What is your estimate for climate sensitivity, and where does it come from?

Your estimate is worthless IMO. Your confidence in cloud feedback is unwarranted. Alarmists always fall back on their alter which is computer models because the real world isn't their ally. It seems you are being purposefully ignorant. CO2 follows temperature, not the reverse. This is really very very basic. If you are trying to delude yourself, you are doing an excellent job. Since I know we don't know enough to provide estimates, I would be foolish to try. I do know that your estimate is worth less than a grain of salt.

Ottawa, there have been a number of studies done recently that have shown a potentially lower climate sensitivity, but d/dx's critique is one I have seen from several climate scientists. I do commend you for asking a reasonable question, even if the information is presented in a misleading way to indicate the thrust of current climate research is indicating significantly less sensitivity. What's left out is the other studies that show higher sensitivities, and also, exactly what d/dx and Baccheus have said. The studies are using historical records, which do not cover the range of CO2 and temperature change we are experiencing and will experience in the future. They do not take into account the seeps of methane which are being found in more and more places, nor the effects of removing the majority of the northern icecap for significant lengths of time. I am no expert. I am an interested citizen with a chemistry background. But I can read a critique, and when it's as obvious as d/dx or Baccheus points out, understand how the two might fit together in general terms. If one can't, one really shouldn't be arguing a science-related question, in my opinion. Ottawa, you are suggesting only the low end of the climate sensitivity studies should be reported? I don't believe that. You must mean all the studies should be reported, including those that show 10C of warming, no? So now I ask you the same question, from the upper end of the projections: So if you're in a policy meeting and somebody pulls out the IPCC scenarios and somebody else says newer studies indicate those numbers are 400% or more too low, wouldn't that change the discussion? Well, wouldn't it? :D ********************* EDIT: Darn, bob326, yes, I noticed the "transient" and "21st century climate" bits, and neglected to comment on them. It seems to me some of these people are saying it won't get really bad until the 22nd century. [Subtitle: Well, we'll all be dead then, so who cares, right?] Looking further, I see: :Assuming paleoclimatic constraints apply to the future as predicted by our model, these results..." The argument here is based on one lousy computer model of climate, and they're making a real whopper of an assumption in neglecting the current methane releases, for example. The simple Aldrin climate model has been criticised by climate scientists as being too simple. [And Padilla - recognize that name - that paper also came in for criticism.] Why do you show only 1 side?

My answer is almost identical to yours. 2 to 4.5°C with a most likely value of 3°C. My sources are in the article I wrote linked below. There are many independent investigations using different types of data (paleoclimate, responses to recent volcanic eruptions, climate model runs, etc.) which all put the sensitivity parameter in this same range.

I certainly agree on 1.2°C for CO2 alone, and another ~2°C from clouds, water vapor, albedo, and other feedbacks. I agree that clouds are the largest uncertainty - I think the positive water vapor feedback is becoming fairly well-established based on observational data (believe it or not Ottawa, we've learned something since 2004).

The evidence of 2 to 4.5°C sensitivity to 2xCO2 is pretty overwhelming. I see no reason to doubt it. Spencer's "internal forcing" hypothesis is really the only plausible alternative, and I think even he puts the sensitivity in the ballpark of 1.5°C.

I'm not surprised that most 'skeptics' don't even attempt to answer the question. The closest is Ottawa who merely says "I don't know". This is obviously not a good excuse for inaction. I don't know if I'll ever be in a car accident or if my house will ever catch on fire, but I own auto and home insurance. Uncertainty is not the friend of those who argue that we should take no action. The only way to justify taking no action is if we have certainty that no action is necessary. If my house were comprised entirely of non-flammable materials, then I could get away with justifying not having fire insurance.

Without certainty, as you put it MTR, if you assume the bad scenario won't happen and you're wrong, you're "in serious trouble." That's why you get the insurance. That's why you reduce carbon emissions. And there's only about a 1% chance that you're home will catch on fire, but there's a much, much higher risk that failing to reduce carbon emissions will result in extremely dangerous and damaging warming and climate change. I bet Ottawa has fire coverage in his home insurance though.

I think you've summarized the evidence for a CS of ~ 3K pretty well.

Ottawa

"However, even Dessler himself had stated in 2004 that..."

Yes, but that was before both Soden et al. 2005 [1] and his own study in 2008, both of which found evidence for roughly constant RH. Lindzen himself admits that more evidence is pointing in that direction in his most recent paper on the role of cirrus clouds in the faint young sun paradox [2], though perhaps without strict conservation of the vertical profile. In fact, through a majority of that study he supposes an invariant RH.

Scientists, Dessler included, will alter their positions as better data comes out. Indeed, Dessler now seems convinced that the constant RH assumption is entirely reasonable [3], [4].

[1] http://www.gfy.ku.dk/~kaas/forc&feedb2008/Articles...

[2] http://www.agu.org/pubs/crossref/2010/2009JD012050...

[3] http://geotest.tamu.edu/userfiles/216/dessler09.pd...

[4] http://geotest.tamu.edu/userfiles/216/Dessler10.pd...

"The water vapour/lapse rate combined feedback is as calculated by models ending up with a constant relative humidity..."

I don't like this statement. Models do what they are told, the don't calculate anything new for us. And constant relative humidity is an assumption which has not been verified.

So my answer is: I don't know. And I'm skeptical of those who think they do know, including Spencer and Lindzen. And I'm extremely skeptical that anyone who says they can accurately model the Earth's ocean-atmosphere climate system.

Edit: Yes, I realize that Dessler 2008 is a paper which claims constant relative humidity globally averaged: http://www.agu.org/journals/ABS/2008/2008GL035333....

However, even Dessler himself had stated in 2004 that "...some climate models might be overestimating the amount of water vapor entering the atmosphere as the Earth warms. " http://www.nasa.gov/centers/goddard/news/topstory/...

There are other studies about this issue that also question the constant relative humidity assertion: http://meteora.ucsd.edu/~pierce/papers/Pierce_et_a...

http://climaterealists.com/attachments/ftp/AMS-Fin...

http://arxiv.org/ftp/arxiv/papers/0809/0809.0581.p...

http://www.esrl.noaa.gov/psd/people/dezheng.sun/ds...

That's just a small sampling of evidence for my case of "I don't know".

I think you've summarised the evidence very well, and come up with an estimate that's as good as we're going to get.

Two notes of caution. One is that there may be effects from the present rapid rate of warming not properly included in our models. For example, and most urgently, release of methane from thawing methane hydrate, and from increased biological activity in recently frozen tundra. The other one is that your 4.5° C is not an absolute upper bound, but leaves, perhaps, a 10% chance of being above that; very bad news indeed.

The denialists will of course jump on these uncertainties. But the current recession is the clearest demonstration you could wish for of what can happen if you ignore unpleasant possibilities because they are uncertain.

My estimate is that we will have weather; based on experiencing weather for the last 16,073 days. I estimate some weather occurances will be worse than others; based on experiencing some of the worse mother nature has to offer and reading evidence of cataclysmic storms during human history.

The longer you study your tarot cards, the more conclusions you can draw.

Your post was actually quite entertaining.

You realize that those who deny the moon landings look no less stupid.