Scientists fear they've oversold global warming

As public debate deals in absolutes, some experts fear predictions 'have created a monster'

Scientists long have issued the warnings: The modern world's appetite for cars, air conditioning and cheap, fossil-fuel energy spews billions of tons of carbon dioxide into the atmosphere, unnaturally warming the world.

Yet, it took the dramatic images of a hurricane overtaking New Orleans and searing heat last summer to finally trigger widespread public concern on the issue of global warming.

Climate scientists might be expected to bask in the spotlight after their decades of toil. The general public now cares about greenhouse gases, and with a new Democratic-led Congress, federal action on climate change may be at hand.

Problem is, global warming may not have caused Hurricane Katrina, and last summer's heat waves were equaled and, in many cases, surpassed by heat in the 1930s.

In their efforts to capture the public's attention, then, have climate scientists oversold global warming? It's probably not a majority view, but a few climate scientists are beginning to question whether some dire predictions push the science too far.

"Some of us are wondering if we have created a monster," says Kevin Vranes, a climate scientist at the University of Colorado.

Vranes, who is not considered a global warming skeptic by his peers, came to this conclusion after attending an American Geophysical Union meeting last month. Vranes says he detected "tension" among scientists, notably because projections of the future climate carry uncertainties — a point that hasn't been fully communicated to the public.

For example, last summer, Ralph Cicerone, president of the National Academy of Sciences, told the U.S. House Committee on Energy and Commerce: "I think we understand the mechanisms of CO2 and climate better than we do of what causes lung cancer. ... In fact, it is fair to say that global warming may be the most carefully and fully studied scientific topic in human history."

Nearly all climate scientists believe the Earth is warming and that human activity, by increasing the level of greenhouse gases such as carbon dioxide, has contributed significantly to the warming.

But within the broad consensus are myriad questions about the details. How much of the recent warming has been caused by humans? Is the upswing in Atlantic hurricane activity due to global warming or natural variability? Are Antarctica's ice sheets at risk for melting in the near future?

To the public and policymakers, these details matter. It's one thing to worry about summer temperatures becoming a few degrees warmer.

It's quite another if ice melting from Greenland and Antarctica raises the sea level by 3 feet in the next century, enough to cover much of Galveston Island at high tide.

Models aren't infallible Scientists have substantial evidence to support the view that humans are warming the planet — as carbon dioxide levels rise, glaciers melt and global temperatures rise. Yet, for predicting the future climate, scientists must rely upon sophisticated — but not perfect — computer models.

"The public generally underappreciates that climate models are not meant for reducing our uncertainty about future climate, which they really cannot, but rather they are for increasing our confidence that we understand the climate system in general," says Michael Bauer, a climate modeler at NASA's Goddard Institute for Space Studies, in New York.

Gerald North, professor of atmospheric sciences at Texas A&M University, dismisses the notion of widespread tension among climate scientists on the course of the public debate. But he acknowledges that considerable uncertainty exists with key events such as the melting of Antarctica, which contains enough ice to raise sea levels by 200 feet.

"We honestly don't know that much about the big ice sheets," North says. "We don't have great equations that cover glacial movements. But let's say there's just a 10 percent chance of significant melting in the next century. That would be catastrophic, and it's worth protecting ourselves from that risk."

Much of the public debate, however, has dealt in absolutes. The poster for Al Gore's global warming movie, An Inconvenient Truth, depicts a hurricane blowing out of a smokestack. Katrina's devastation is a major theme in the film.

Judith Curry, an atmospheric scientist at the Georgia Institute of Technology, has published several research papers arguing that a link between a warmer climate and hurricane activity exists, but she admits uncertainty remains.

Like North, Curry says she doubts there is undue tension among climate scientists but says Vranes could be sensing a scientific community reaction to some of the more alarmist claims in the public debate.

For years, Curry says, the public debate on climate change has been dominated by skeptics, such as Richard Lindzen of the Massachusetts Institute of Technology, and strong advocates such as NASA's James Hansen, who calls global warming a ticking "time bomb" and talks about the potential inundation of all global coastlines within a few centuries.

