"The subset of experts who understand what they're doing is considerably smaller than the set of amateurs who don't -- even if the basic skills are similar. "
An amateur is someone who isn't being paid for his work. Pick up a history book and see how many technical and scientific advances have been made by amateurs -- people who fund their own research. Einstein did some of his best work as an amateur when he was a patent clerk.
But you raise a good question -- who should we trust? Honesty is more of an issue than competence. I would trust the man who is not being paid, he has nothing to lose by telling the truth and nothing to gain by distorting the truth to please his politically motivated patrons.
I will grant that the opinions of informed individuals on subjects that lie outside of their area of expertise can be valuable, if those individuals are truly well-informed. However, the ability to be "truly" well-informed on a particular topic becomes more difficult as the complexity of the topic increases. An uninformed opinion, or an opinion based only a skewed awareness of the information that is relevant to the topic, is not very valuable. If uninformed or skewed opinions are presented as being equivalent to well-informed opinions, then the validity of any conclusions that can be drawn about the topic from a compilation of such opinions is marginal.
Let's break this down to a very simple example (which happened to me recently). You're driving through an unfamiliar town trying to find the local post office. You can ask anybody you see how to get there. You happen to spot a mail carrier delivering mail to the businesses along a central street in the town.
Would you prefer to ask the mail carrier for directions to the post office, or just anyone who is walking on the street? (You can assume that most people walking on the street are local and might know how to get to the post office.) Explain your answer.
Extend this a bit. Just imagine that you can survey two groups in the town to get directions to the post office from a given location. One group is made up exclusively of the town's mail carriers; the other is a random group of town citizens. When you examine the two sets of responses, would you expect the quality of the responses from the mail carriers to be generally better than the quality of responses from the random group of citizens?
Now let's go one step further. Imagine that instead of mail carriers, you have one group made of citizens of the town, and another group composed of citizens from the state that the town is located in. The only criterion for inclusion in the latter group is that the citizens can locate the town on a map of the state.
Which of these groups will likely provide the better set of directions to the post office?
Who do you think might have a better understanding of global warming -- a meteorologist or a microbiologist? Now, the meteorologist could be a dunce and the microbiologist a brilliant person who knows a lot about a lot of things (including global warming): but without knowing anything other than their profession, who would you choose?