels of the atmosphere. (Which has a chance of being much higher, as recent data seem to indicate).
Along with that theoretical calculation, we have certain actual measurements: the solar flux in the lower atmosphere has been dropping in recent decades, the outer atmosphere has been cooling not heating, the measured warming trends have been most pronounced during winter nights rather than summer days, and so forth.
All these actual measurements conflict with theoretical ascription of the recent climate patterns to what in any case would be an inadequate change (by itself,without amplification by other factors) in solar flux.
To use that for comparison, you need one more very important set of factors: what should be the effects (on that natural greenhouse regime) of our 50% boost in CO2 concentrations in the lower atmosphere ?milkweed said:
They would of course be extremely complex, but a general warming trend such as we are experiencing is well within the likely expectations - as is a fairly dramatic feedback warming in the near future, so far right on schedule. We will see how things play out in reality.
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Yes I know and that solar irradiance factor was pooh pooh'd by the IPCC as insignificant.
Our 50% boost?
95% (or so) is due to water vapor. It is the dominant factor in ghg. The IPCC calculates 1.6 W/m2 (+/-) as the impact of people induced ghg. It should be obvious to those looking for answers from the IPCC the fact that they obscure the Natural ghg forcings is very telling in their "conclusions" because when you hold the two up in comparison, human induced ghg is insignificant when compared to 150 W/m2 that occurs naturally. But you dont win Nobels for saying hmmm... we cannot reach a conclusion based on the data with our understanding of how Nature works in this wide world of sciences. Nor do you get more funding for your research into such things.
Read up on the IPCC Ch 2 page 130 or so. See how much scientific understanding of the various factors are described by they themselves. Low level of understanding comes up alot but yet they declare 90% significant impact on climate?
Does anyone else find it odd how the idea of Global Warming, especially the impact from of our consumption of fossil fuels, is heightened just as we start to realise we're running out of those same fuels?
Maybe the world is so supremely balanced that there is only enough fossil fuel in existence to bring us to the brink of self-inflicted demise before they run out, forcing us to use clean fuels while new fossil fuels are created deep in the bowels of the earth - waiting for another civilisation, millions of years from now, to discover and use, so perpetuating yet another cycle?
Maybe the world is so supremely balanced that there is only enough fossil fuel in existence to bring us to the brink of self-inflicted demise before they run out, forcing us to use clean fuels while new fossil fuels are created deep in the bowels of the earth - waiting for another civilisation, millions of years from now, to discover and use, so perpetuating yet another cycle?
And you can't see why ?milkweed said:
According to your own data it's at most less than a sixth the anthro effect, which you are claiming is negligible.
Please reread. We are in the process of boosting the CO2 concentration by 50% over a period of about a hundred years.milkweed said:
The water vapor effect, although larger than the direct CO2 effect, is derivative, not dominant. Take away the CO2 warming, and the water condenses out - eventually freezing out. The air dries, and we lose that 35 degrees of greenhouse warming. Put more CO2 in, and the hotter air takes up more water vapor, amplifying the direct CO2 effect. And the hotter air doesn't give it back as fog and rain nearly as easily - although wetter air in the higher latitudes may snow more.
The effect on water vapor in the air is maybe the single most likely source of quick and dramatic disaster from CO2 accumulation.
B
BlueMoose
Guest
Whats sickens me is that this Global Warming is now business/politics, convinient smokescreen to allow big-biz keeping on to rape nature all the other possible ways, nukewastes, toxic chemicals, cutting down the rain-forests, draining lakes, you name it.
Certainly every vested interest will use everything it can to advance its own interest, but it is possible that no action to control the unprecidented RATE of change in the green house gases man is making (without any doubt that he is accelerating the RATE of change) may be able to erradicate all life on Earth. The risk that this will happen is unknown. The problem is too complex and too little is known for sure, but Earth could be switching to the other, very hot, stable state. (In which the oceans slowly boil away into space as they did on Venus when it switched from the cooler to the hotter stable state it is now in.)
