Another thought about the "global warming attracts asteroids" nuttiness: Leftists understand that even patently absurd propaganda can be effective when it's wall-to-wall. Global warming (and later, when that term became too specific and untenable, "climate change") has been a flood-the-zone affair, since its inception. It's everywhere. It's difficult to get through a few days of nothing but light entertainment programming without being hit by global warming ideas and terminology, dropped casually by characters in everything from breezy comedies to sci-fi epics. And of course, it's perpetually appearing in news broadcasts, too. I vaguely recall a story years ago in which some meteorologist urged his peers to mention global warming at every opportunity, precisely because he understood the value of repetition, particularly casual mentions.
This has the effect of programming a leftist meme into the public mind as something "everyone knows." It's hardly a sin to care about the Earth too much, right? So it was presented as received wisdom that people who really care about the earth casually, uncritically accept global warming mythology. If certain advocates went a bit overboard with their cooked-up graphs and "Day After Tomorrow" hysteria marketing... well, they clearly meant well.
You can see something similar happening with class warfare and anti-capitalist themes. Long after last rites were performed over the Occupy movement, its rhetoric and ideas pop up casually in entertainment media; there's even going to be a DC Comic book expounding their themes. The Manichean social model of an overworked, underpaid "99 Percent" cheated by greedy, treacherous "One Percenters" who never "pay their fair share" is presented as an unchallenged core assumption, and it's coded into public discourse through constant low-intensity repetition, as something news reporters or fictional characters simply accept. Needless to say, no conservative ideal will ever be presented as an offhand truth, accepted as casual wisdom by real and fictitious people who might not even claim to be politically active.