Water vapor feedback - positive or negative (Dessler vs. Spencer)?

Question

With virtually all scientists (including 'skeptics' like Christy and Lindzen) in agreement that humans are causing global warming, the main scientific question is how much will our greenhouse gas emissions cause the planet to warm in the future.  One of the main determinants is whether water vapor will be a positive (warming) or negative (cooling) feedback.  Water vapor forms clouds, certain types of which reflect sunlight causing cooling, but it's also a greenhouse gas itself.  As other greenhouse gases like CO2 warm the planet, the atmosphere can hold more water vapor.

Roy Spencer has claimed "Three IPCC climate models, recent NASA Aqua satellite data, and a simple 3-layer climate model are used together to demonstrate that the IPCC climate models are far too sensitive, resulting in their prediction of too much global warming in response to anthropogenic greenhouse gas emissions."

However, he makes this claim on his personal website, and I can't find any peer-reviewed scientific publications in which he presents this argument.
http://scholar.google.com/scholar?hl=en&lr=&q=RW+Spencer+climate+change
http://www.drroyspencer.com/research-articles/satellite-and-climate-model-evidence/

However, a peer-reviewed paper published by Dessler et al. in Geophysical Research Letters in 2008 concluded "Height-resolved measurements of specific humidity (q) and relative humidity (RH) are obtained from NASA's satellite-borne Atmospheric Infrared Sounder (AIRS)...The water-vapor feedback implied by these observations is strongly positive, with an average magnitude of λ_q = 2.04 W/m2/K, similar to that simulated by climate models."
http://www.agu.org/pubs/crossref/2008/2008GL035333.shtml
http://www.sciencedaily.com/releases/2008/11/081117193013.htm

Who do you think is right - Dessler et al. that water vapor is a positive feedback and climate models are correct, or Spencer that climate models exaggerate climate sensitivity, and why?

bucket22 · Accepted Answer

Direct observations have confirmed the positive water vapor feedback, as the Dessler study shows.  There's actually not much scientific uncertainty with regards to this feedback, although note that Dessler found a feedback that is a bit stronger than the IPCC estimate.

There's more uncertainty with regards to the cloud feedback, and this makes up the bulk of the uncertainty in the IPCC climate sensitivity range (2-4.5 C likely range).  There have not been robust estimates of this, although Spencer has been working hard to find a negative feedback from this variable.  Given his scientific track record to date and his known biases, I wouldn't take him too seriously.

Studies of past climate change (i.e. ice ages), imply that cloud feedback is likely to break somewhat on the positive side.

EDIT:

John Terry:

Actually, climate models on balance have been remarkably accurate.

http://www.skepticalscience.com/climate-models.htm

http://www.realclimate.org/index.php/archives/2008/12/2008-temperature-summaries-and-spin/langswitch_lang/fr

http://www.realclimate.org/index.php/archives/2008/05/what-the-ipcc-models-really-say/

Predictions of ENSO tends to determine how accurate they are in the short-term.  Then again, global climate models in the context above are not intended to be evaluated short-term.  Think long-term.

EDIT2

John Terry,

No.  I'm not joking.  Considering the sources you're citing, I wonder if you're joking.

J S · Answer

What happened after 800 years of warming in the past?

In many cases it was 4200 more years of warming, correlating with increased CO2 levels, and due to increased evaporation, higher water vapor levels.  

So whatever caused initial warming, CO2 released from warming oceans, methane released from melting tundra and ocean sediments, and water vapor obviously overpower any (theoretical) cooling affect that some types of clouds may have.

The historical evidence seems rather obvious.  If clouds magically counteracted warming with a cooling influence of their own, past warming trends would reverse sooner and the temperature record of the earth would be far more stable.

It's possible that some threshold of heating must be reached before a magic cloud cooling then kicks in (and through some equally mysterious process then dominates for a long cooling period), but the size of past warmings and the rate of curent warming do not bode well for our chances at survival while waiting for a miraculous event to happen.

bob326 · Answer

water vapor ≠ clouds

Interesting papers dana, but they look like they are answering different questions that involve different processes (i.e. Spencer argues that clouds are a significant forcing rather than a feedback, and Dessler provides evidence for a positive water vapor feedback).

JimZ · Answer

Clearly Spencer is correct.  There are probably both negative and positive feedbacks.  Clearly there is a negative feedback in water vapor and probably related to cloud formation.  If there wasn't, there should be runaway warming. There is plenty of water to make that happen.   Since this doesn't happen, we can safely conclude that the mechanism exists. It stands to reason that temperature plays a primary role.   Water, particularly in evaporation and condensation, dwarfs CO2 in regard to its effect on climate.  Not understanding the exact nature of the mechanisms in no way should be a license for alarmists to claim then that humans must somehow be responsible for all or most warming because we don't understand  how it all works.  You don't need models for that.  You just need a dose of common sense.

JOHNNIE B · Answer

The Global Warming is a scam so the oil co. can charge U 3 prices and field justified because they are protecting the environment.The great green house gas is not there . The CO2 is a very heavy gass and if the plants did not recycle it we could be in trouble.Methane is just an out right lie. Methane is a very light gas so how can U measure it. It has completely dissipeared because methane is flamible. As the methane gets high enough the sun light is high enough that methane just oxidizes.

Water vapor feedback - positive or negative (Dessler vs. Spencer)?

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