For more on the mechanism of the switch, and the importance of RATE, see the exchange betteen MetaKron and myself at:
http://www.sciforums.com/showpost.php?p=1691131&postcount=41
This exchange begins with post 24, where I note that Japan is not only planning to harvest methane from the deep sea Clathrates, but and this is the very scary part, Their researchers have already observed methane bubbles rising up from the sea bottom "methane ice" (Clathrates). That suggests that it may already be too late to stop Earth from switching to the hot stable state, with high-pressure, scalding-steam atmosphere at ground level (and several kilometers up from that at least.) If Earth switches to the hot stable state, it will be steril - No form of life on Earth.
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Yes I think the anthro effect on ghg is insignificant.
50% (max estimate, realistic is 30% or so) of .3% or .4% by volume over 100 years is nothing substancial. CO2 is not dioxin.
I am pretty sure CO2 is not needed to create water vapor. If you take all the CO2 out of the atmosphere (ignoring the other negative effects) there will still be water vapor and I can only guess as to the amount.
When CO2 concentrations where much higher than now, the surface of the earth did not burn away, the skies didnt become so laden with clouds that life ceased to thrive and the temps (as best as we can reconstruct them) allowed for a host of life for a much longer time span than the space between our current cycle of ice and warming. But thats getting a bit off topic from the intent of the thread.
And it still avoids the W/m2 provided naturally (estimated at 150 W/m2) compared to all the human caused ghg (estimated high end 2.4 W/m2). It doesnt matter which way one slices it. 2.4 W/m2 is nothing compared to the whole and is within what I would guess is a natural adaptivity factor built into all surviving life from cockroaches to humans.
These are comforting thoughts and true, but what is different today is the RATE at which CO2, CH4, etc are being added to the atmosphere. If these ghg were added more slowly then natural processes would remove them or destroy them at essentially the same rate. (especially CO2 disolving in the oceans and UV + oxidation removing the methane etc.) Rates can be important (I am not stating that they are in this ghg case). For example, a centimeter cube of glass can support many tons if applied slowly, but is easily shattered if a 3 pound hammer strikes it.
I think it not very likely that Earth is currently in the process of switching to the hot stable state, as Venus did long ago when it too had water oceans. (Now it may have "lead lakes" on the surface as the surface temperature is higher than the melting point of lead.) Even in the hot stable state Earth will not have "lead lakes" but the atmosphere will be dense scalding steam several kilometers thick until the oceans boil away into space.
We worry about other low probability disasters which we understand much better that can not make Earth steril as a switch to Earth's hot stable state would do. For example, we can calculate or at least estimate the probability that a nucelar power plant will blow up or that a modest sized asteroid will collide with Earth. We need much more study of why and how Venus switched to the hot state from one very much like Earth today. The fact that the Japanese researchers, planing to use the methane trapped in sea bottom methane Calthrates, have observed methane bubble already rising from it is scary. There is more ghg stored on the ocean floor in these marginally stable calthrate "methane ices" than all the ghg in CO2 that man has released in all of his use of all fossil fuels. If they come up in a decade or so that could be like the 3 pound hamer hitting the glass cube. We simply do not know.
If man's small contribution, but great rate excursion, is why that methane is now coming up, things could be very serious, due to sensitivity to RATE effects. Thus, fact there has been more CO2 in the atmosphere in Earth's past is hopeful, but not proof, that Earth is not starting to switch to the hot stable state.
Again: much more effort needs to be put into understand all this and especially Venus's history. NASA should be doing this instead of planning PR joy rides to Mars.
See post 52 here and the set of posts in 52's link.
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It's already over 30%, and there is evidence the accumulation may be accellerating - presumably the natural sinks might saturate, which would be fairly disastrous. Max estimate is well over 100%.milkweed said:
And most of the scientists who have studied the matter are pretty sure that it is - the water vapor greenhouse effect is not self-sustaining. A little cooling causes a little depletion, which results in a little more cooling, and so forth.milkweed said:
This is a very good thing, actually - or the positive feedback from a summer's evaporation would long ago have dried the freshwater lakes and rivers, and a good share of the oceans.
