GMAT Sentence Correction (SC) | Those skeptical of the extent of global warming argue that

In choice E, are "whether significant...occur" and "what impact...have" noun phrases? According to the structure of the sentence, they must be noun phrases because they come after a preposition (over), and only noun and noun phrases can follow a preposition.
However, both don't seem nounphrases:


Can "whether + clause" be a noun phrase?
Also, I cannot identify what kind of structure "what impact it would have" is .

It seems a clause. Is "what"