Arn, I may be wrong here but it seems to me you are making two arguments that are somewhat contradictory.  If I understand you correctly you believe that we can not accurately predict the future and that we should be skeptical of computer models and scientists who do so. 

On the other hand your CBA assumes 2 things about the future.  First, that the problem is likely to be less significant (rather than more) and second, that science will solve the problem.  It seems incongruent to me to simultaneously base your CBA on assumptions about the future when you do not believe that the future can be accurately predicted.  I prefer a more conservative approach. 

Also, I am not particularly pessimistic about the ability of science to solve problems.  I am just not willing to place all my eggs in that basket.  It would for example be a mistake to smoke cigarettes based on a belief that there will be a cure for lung cancer sometime in the future.  

Also, I think it might be worthwhile to review the fallacies of presumption found here:   http://logicalfallacies.info/

I find it useful to reference these occasionally particularly when making predictions/assumptions about the future.  

Also Arn, I agree that Jeff's statements are value laden.  While we should not pass judgement on the behavior (whether it is good or bad in the moral sense is indeed irrelevant) we do need to know if it has caused the problem at hand because if it has, we need to modify that bahavior.  I think Jeff's response was overly emotional but accurate in the sense that we are not changing our behavior bacusse we are looking at the situation from a short term ecenomic  perspective. 

Tom,
My statements aren't contradictory.  My CBA isn't dependent on science coming through for us and I am willing to accept an expected scenario with respect to forecasts for the purposes of having a discussion on where to spend money.

I simply want to look at the predicted effects - irrespective of causes - and determine how best to mitigate those effects.  So, talking about disease, flooding, etc. is all fair game.  Let's put all that on the table and say how do we effectively deal with it.  My supposition is that there are usually more cost effective ways of dealing with the effects than what has been proposed.

So, for example, with respect to flooding in the 3rd world, I would argue that wealth is more important than sea level.  I say this because dikes and levees, barriers, etc. are relatively cheap investments - even in the 3rd world.  But, if you take actions that limit economic development - especially in the 3rd world - you may be making it difficult to afford that cheap protection.

I think part of the problem with the discussion is that I don't view our current climate as a good thing or a bad thing - just a state.  Just like I don't view our lifestyles with a moral judgment.

So, when I talk about CBA or GW, I just want to talk about effects and how best to manage those effects.  And, unfortunately, a lot of the discussion is often driven in more of a "purity of essence" way (not from you Tom, just in general).  Knowing the cause is interesting and important because modifying the cause can be one mechanism for cost effectively managing the effect.  But, ultimately if the effect is the issue, the cause doesn't matter.

And, Jeff, I am glad that you are passionate about your beliefs - that is great and that is what I admire most about the people in the environmental movement.  Unfortunately, that passion is what often clouds judgment - which is what I admire least.  So, it is a double edged sword.

Fair enough, though I think the passion has come from both sides of this argument.  I'm going to end my posting on this thread with one final comment...

It sounds like our primary difference is this.  I believe prevention is more cost effective than mitigation and you beleive the opposite.