That may be changing, Curry says. As the public has become more aware of global warming, more scientists have been brought into the debate. These scientists are closer to Hansen's side, she says, but reflect a more moderate view.

"I think the rank-and-file are becoming more outspoken, and you're hearing a broader spectrum of ideas," Curry says.

Young and old tension Other climate scientists, however, say there may be some tension as described by Vranes. One of them, Jeffrey Shaman, an assistant professor of atmospheric sciences at Oregon State University, says that unease exists primarily between younger researchers and older, more established scientists.

Shaman says some junior scientists may feel uncomfortable when they see older scientists making claims about the future climate, but he's not sure how widespread that sentiment may be. This kind of tension always has existed in academia, he adds, a system in which senior scientists hold some sway over the grants and research interests of graduate students and junior faculty members.

And if it is worse? Would junior scientists feel compelled to mute their findings, out of concern for their careers, if the research contradicts the climate change consensus?

"I can understand how a scientist without tenure can feel the community pressures," says environmental scientist Roger Pielke Jr., a colleague of Vranes' at the University of Colorado.

Pielke says he has felt pressure from his peers: A prominent scientist angrily accused him of being a skeptic, and a scientific journal editor asked him to "dampen" the message of a peer-reviewed paper to derail skeptics and business interests.

"The case for action on climate science, both for energy policy and adaptation, is overwhelming," Pielke says. "But if we oversell the science, our credibility is at stake."

go on – try to find a source for this – I have and can’t

You might try this one, which demonstrates the relative magnitudes of H2O vs CO2 forcing implemented by current OAGCMs to operate in the manner described by the AGW hypothesis.

The relative magnitudes of the contributions of H2O radiative forcings vs CO₂ for a doubling of CO2 concentration is demonstrated in Collins et al. 2006,:

Radiative forcing by well-mixed greenhouse gases:
Estimates from climate models in the
Intergovernmental Panel on Climate Change
(IPCC) Fourth Assessment Report (AR4)
http://www.agu.org/pubs/crossref/2006.../2005JD006713.shtml

Collins, W.D., V. Ramaswamy, M.D. Schwarzkopf, Y. Sun, R.W. Portmann, Q. Fu, S.E.B. Casanova, J.-L. Dufresne, D.W. Fillmore, P.M.D. Forster, V.Y. Galin, L.K. Gohar, W.J. Ingram, D.P. Kratz, M.-P. Lefebvre, J. Li, P. Marquet, V. Oinas, Y. Tsushima, T. Uchiyama, and W.Y. Zhong 2006. Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). J. Geophys. Res. 111, D14317, doi:10.1029/2005JD006713.
The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO₂, CH₄, N₂O, CFC-11, CFC-12, and the increased H₂O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH₄ and N₂O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.
*** SNIP ***

[12] The specifications for each of the calculations requested from the LBL and AOGCM groups are given in Table 1. The concentrations of WMGHGs in calculations 3a and 3b correspond to conditions in the years 1860 AD and 2000 AD, respectively. The concentrations in 1860 are obtained from a variety of sources detailed in IPCC [2001]. Differences among these calculations cover several standard forcing scenarios performed by all AOGCMs in the AR4, including (1) forcing for changes in CO₂ concentrations from 1860 to 2000 values (case 2a-1a) and from 1860 to double 1860 values (case 2b-1a), (2) forcing for changes in WMGHGs from 1860 to 2000 values (case 3b-3a), and (3) the effect of increased H2O predicted when CO₂ is doubled (case 4a-2b). In addition to the four forcing experiments listed above, there are three additional forcing experiments for combinations of CH4, N2O, CFC-11, and CFC-12. The changes in WMGHGs and H2O in these seven forcing experiments are listed in Table 2.

*** SNIP ***

*** SNIP ***

The comparative results of the above study for a doubling of CO₂ and the expected water vapor feedback in the IPCC suite of AOGCMs demonstrates the relative contributions of water vapor & CO₂ to the anthropogenic global warming hypothesis as envisioned by the UN/IPCC's ensemble of climate models used in the be published IPCC AR4 report.

