I've noticed a pattern recently. Some of the most vociferous skeptics of AGW are professors emeritus. No doubt, some people will argue that these are the most trustworthy sources, since their career is over and they are not dependent on getting funding.
However, I will advance a second theory. I was at a meeting a few weeks ago where I ran into Bill Gray, a famous emeritus skeptic. He gave his standard stump speech in which he claims that the water vapor feedback is negative. I followed up on this with him and it became quite clear to me that he is unfamiliar with all of the peer-reviewed literature on this subject that has been published in the last five years. This makes sense. Reading the literature is a difficult and full-time job, and emeritus faculty simply don't need to do that. Especially (in the case of Gray) when your time is occupied being interviewed and screaming at people. As a result, my sense is that the views of emeritus skeptics are often substantially out of date.
But the story goes on. After arguing with him for a few minutes, it became clear that Bill Gray has no scientific theory of his own *why* the water vapor feedback is negative, and no data to support his non-theory. He has no manuscript describing his non-theory and no plans to attempt to publish it. After I pointed out all of the evidence supporting a positive feedback, he looked confused and finally said, "OK, maybe the feedback isn't negative, maybe it's neutral. I'll give you that." I quickly concluded that he has no idea what he's talking about. I wish everyone that considers him credible could have witnessed this exchange.
Thus, we have two explanations for the emeritus-skeptic phenomenon: 1) only they are credible because their career is over, vs. 2) their knowledge is substantially out-of-date. My personal experience is that the second explains the phenomenon far better than the first.
And anyone with additional theories, please leave a comment.
My point is not that all emeritus are out of touch with the scientific literature. Some continue to be extremely credible (anyone swinging at that strawman will have their comment unceremoniously dumped into the electronic ether). My point is that we have to consider whether any particular emeritus scientific (either pro or con AGW) is familiar with the most current science. If their knowledge is 10-years old, then their statements might be quite incorrect.