Stossel saying "sensational media reporting about a so-called ‘climate apocalypse'" reads to me that he's a climate denier and his reporting was to enforce that (and double down on his favorite hate: "government").
The phrase "climate apocalypse" comes from his interview of Michael Shellenberger, a pro nuclear environmentalist. Shellenberger's core points aren't climate denial, but instead optimism that we as a species will prevail against the challenges posed by climate change, and that the apocalyptic rhetoric of some environmentalists hurts that effort.
I want to make it really clear though; it doesn't matter whether or not Stossel is a climate denier, it's still probably not defamation. The fact checkers don't need to prove that Stossel is a climate denier.
