"On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity."
Climate change definitely fits that description. Lets search for climate change: https://www.hnsearch.com/search#request/submissions&q=climat... None of those articles look any more inappropriate for HN to me, compared to what you'd find for any of the other categories I mentioned.
If you think a comment in a discussion is inappropriate, you shouldn't flag the story, you should flag the comment.
How many articles are there about uncontroversial, but interesting aspects of climate science? Do any of them ever get upvoted, ever?
But whether or not YOU should find these articles intellectually gratifying is beside the point. We already have a feature for those kind of articles: don't upvote them. Flagging is not for those kind of stories. Again, if you find a discussion inappropriate, flag the comments in the discussion. Anyway, I don't think this discussion is going to be productive any more, so this will be my last comment.
I probably would not have flagged the perl/gnuplot one, I'll grant you that one.
Node.js articles are on topic, even if one or the other happens to be boring. So I would not flag them, even the most uninteresting ones.