Of course.milkweed said:
It's human civilization that would take the damage, plus a fair share of the animals and plants adapted to current conditions in various places.
Link for this please.
I too would like the link to read the arguments that water vapor in the air is a helpful factor pushing back towards a cooler surface temperature. Certainly under some conditions (formation of clouds on sunny days) this would seem to be the case as the albedo is locally increased, but the net effect of clouds is much harder to ascertain and depends upon many factors.
Also, the atmosphere up to high altitudes becomes cooler adibatically. (As some air rises, it expands and adibatically cools with the expansion is why the temperature drop is adibatic curve on the PV plot). Thus, water, in the form of falling rain is an efficient way to transport "cold" to a lower level. Anyone who has taken long high altitude plan ride with luggage in the unheated space or experienced a cold rain on a hot summer afternoon knows the truth of these effects.
However, if there is so much water vapor in the atmopsphere that IR radiation can not escape from the surface, but only from the very highest layers of the atmosphere, where the adibatic cooling has decreased the surface temperature down to approximately 90C, and the atmosphere is much thicker (very much higher pressure at surface than Earth now has), then I would expect that it would almost always be raining in the top layers of the atmosphere, but the cooler rain falling down would never come close to the surface. I.e. high altitude only rain might make the temperature fall off with altitude more complex than a simple adibate, but the surface would still be scalding steam several kilometers thick, at least until the oceans boil away into space as they did on Venus long ago.
I hope my physics/ understanding of this is all wrong, but with the example of Venus having been very "Earth like" in the cooler stable state for most of is early history and then switching to the current hot stable state, I do not see any reason Earth can not do the same.
What? Water vapor is a greenhouse gas - it traps infrared, much as CO2 does.billy said:
But water condenses out as rain, freezes out as ice and snow, etc., and thus transfers heat to the higher atmosphere from the surface - from whence a certain percentage is lost to space as infrared radiation. So it cannot keep itself at high concentrations in the atmosphere. It can amplify an initial warming from other causes, even (from outgassing by volcanos etc) touch one off, but cannot be the principal cause of a global climate change.
A little thought will make that clear - if it could, the earth's atmosphere would have superheated from solar driven evaporation feedback long before now.
Clouds are a different matter - probably the main current known cause of uncertainty (rivaled by the saturation levels of the carbon sinks) in the global warming predictions. Nobody knows what's going to happen with clouds - not even jet contrails, the most easily studied and handled.
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That is true IF the atmosphere temperature less than the boiling point as is the case now with Earth in the cooler of two stable states, but not true when it is in the hotter stable state. Then rain is present only in the very highest layers of the atmosphere (For the physics reasons I gave in last post related to the adibatic drop of temperature with altitude.) In the hotter stable state of Earth, the lower atmosphere is dense, hot, high-pressure, steam. I.e. no rain as certainly no ice or snow is forming.
The critical question, not curently well answered is:
What are the cause of a switch for the cooler to the hotter stable state? This switch is certainly possible as it happened to Venus. I suspect that if Earth is now in damger of switching, it is that the RATE of release of ghg, other than H2O, which can rapidly condense out and fall back to rejoin the oceans, is the threat. For example, CH4 is not soluable in the oceans to any appreciable extent. In contrast huge quantities of CO2 that do easily disolve in water. Atmospheric Ch4, I think, is mainly decomposed or oxidized. I do not know the mean lifetime of a CH4 molecule released at the surface. (Perhaps some one will research this?)
If it is a few months or less, then methane now begining to bubble up for the vast store on the ocean floor may not be too scary; but if the rate of release of the sea floor methane is faster than the rate it is removed from the atmosphere, then Earth may well switch to the hot stable state as Venus did.
That is why in my posts I have tried to focus attention on the role RATES of addition of ghg to the atmosphere may play. - Fact that Earth has in the past had higher concentrations of CO2 is little comforth, if they built up slowly over thousanda of years. - E.g. If the associated warming (by more IR trapped) was slow and released only small amounts of CH4 annually, which natural processes could remove more quickly than it was being released.