Note the IPCC ensemble AOGCM's implementation of ~10 to 1 LW(i.e. IR) forcing of water vapor over CO₂ at the surface where global climate change has its most meaningful effect on us poor surface dwelling creatures.

The information contained in Collins et al. 2006 above is indeed eye opening and answers many questions about the climate model implemented forcings of water vapor vs CO₂ and where in the atmosphere they each dominate in relation to the surface of the earth.

Eyeballing the graphs, it is interesting to note that the net LW,SW CO₂ forcing at the tropopause works out to be ~(5.2 -0.8) = ~4.4 w/m².

Danged close to the figure for CO₂ forcing IPCC quotes in IPCC (1990) and subsequent SAR when discussing CO₂ doubling with respect to "pre-industrial times", from the DF = 6.3 ln(C/C₀) w/m² parametric CO₂ forcing equation for tropopause derived from Wigley(1987), from which the oft quoted value of 4.37 w/m² for doubled CO₂ forcing is taken. Probably coincidental, but a curious result nevertheless when regarding the muddled explanations of the IPCC in its third assessment, "Scientific Basis" concerning forcing in the CO₂ arbitrary doubling scenario.

When one considers the statments of UN/IPCC's TAR and the broad span of outputs of the AOGCMs picked by IPCC for reference in the coming AR4, one does wonder just how "settled" the science really is, even within IPCC's pick of the litter climate models.

Finally managed to tracked the genesis of that 12% for CO2 contribution to AGW down.

Wikipedia and several blogs quoting the 12% CO2 figure claim the numbers are derived from values of GHG forcings observed as individual components and combinations are removed from a modeled atmosphere represented in a simple 1 dimensional vertical column climate model evaluated by Ramanathan.

The tabular results of relative contributions quoted appear to be from adhoc runs on Ramanathan's focus model done by individuals using an a somewhat inadequate tool for estimating forcings of perturbations from present atmosphere and do not appear to come from any definitive study subject to peer review. The 1 dimensional model used is hardly one to place much credence in to begin with, as attested to by Ramanathan himself:

Ramanathan, V. and J. A. Coakley, Jr., 1978: Climate Modeling through Radiative-Convective Models. Rev. Geophys. and Space Physics, 16: 465-490.

Ramananathan paper is is not a definitive study of the atmosphere or components of the atmosphere. It is a evaluation of a specific type of model describing the limitations and uses of such, discussing the short comings and potential for use in climate studies.

Quoting from Rathmanathan's concluding remarks concerning the central model he evaluates in his paper:

"Here we will discuss the future use of radiative-convective models for climate studies. Such models should be continued to be used for obtaining first estimates of the potential sensitivity of global temperture to perturbations in radiatively active gases for several reasons. First, the global surface temperature changes prediced by the model are in reasonable agreement with those obtained from the more complex three dimensional general circulation models (GCM). For example, for a doubling of CO2, Manabe and Wetherland [1976] obtained 2.24^oK from a radiative-convective model, while the obtained 3^oK from a GCM, [Manabe and Wetherald, 1975] It is important to not that both studies used the same radiation model. Second, because of its simplicity a radiative-convective model is capable of including many details of the radiative proceses without overburdening the computer resources, and thereby it can give valuable information on the importance of such processe. Third, for climate change experiments, analysis radiative-convective model results would be useful for GCM studies, since it is much more difficult to infer cause-effect relationships in a GCM model.

The important limitation of the model is that the model results are mostly of academic interest, since the model does not give any information about regional and latitudinal temperature ranges. Furthermore, many of the model parameters (cloud amounts, surface albedo, relative humidity, and critical lapse rate, to name a few) are prescribed on the basis of present day conditions which may not apply for large departures from present conditions. For example, the study by Wetherald and Manabe [1978] indicates that for a 2% increase in solar constant the radiative-convective model results for DT_s, are within 20% of the GCM results, while for a 4% decrease in solar constant the two models differ by a factor of 2 in the estimated value of DT_s. Clearly, radiative-convective models cannot be applied for large perturbations from present conditions."

