No, eutectics are vary real and useful in industry.

I'm sure they they are, but "eutectics" wasn't what I was referring to as "highly speculative".  It was the theory you were stating:  

"But, is there a eutectic point such that the heat capacity of the atmosphere becomes more, not less, opaque to heat. This would cause a very rapid rise in temperature. I had not considered that one. I would hope that Hansen and others have thought about that one." 

And I'll have to admit I was, rather lamely trying to be funny.

I guess you were talking about something like what's going on on Venus, where the cloud cover is actually holding in the heat?