With man accelerating the RATE of CO2 release, this accelerates the rate of temperature rise, which accelerates the rate of CH4 release. Thus it is then possible that these natural removal processes may NOT act fast enough to keep the CH4 concentration in the atmowsphere low. If that is the case, since CH4is a much more powerful ghg than CO2, the increasing atmopsphereic concentrations of CH4 will further accelerate the temperature rise, and that will accelerate the release still more CH4, etc. in a "run away" instability that swiches Earth to the hot stable state. I.e. converts Earth into a steril planet with dense scalding steam atmosphere, kilometers thick, at least until the oceans boil away into space.
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Very interesting discussion everyone, and a few points worth discussing. I hope you do not mind if I throw in some input.
I think there is some confusion with number usage here. The first being the numbers the IPCC use which are a radiative forcing, and the second being the amount of radiation intercepted by the Earth or the downward infrared flux. The 150 W/m^2 is the contribution of the greenhouse effect, a number that would be zero in the absence of greenhouse gases, which from εσT^4 corresponds to a global temperature of 255 K (the actual temperature is around 288 K due to the greenhouse effect). The *net* downward solar irradiance at the top-of-atmosphere (TOA)(where forcing is defined) is ~240 W/m^2 taking geometry and and taking albedo into account).
The "extra" 2 W/m^2 from the sun is NOT a radiative forcing, it is simply ΔTSI. If you want to make this a radiative forcing you need to multiply by 0.69 to factor in albedo, then divide by 4 to compensate for the cross-sectional area of the Earth.
The next step is to realize that the 1.6 W/m^2 radiative forcing from CO2 (not ALL greenhouse gases, they add up to more, but you have aerosols with a negative radiative forcing which essentially cancels out the other stuff so the net forcing and CO2 forcing are about the same) is a "change" from the pre-industrial to industrial conditions. The forcings are a measure of the imbalance at the TOA that would occur if the atmosphere was changed instantaneously. Comparing this to "150" and saying "the effect is negligible" is not right: you need to look at the imbalance's effect on global T as a new equilibrium is reached over a time lag. As an analogy, suppose we have a bathtub with 100x units of water, and we are adding x units of water, while x units are being drained. The water level will remain roughly constant, because the same is going in as going out. Now suppose we "force" another x units of water in so that 2x is going in, and x is going out. You can't simply say that "x" compared to "100x" is negligible, but rather you will observe a steady rise in water level because more is going in than going out (it is out of equilibrium). If the water rises a few feet, and then you return the "in and out" to equilibrium, then you have your "radiative" forcing which is defined as the final conditions relative to initial conditions. Absolute values are not relevant, global warming is only concerned with the changes in the numbers. If the current literature is correct, then obviously CO2 makes up a relatively small amount of the actual observed temperature, but is nearly completely responsible for the *change in* temperature from pre-industrial to present conditions. Of course time-frames matter, since solar and anthropogenic and lack of volcanoes and internal variability played a large role from 1900-1950, but anthropogenic dominates from 1950-present (See ex. Meehl et al 2004; Ammann et al 2007). Since 1950 there is no trend in solar (maybe even negative)(Max Plank Institute 2004; Benestad 2005; Foukal et al 2006; Ammann et al 2007; Lockwood and Frohlich 2007) iceaura is also right in that if solar changes were the dominant factor, we'd expect stratospheric warming as well as tropospheric warming, when we are actually getting tropospheric warming and surface warming, with stratospheric cooling (U.S. Climate change Science Program 2006; Lastovicka et al 2006). Explanatory and predictive power is a big part of the scientific method, and this is just one example of where solar explanation fail- changes in night time vs. day time temperatures are another.
Regarding water vapor, iceaura is right again. Water Vapor is a feedback to initial climate change, it does not initiate climate change (i.e. it is not a forcing), although once a climate change has been initiated it can further amplify or dampen the initial forcing. Since a warmer atmosphere can hold more water vapor and a cooler climate can hold less, if conditions are pushed by some climate forcing (ex. solar increase, a change in CO2, increase in anthropogenic aerosol concentration, volcanic eruption), the water vapor concentration will adjust accordingly to the climate adjustment a make it either warmer or colder (in the first two examples with all other things equal the effect is warmer temperatures, so a positive feedback from water vapor making it still further warmer; in the latter two examples with all other things equal the net effect is cooling, and so a positive feedback from water vapor gives still further cooling).
Vapor pressure in equilibrium with a water surface increases exponentially with temperature at a rate in accord with Clausius-Clapeyron. If the relative humidity remains about constant as temperature and specific humidity increase, then water vapor greenhouse feedback roughly doubles the sensitivity of climate. The changes in specific humidity, with little change in relative humidity, have been documented recently and in accordance with our understanding of modern global climate change (Santer et al 2007; Willett et al 2007).
All gases are condensible at low enough temperatures and/or high enough pressures. CO2, for example, is condensible on Mars though not in present-day Earth climate. This happens when the partial pressure of a gas is equal to the saturation vapor pressure (Psat). Psat increases with temperature, since molecules move faster and it becomes more difficult for condensation. This temperature dependence of Psat comes from the Clausius-Clapeyron relationship (you guys can look this up). For ground temperatures below 240K there is so little water vapor in the air that the exact amount of water vapor has little effect on the Outgoing Longwave Radiation (Pierrehumbert, climate book). In essence, increase of water vapor with temperature reduces the slope of the Outgoing Longwave Radiation vs. temperature curve (in a dry atmosphere, the OLR is just σT^4) and so the climate is more sensitive to radiative forcing, whatever the forcing is (solar, GHG, albedo, etc).
The point to take is that water vapor, despite its strength of a greenhouse gas, is not efficient at causing a climate change (and really, there is little water vapor in high, cold part of the atmosphere where the heat balance is determined), however once a climate change is underway by CO2 or the sun, or whatever, water vapor will change in accordance with the new climate pushing it further in that direction. Kump (2002) in Nature will do a good overview of this. Also- despite what Monte Hieb says on his personnel blog, 95% of the greenhouse effect is not due to water vapor, more around 60% (see Kiehl and Trenberth 1997), but the more important point is that H2O only stays in the atmosphere for around 10 days, while an extra input of CO2 will stay around for over 100 years with some extending for tens of thousands of years, and so is just a feedback.
Chris
I think there is some confusion with number usage here. The first being the numbers the IPCC use which are a radiative forcing, and the second being the amount of radiation intercepted by the Earth or the downward infrared flux. The 150 W/m^2 is the contribution of the greenhouse effect, a number that would be zero in the absence of greenhouse gases, which from εσT^4 corresponds to a global temperature of 255 K (the actual temperature is around 288 K due to the greenhouse effect). The *net* downward solar irradiance at the top-of-atmosphere (TOA)(where forcing is defined) is ~240 W/m^2 taking geometry and and taking albedo into account).
The "extra" 2 W/m^2 from the sun is NOT a radiative forcing, it is simply ΔTSI. If you want to make this a radiative forcing you need to multiply by 0.69 to factor in albedo, then divide by 4 to compensate for the cross-sectional area of the Earth.