As can be noted above in reponse #142, even modern 3 dimensional GCM's have their own large variability in comparative studies among each other. Saying a 1 dimensional model only manages to stay within 20% with a small perturbation and 200% for a modestly larger one comparing with a GCM output leaves one whale of a lot to be desired in the confidence and validity departments.

This 12% number of does not have the appearance of being anything like a definitive empirical measurement of CO2 or water vapor's capacity as greenhouse gases.

By the author's own characterizations any results from the model described can by no means be said to represent a definitive result.

Making large perturbations like zeroing GHG concentrations wholesale to build a table of contributions by subtracting component gasses is a questionable process on such a model which shows large percentage varations from other models in radiative flux and temperature measures. Being a model and not an empirical measure takes it totally out of the class of definitive science.

Regardin 149:

No, you are agreeing with your own straw-man

It's not a strawman to point out a flaw in the AGW hypothesis as regards the capacity of its prime component to effect a forcing the hypothesis is predicated on:

http://www.meteor.iastate.edu/gccourse/model/direct/direct_lecture_new.html

Water-vapor/greenhouse feedback.
Relative humidity is reasonably constant despite variations in absolute humidity. That is, an increase in temperature leads to more evaporation and increase in the absolute amount of water vapor in the air (increase absolute humidity). But since the warmer air has a higher saturation vapor pressure (can hold more water vapor), the relative humidity stays approximately constant. The increased absolute humidity, however, increases absorption of infrared radiation by the atmosphere and hence increases the greenhouse effect. Note that this increased greenhouse effect raises the surface temperature, which further increases evaporation. This feedback mechanism is not self-regulating, so we call it a positive feedback.
Cloudiness / surface-temperature feedback.
As temperature increases, evaporation and absolute humidity both increase leading to more cloudiness. But the increase in clouds leads to increased reflection of solar energy and also leads to more trapping of infrared energy from the surface of the earth. The net effect (which depends on the altitude of the clouds) is thought to lead to a cooling, which makes this a negative feedback process.

as quantified by Ramanathan:

"the direct radiative effects of doubled CO2 can cause a maximum surface warming [at the equator] of about 0.2 K, and hence roughly 90% of the 2.0-2.5 K surface warming obtained by the GCM is caused by atmospheric feedback processes described above.
--- "Increased Atmospheric CO2: Zonal and Seasonal Estimates of the Effect on the Radiation Energy Balance and Surface Temperature"
(V. Ramanathan and M. S. Lian), J. Journal of Geophysical Research, Vol. 84, p. 4949, 1979.

Regarding #150.

I don't see how that justifies the 98% figure by Lindzen

I didn't say the GCM evaluations did. The outputs of models implementing the hypothesis being tested can't be used to validate its driving hypothesis, only independant empirical measure can be used for such purposes.

I provided the paper to demonstrate that the models implementing the currently accepted AGW hypothesis reflects the same level of water vapor forcing bracketed by the results of Barret and the statements of Lindzen about water vapor's contribution to surface warming.

The paper in #142 is merely an analysis comparing responses of models implementing the AGW hypothesis with the assumptions of a strong water vapor feedback component.

I submit the AGW hypothesis watervapor forcings are operating at around ten times the forcing of CO2 at the surface with magitudes equal and greater than CO2 responses in high altitude topopause & mesosphere low temperature conditions, where the water vapor should be nil and of no significance according to what you have been pointed out to me.

If I take the AGW people's claims that the 80's & 90's worth of surface temperature rise:

http://data.giss.nasa.gov/gistemp/graphs/Fig.E_lrg.gif

as being due to CO2 forcing driving water vapor and cloud feebacks as opposed to solar effects adding forcing into the system.

I have to challenge the AGW model implementations on the basis of an erroneous water vapor feedback that is obviously too high and very likely the wrong sign on the basis of empirical measurements of changes in cloud cover and precipitatable water vapor column data in the ISCCP and NAVP datasets for the period.

