One of the more heavily abused terms around blogs and such is ad hominem, which is unfortunate because it is actually a useful method for weeding sources -- if you understand what it really means. The bare translation of the Latin, against the man, doesn't help us much unfortunately. But it's a start. You know something is up when someone starts talking about a person instead of the science in what is supposed to be a scientific discussion.
The classic form of an ad hominem is:
"X is a bad person, therefore they're wrong about Y".
'bad person' is usually substituted with something else ('travels', 'has a big house', ...), and the 'therefore' is often omitted. One I commonly see is, for example "AGW is a scam. Al Gore has a big house and uses a lot of electricity. "
The logical fallacy being that comments about Al Gore say exactly nothing about the science on anthropogenic global warming. But if someone says it loud enough, often enough, and maybe shouts it at you, you might get swept up into their emotional argument. They're hoping so.
My wife occasionally reminds me that there's more to the world than science. So I'll step outside that and look at the policy implications the ad hominem people want us to draw. One is obviously that they want us to think that doing anything about AGW requires us to give up having large houses and using 'a lot' of electricity. Except this is false. See my comments about keeping your vehicle how you choose for more. Part of how Gore is dealing with the carbon produced in his energy use is to buy carbon credits (the idea being that if you cause 10 units of carbon to be released, and you also cause 10 units to be drawn back out of the atmosphere, then your net effect is zero -- and you can do what you like as long as your net is zero). If that's brought up, the ad hominem folks then say something about Gore owning (part of? I don't know) the company he buys the carbon credits from. As long as the carbon is buried, though, it doesn't matter -- to the science, or honest policy -- who gets the corporate profit. Their complaint, then, is that Gore is able to make money while living large and not contributing to climate change. Huh? Shouldn't he be getting a medal for that? There are jobs to be had in carbon credit sorts of activities -- planting trees, building windmills, whatever. But that means a common whine from them -- that doing anything about climate change would bankrupt the world (talk about folks who want to scare us!) -- would also be false.
If you've got examples of people whose opinions are quite different from Gore's on climate and are regularly attacked in similarly ad hominem ways, do submit names and examples. It was a Gore ad hominem article I just saw that prompted this note. He, Jim Hansen, and Michael Mann, in that order, are the three I see the most ad hominem attacks against. No doubt a function of where I read.
Back to the science. When you see a note that talks about the person who holds an idea, rather than the idea, you can be fairly confident that the source isn't concerned about the idea. If there is no such thing as a greenhouse effect, present evidence against that; don't tell me about Al Gore's house. And so on. A google search on "al gore" agw scam brings up the following examples of folks doing the ad hominem game against Gore instead of saying anything substantive about the science), so can be added to the list of unreliable sources
(This site showd up earlier as an unreliable source -- as I mentioned early on, there's a lot of consistency in being unreliable. Once you've found one such example, you're likely to find multiple if you spend more time. That's the value of discovering at some point that a site is unreliable.)
John Coleman is the person quoted above.
And you'll see quite a lot of examples in blog comments, but I excluded sites where such things only appeared in the comments.
There's a converse mistake about ad hominem that's often made (usually by the same people) -- that to say anything about a person is ad hominem. Of course they don't apply that rule consistently. For instance, it is certainly important to consider the qualifications (a personal attribute) of a speaker. So a comment that Al Gore is a (retired) politician, rather than a climate scientist, is perfectly reasonable. I don't get my science from him for that reason. If you are, then please read some science books as well, say Spencer Weart's Discovery of Global Warming. Equally, though, it's not ad hominem to observe that John Coleman is a TV weather forecaster, not a climate scientist. So it probably isn't a good idea to rely on him for your climate science either.