The next step is to realize that the 1.6 W/m^2 radiative forcing from CO2 (not ALL greenhouse gases, they add up to more, but you have aerosols with a negative radiative forcing which essentially cancels out the other stuff so the net forcing and CO2 forcing are about the same) is a "change" from the pre-industrial to industrial conditions. The forcings are a measure of the imbalance at the TOA that would occur if the atmosphere was changed instantaneously. Comparing this to "150" and saying "the effect is negligible" is not right: you need to look at the imbalance's effect on global T as a new equilibrium is reached over a time lag. As an analogy, suppose we have a bathtub with 100x units of water, and we are adding x units of water, while x units are being drained. The water level will remain roughly constant, because the same is going in as going out. Now suppose we "force" another x units of water in so that 2x is going in, and x is going out. You can't simply say that "x" compared to "100x" is negligible, but rather you will observe a steady rise in water level because more is going in than going out (it is out of equilibrium). If the water rises a few feet, and then you return the "in and out" to equilibrium, then you have your "radiative" forcing which is defined as the final conditions relative to initial conditions. Absolute values are not relevant, global warming is only concerned with the changes in the numbers. If the current literature is correct, then obviously CO2 makes up a relatively small amount of the actual observed temperature, but is nearly completely responsible for the *change in* temperature from pre-industrial to present conditions. Of course time-frames matter, since solar and anthropogenic and lack of volcanoes and internal variability played a large role from 1900-1950, but anthropogenic dominates from 1950-present (See ex. Meehl et al 2004; Ammann et al 2007). Since 1950 there is no trend in solar (maybe even negative)(Max Plank Institute 2004; Benestad 2005; Foukal et al 2006; Ammann et al 2007; Lockwood and Frohlich 2007) iceaura is also right in that if solar changes were the dominant factor, we'd expect stratospheric warming as well as tropospheric warming, when we are actually getting tropospheric warming and surface warming, with stratospheric cooling (U.S. Climate change Science Program 2006; Lastovicka et al 2006). Explanatory and predictive power is a big part of the scientific method, and this is just one example of where solar explanation fail- changes in night time vs. day time temperatures are another.
Regarding water vapor, iceaura is right again. Water Vapor is a feedback to initial climate change, it does not initiate climate change (i.e. it is not a forcing), although once a climate change has been initiated it can further amplify or dampen the initial forcing. Since a warmer atmosphere can hold more water vapor and a cooler climate can hold less, if conditions are pushed by some climate forcing (ex. solar increase, a change in CO2, increase in anthropogenic aerosol concentration, volcanic eruption), the water vapor concentration will adjust accordingly to the climate adjustment a make it either warmer or colder (in the first two examples with all other things equal the effect is warmer temperatures, so a positive feedback from water vapor making it still further warmer; in the latter two examples with all other things equal the net effect is cooling, and so a positive feedback from water vapor gives still further cooling).
Vapor pressure in equilibrium with a water surface increases exponentially with temperature at a rate in accord with Clausius-Clapeyron. If the relative humidity remains about constant as temperature and specific humidity increase, then water vapor greenhouse feedback roughly doubles the sensitivity of climate. The changes in specific humidity, with little change in relative humidity, have been documented recently and in accordance with our understanding of modern global climate change (Santer et al 2007; Willett et al 2007).
All gases are condensible at low enough temperatures and/or high enough pressures. CO2, for example, is condensible on Mars though not in present-day Earth climate. This happens when the partial pressure of a gas is equal to the saturation vapor pressure (Psat). Psat increases with temperature, since molecules move faster and it becomes more difficult for condensation. This temperature dependence of Psat comes from the Clausius-Clapeyron relationship (you guys can look this up). For ground temperatures below 240K there is so little water vapor in the air that the exact amount of water vapor has little effect on the Outgoing Longwave Radiation (Pierrehumbert, climate book). In essence, increase of water vapor with temperature reduces the slope of the Outgoing Longwave Radiation vs. temperature curve (in a dry atmosphere, the OLR is just σT^4) and so the climate is more sensitive to radiative forcing, whatever the forcing is (solar, GHG, albedo, etc).
The point to take is that water vapor, despite its strength of a greenhouse gas, is not efficient at causing a climate change (and really, there is little water vapor in high, cold part of the atmosphere where the heat balance is determined), however once a climate change is underway by CO2 or the sun, or whatever, water vapor will change in accordance with the new climate pushing it further in that direction. Kump (2002) in Nature will do a good overview of this. Also- despite what Monte Hieb says on his personnel blog, 95% of the greenhouse effect is not due to water vapor, more around 60% (see Kiehl and Trenberth 1997), but the more important point is that H2O only stays in the atmosphere for around 10 days, while an extra input of CO2 will stay around for over 100 years with some extending for tens of thousands of years, and so is just a feedback.
Chris