Regarding #150:

I have not found anything that would substantively alter their conclusions

What's to alter in their conclusions? its a pretty straight forward evaluation:

The important limitation of the model is that the model results are mostly of academic interest, since the model does not give any information about regional and latitudinal temperature ranges. Furthermore, many of the model parameters (cloud amounts, surface albedo, relative humidity, and critical lapse rate, to name a few) are prescribed on the basis of present day conditions which may not apply for large departures from present conditions. For example, the study by Wetherald and Manabe [1978] indicates that for a 2% increase in solar constant the radiative-convective model results for DT_s, are within 20% of the GCM results, while for a 4% decrease in solar constant the two models differ by a factor of 2 in the estimated value of DT_s. Clearly, radiative-convective models cannot be applied for large perturbations from present conditions."

It's too bad the folks using models to get their table values don't pay attention to caveats in the conclusions about the performance of the models in use.

Regards 151:

Gavin Schmidt seems to have calculated his own number and it seems to be in the ball part of Ramanathan. But I will note that if I read it correctly it is actually closer to the number that you quote from Barrett (91% as opposed to 92%). You gotta love science.

Taking a look over there he appears to be using NASA's GISS GCM for his modeling, I believe that one to be one of Jim Hanson's implementations.

Using AGW model output as if it were an empirical measurement rather than merely a projection of the model's implemented hypothesis, is simply not science.

Such output validates nothing about the hypothesis on which the model is based, the output is preordained by the hypothesis implemented in the model.

In the particular case of AGW, this process becomes a particularly seductive form of circular reasoning. A trap that many fall for because its a computer doing the arithmetic and thus must be precise.

Precise a computer may be, but accurate is a different ball game. Accuracy depends on the basis of the implementation as a valid description of reality. Being from a programming background with more than a little modelling experience, complex models often are a wonderful way of calculating grossly inaccurate answers to levels of ridiculas precision.

Gavin Schmidt seems to have calculated his own number and it seems to be in the ball part of Ramanathan. But I will note that if I read it correctly it is actually closer to the number that you quote from Barrett (91% as opposed to 92%). You gotta love science.

Looking in the Ramanathan paper, Ramanathan states the CO2 contribution as being 9% as well. refer:

7. Role of H2O, CO2, O3 and Clouds starting page 17 of the PDF ( page 481 of the paper) Table 6.

Ramanathan, V. and J. A. Coakley, Jr., 1978: Climate Modeling through Radiative-Convective Models. Rev. Geophys. and Space Physics, 16: 465-490.

We find that Ramanathan's model calculates 9% increase in outgoing flux with CO2 removed from the atmosphere (my calculation using the flux numbers provided by Ram. in the table makes it 8.88%).

Looking around abit on the internet with google, I do find a lead as to where the 12% figure that folks are repeating may actually have come from, Richard Wayne's, Chemistry of Atmospheres, per this website:

http://www.radix.net/~bobg/climate/halpern.trap.html

Your guess is a good as mine as to how that 12% came about.

There's research, and then there's models. Only empirical observation can validate model projections.

I just ran across the following recent contribution by Ramanathan to the climate debate that centers on recent empirical measures rather than models:

Ramanathan, V. and A. Inamdar, 2006: “The Radiative Forcing due to Clouds and Water Vapor” in Frontiers of Climate Modeling, J. T. Kiehl and V. Ramanthan, Editors, (Cambridge University Press 2006), pp. 119-151.

It'll take awhile to digest the meat in this one.

This article along with the GCM evaluations of Collins 2006 above, looks to provide valuable insights on the issues we are discussing as well as provide solid empirical measures relating to water vapor and cloud effects on the radiative budget.

After a quick perusal to locate caveat etc. to maintain a better context as I study this document, I noted a couple of things that caught my eye and triggered some ruminations in regards water vapor content of the atmosphere (w):

[p. 19] "Before summarizing the results, we will clarify an oft-held misconception that only the lower layers of the troposphere contribute to G_a.

