From a naive point of view, uncertainty clearly tips the scales against costly action.  If you’re only 50% sure that your transmission if broken, for example, you have less reason to replace it than if you are 100% sure that it’s broken.  You could object, “If you don’t act now, it may be too late”; but this argument is still weaker than, “If you don’t act now, it will definitely be too late.”

Global warming skeptics often appeal to this intuition.  The less sure we are that we actually have a problem, the weaker the case for action.  As far as I can tell, their argument is completely solid.

Nevertheless, Tyler argued back in 2006 that uncertainty about global warming is an argument in favor of doing something about it:

Like Arnold Kling, I do not much trust climate models.  Perhaps I have
spent too much time doing macro, and the experience carries over. 
Nonetheless uncertainty about final effects gives us more to worry
about, not less.  It is the worst-case scenarios for global warming
which worry me, not the middling scenarios.  Variance is our enemy in
this matter.

Brad DeLong approved:

As Tyler Cowen said a while ago, uncertainty in the accuracy of climate
models is not our friend and not an argument for inaction.

What’s going on?  If you read Tyler and Brad’s words carefully, their argument is also completely solid.  The problem is an equivocation on “uncertainty,” with each side using the meaning that supports its case.

On the first meaning, “uncertainty” means “uncertainty that a problem exists”  The higher the uncertainty, the lower the expected severity.  If this is what you mean by uncertainty, then uncertainty about global warming (or anything else) is a valid argument for inaction.

On the second meaning, “uncertainty” means “variance around a problem’s expected severity.”  The higher the uncertainty, the greater the variance.  That’s why Tyler explicitly said “variance,” and Brad talked about “accuracy.”  If this is what you mean by uncertainty, then uncertainty about global warming (or anything else) is a valid argument for action.

In the past, I’ve affirmed my willingness to defer to the expect consensus on global warming.  Lately, I’ve been disturbed by hard evidence of the experts’ ideological biases.  Unfortunately, this means that while I’d like to ask some climate experts to specify which kind of uncertainty they’re talking about, I no longer know who to turn to for an objective explanation.  Got any pointers?