Q. I hate defining acronyms in the first paragraph of a paper because they impede flow. Therefore, is it acceptable to repeat the whole phrase, in this case, greenhouse gases, in the second use, and then define it there as (GHG)?

A. See CMOS 10.3: “The abbreviation usually follows immediately, in parentheses, but it may be introduced in other ways.” Your approach seems like a sensible and reader-friendly alternative to introducing the abbreviation immediately after the first mention.