I am reading Frederick Crews' The Follies of the Wise, which collects essays he has written over the years attacking creationists, UFO believers, Freudians, and others. The common thread is his unrelenting empiricism.
Greg Mankiw exposed my skepticism on Global Warming to his audience, and got a couple of predictable comments.
Questioning the validity of scientists on global warming is like questioning the validity of economists on the economy.
I would say that it is more like questioning the validity of creation scientists on creation science. You could argue that Crews, a literature professor, has no right to criticize the people that he does. But he argues as an empiricist. So do I.
Another commenter chides Mankiw,
Review the scientific literature. Don't be lazy. Inform yourself. This is too important an issue for smart and influential people like you to remain ignorant about.
I felt the same way, which is why I looked into the literature, and why I came out the way that I did. Readers who have not been following my thought process can browse the energy-environment archive.
Basically, I believe that the best evidence for global warming is global warming. That is, the rise in observed temperatures is the most important data.
That rise began around 1900, and amounted to about 0.3 degrees centigrade by 1940. Temperatures leveled off until 1980, and since then they are up 0.4 degrees centigrade.
Much, much more of the human activity that would cause global warming has occurred in the last 20 years than took place between 1900 and 1940. Also, much, much more of the greenhouse gas layer on earth consists of either water vapor or pre-industrial levels of carbon dioxide.
Thus, the link between human activity and global warming depends not on simple, obvious relationships in the data. It depends entirely on climate models of how these tiny (relative to the overall volume of greenhouse gases) human activities produce "feedback loops" on the rest. They are models of how much less than one percent of a phenomenon affects the entire phenomenon. They are much more faith-based than empirical.
It is possible that the models underestimate human-caused global warming. However, I believe that this is far less likely than that they over-estimate the human causal factor.
I believe that average temperatures have been rising. I have no reason to believe that they will stop rising. However, the most sensible position an empiricist can take is that human activity is not going to make much difference to global warming, one way or the other.