It may or may not be illegal (it probably would be in the case of fraudulently obtained grant funding), but academic fraud is career-ending. It is a very serious accusation. Besides, you are completely misunderstanding how defamation works. Defamation does not require that an allegation of a crime. Heck, there's even a term for this kind of thing: defamation of character.
Indeed. It seems fraud is table stakes at this point.
Typically however, climate 'scientists' abstract their fraud. They take raw data, fit it to their 'model', and then produce a conclusion. Sometimes they get over zealous, and manipulate the raw data, like the NOAA was caught doing.
https://www.alllaw.com/articles/nolo/civil-litigation/defama...
Another term you might want to look up is "Dunning-Kruger."