Figure 5.9 shows the sensitivity parameters dFc/d(lnw) and dGa/d(lnw) as a function of altitude. In each layer of the atmosphere, we change the water-vapor concentration by 1% and estimate the change in OLR and G_a. The figure clearly demonstrates that both G_a and OLR (i.e., F_c) receive comparable contribution from all layers of the troposphere."

Something bearing on what I have been attempting to point out in regards the relative contributions of water vapor as compared to CO2 at differing layers of the atmosphere and interesting to note, observing the graph at Figure 5.9, that with its band saturation, like CO2, water vapor radiative forcing is treated as a logarithmic function of concentration.

Doubling or halving water vapor concentration provides a linear decrement/increment to forcing in much the same manner as CO₂. Strong lines limiting out at saturation with wings and weak absorption lines providing a small linear element. The whole taken together yield a close approximation of a logarithmic relationship, as opposed to a resource limiting function of the form (1-e^-c/c0) which asymptotically approaches a hard maximum limit. A log function continues to rise albeit ever slowing with increasing values of its independent variable.

Ramanathan provides a graphic illustration for CO₂ in:

Augustsson, T. and V. Ramanathan, 1977: A Radiative-Convective Model Study of the CO2-Climate Problem. J. Atmos. Sci., 34: 448-451;

PDF page. 2-3 referring to Figure 1 on page 3.

The near logarithmic relationship is clearly an explanation as to why we see such high residual forcings for CO2 and H₂O for regions where one would be inclined to expect there should be nil due to vanishing small absolute density.

With increasing altitude, relatively small amounts, in terms of density measured as molecules/m³ at low atmospheric pressures, still yield appreciable radiative forcing at even the high reaches of the atmosphere.

[p. 23] "Both Figures 5.10 and 5.11 reveal that g_a,w₁,w₂,w₃, and T_s are positively correlated. The figures again reveal the consistency between the radiation-budget data and the water-vapor data. When w and g_a are correlated with T_s, the best correlation coefficient is obtained for a phase lag of less than a month (as shown later). This near-zero phase lag rules out the possibility of variations in g_a or w driving variations in T_s; were this to be the case, T_s should lag behind the forcing by at least more than a month. On the other hand, since convective time scales are less than a month, it is reasonable to expect that variations in g_a and w are driven by variations in T_s without much phase lag. The deduction is that the correlation coefficient between g_a and T_s or that between w and T_s is the feedback parameter, valid at least for annual time scales."

The reference to near zero phase lag calls to mind similar observations by many researchers concerning the observed lags of decades and centuries in CO2 concentration changes with respect to changing surface temperature, T_s, as well.

The above holds another interesting observation indicating an increased surface temperature gives rise to increased water vapor enhancing the global warming effect, G_a, of the system as a whole. However from his statement:

"This near-zero phase lag rules out the possibility of variations in g_a or w driving variations in T_s;"

it would appear the latter enhancement does not in turn act to increase surface temperature so much as to facilitate the transfer of surface heat to the upper layers of the atmosphere.

Water vapor would appear not to provide feedback to a surface manifestation of global warming effects as much as it provides a linear enhancement to absorption of radiant flux of the atmosphere above effecting transfer of energy upward towards space.

In the Global system water vapor, it appears, merely adds to G_a in response to radiative heating from the surface as opposed to a non-linear effect invoking a so-called thermal runaway effect so frequently speculated about by many.

Consider for argument's sake that water vapor is a very light molecule in regards the atmosphere with a molecular weight of 18 as opposed to the primary constituents of the atmosphere of 32 for O₂ 28 for N₂. The migration of water vapor, even in absence of heating, is to rise in the atmosphere towards cool regions where it reaches dewpoint temperature and condenses out to form clouds releasing outbound radiative flux, for the most part outside the narrow IR absorption lines of other molecules.

