Between the science and a hard place: The intellectual incoherence of lukewarmism.

This is a mirrored post from Idiot Tracker, that is too good not to share.

Part One: jimming the Overton window.

In recent years, the climate wars have witnessed the rise of a group of self-identified "lukewarmers," people who, to borrow Greg Easterbrook's self-description, believe that:
    [G]lobal warming is scientifically confirmed but exaggerated as a threat; that greenhouse gas regulation is justified, but not an emergency need.

The first use of the term I have found was by David Smith, on "Watts Up With That" (a bit of foreshadowing, that):
    I am a “lukewarmer” who thinks that the world is warmer than it would otherwise be due to anthropogenic gases (but doubts that the impact will be extreme).
Probably the most famous "lukewarmer" is Lucia Liljegren, a mechanical engineer (surprise!) whose blog, The Blackboard, can be found on the blogroll here. The Blackboard entertains many lukewarmers, along with a bunch of deniers and a smattering of pro-consensus folks, including myself.

Another self-described lukewarmer, Steven Fuller, has stepped into the giant floppy red shoes of Steven Goddard at WUWT, now that Steve has metastasized to his own blog. His self-description is the closest to providing a clear, testable proposition, as well as reflecting, by my reading, the central thrust of the comments by the "lukewarmers" on The Blackboard:
    It’s because I am a ‘lukewarmer,’ one who believes that the physics of climate change are not by theselves controversial, but who believes that the sensitivity of the earth’s atmosphere to a doubling of concentrations of CO2 is not yet known, but is likely to be lower than activists have claimed.
Before I get into what the climate sensitivity might be, it's important to note who the lukewarmers are, which is slightly different from their self-definition. The lukewarmers to a person are critical of the IPCC, but when we take a look at the estimates of climate sensitivity cited by the IPCC, it's clear that thinking climate sensitivity is low is not enough to get on the outs with them: