Data schmata. We don't need no stinkin' data.
The main issue with people like Mann (and the EPA) is the data they use to make their predictions (and laws). As the old saying goes, "crap in, crap out".
When people like Steyn criticize these alarmists they most often do so by pinpoint flaws in the data. As an example, say an equation consists of 4 parts and 3 of those parts have some basis in fact. But the 1 remaining party is fuzzy, very fuzzy. It only takes 1 part of an equation to be fabricated to throw off the entire equation.
That is the basis of most of the arguments against these alarmists. In the case of the EPA they keep their calculations secret and have even defied demands from Congress to reveal their secrets.
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The issue illustrated above begets another problem. While the quacks and the quack attackers receive all the headlines there are real scientists out there doing real scientific research. They don't get any headlines, but they do receive death threats. The upshot is we never get to engage in any sort of real adult conversation on the topic.