Any downwelling IR flux arising from condensation of water vapor would be inhibited through absorption by water content in the clouds forming below as well as by upwelling water vapor rising from the surface still in an unexcited state for the paucity of IR flux available, (a consequence of saturation, more molecules than photons of IR at the proper wavelengths for radiative absorption to go around).

[p. 26] "However, our results do not necessarily confirm the positive feedback resulting from the fixed relative humidity models for global warming, for the present results are based on annual cycle.We need additional tests with decadal time-scale data for a rigorous test. Nevertheless, the analysis confirms that water vapor has a positive feedback effect for global-scale changes on seasonal to inter-annual time scales. In addition, the results presented here provide a starting point for validating general circulation models."

I would say, a long overdue validation of the GCM projections as well as their input scenarios.

Climate models are severely lacking in validation of projections and input scenarios, as well as lacking in the verification of the base GCM numeric algorithms and programming such as is incurred in the software QA required of any engineering model on which policy decisions are dependent.

Found a Ramanathan peer review paper detailing the effect of doubling CO₂ on primary direct radiative forcing at tropopause and surface as a consequence of the overlap of water vapor continuum absorption in the 12-18mm region with CO₂ mm absorption, not accounted for in most GCM studies.

refer:

absorption overlap between Water Vapor and Carbon dioxide
http://earthobservatory.nasa.gov/Study/ArbitersOfEnergy/

On the meat, noting especially Table 1 detailing the effect on downwelling LW radiation with regards enhancement of solar heating of the surface and hence surface temperatures resulting from a doubling from a doubling of CO2 concentration.

Kiehl, J. T. and V. Ramanathan, 1982: Radiative Heating Due to Increased CO2: The Role of H2O Continuum Absorption in the 12-18 mm Region . J. Atmos. Sci., 39: 2923-2926.

Introduction:
Within the 12-18mm region, both H2O and CO2 absorb and emit radiation giving rise to the so-called "overlap." This study examines the role of this H2O-CO2 overlap in the CO2-climate proble. The H2O absorpotion, within the 12-18mm region, that has been traditionally included in climate models (Manabe and Wetherald, 1980; Ramanathan, 1981) is the line absorption due to the pure rotational band of H2O. In addition to the pure rotaional band, there is very strong "continuum" absorption by H2O in the 12-18 um region (Roberts et al., 1976). The few climate model studies which include the effect of this continuum (e.g., Wang et al., 1976) have not examined its rl in the increased CO2 radiative effects. In order to isolate the overlap effects of various H2O radiation processes in the 12-18mm region, we comput the radiative heating of the surface/troposphere system due to double CO2 with and without the H2O overlap effects.
*** SNIP ***
[p. 3] We consider three cases in which CO2 is doubled and the changes in longwave fluxes are computed for no overlap between water vapor and CO2. This is achieved by setting the transmissivity of H2O in the 12-18mm region to be equal to 1. In the second case, the H2O overlap due to the rotaional band is included. Finally we include the H2O continuum and calculate the flux changes using both continuum and line transmissivity (for the pure rotation band) described in the previous section. These cases illustrate the most important aspects of the water vapor overlap.
*** SNIP ***

Table 1. The effect of CO2 increase on the hemispherically
averaged net radiative heating (wm^-2). DF_T^N is the change
in the net outgoing longwave flux (at the tropopause) due to
doubling of CO2; negative values of this quantity denote
heating of the joint surface/troposphere system. DF¯_s is the
is the change in the downward longwave flux at the surface.

Case Comments -DF_T^N DF¯_s

1 Without H2O absorption in
12-18 mm region 4.69 3.65

2 H2O line absorption 4.18 1.56

3 Line plus continuum absorption 3.99 0.55

Referring to the value of surface forcing for CO₂ in the Collin's study above, of around 1.5 w/m² it appears only a very few out at the tail end of the current crop of GCMs account for the water vapor's continuum absorption overlap with CO₂.

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Case	Comments	-DF_T^N	DF¯_s
1	Without H2O absorption in 12-18 mm region	4.69	3.65
2	H2O line absorption	4.18	1.56
3	Line plus continuum absorption	3.99	0.55