Michael Mann, the Penn State climate scientist who was heavily featured in the "ClimateGate" email drop, has sued National Review for defamation over this Mark Steyn post. I myself think the post went a little over the top. But I'm quite surprised to see Mann suing. For a public figure (which Mann is), it is extremely difficult to win a libel suit against a publication. You have to prove "actual malice", which is not what it sounds like to laymen--a grudge against the subject--but rather, that the person who wrote the statement was averring a fact which they had good reason to believe was false. The last public figure I can remember winning such a suit in US courts was Carol Burnett, a teetotaller whom the National Enquirer claimed to have seen belligerantly drunk in a public space.
First, Mann will have to prove that the passage at issue (a reference to Mann's famous "hockey stick graph", which Steyn called "fraudulent") is a statement of fact. That alone looks like a high hurdle, since it's far from clear that Steyn is claiming criminal or even scientific fraud, rather than choosing a rather hyperbolic way of saying "misleading". As I understand it, this is National Review's defense, and it seems not unreasonable.
But say he proves that they were making a factual claim, rather than a poor word choice. Now, as I understand it, he has to prove that, first, it's false, and second, Steyn had good reason to believe it was false. Not that he failed to investigate as he should, but that he ignored evidence he already had. (I'm no lawyer, and I'm grossly simplifying for space considerations, so I invite readers to expand in the comments).
Proving that the fraud is false will open up Mann to considerable discovery--much more, as far as I can tell, than NR will be exposed to. His entire work and email history are arguably relevant. This (and the expense) is why even people with good libel cases often don't sue. As a financier of my acquaintance remarked, "The chances of correspondence that looks bad approach one. Just ask Eliot Spitzer." Spitzer, of course, lost every financial case he took to court. But financial firms often preferred to settle rather than see their out-of-context email fragments splashed across the front page of every paper in the country.
Of course, Mann's lawyer has presumably advised him of these difficulties. Which suggests that perhaps he doesn't fear discovery . . . or that he thinks he can use discovery to get something really juicy out of National Review. On the other hand, it wouldn't be the first time that climate scientists had committed catastrophic errors of judgement in an attempt to "expose" the other side. Which means, as Jonathan Adler says, that for those of us on the sidelines, it's popcorn time.