There's an interesting bit of complexity here though because a fair amount of Mann's status as a public figure is because of the climate-conspiracy community's singling him and his work out as a target to be attacked.
If a group elevates someone to be a public figure, and uses the fact that they're a public figure to defame them in ways that would not be permitted for non-public figures, should the group really be allowed the same protections that random members of the public have talking crap about people who are incidentally famous?
(I suppose it gets a bit complicated that Mann didn't retreat from the attacks, and used the notoriety thrust upon him to become a fairly prominent science communicator. Even so, giving his attackers any benefit of the doubt doesn't seem right somehow...)
If you are intentionally trying to cause harm by saying something that is provably false and succeed at causing harm, that's defamation. There are four tests and you have to prove all of them:
> To prove prima facie defamation, a plaintiff must show four things: 1) a false statement purporting to be fact; 2) publication or communication of that statement to a third person; 3) fault amounting to at least negligence; and 4) damages, or some harm caused to the reputation of the person or entity who is the subject of the statement.
Public figures have to prove "actual malice" (a technical legal term- don't use the casual conversation definition for those words) in order to get libel, which is a much higher bar than us regular schmoe's have to clear. But it still exists.
