Two words. Fluctuating orbits. Its quite natural to have a meteor every now and then, given that there are already several regularily passing Terra, but not hitting it. Although they are rarely extinction level, and often burn up in the atmosphere, every so often, maybe once every 50 million years, a big one comes along and majorly screws up the ecosystem. Hell, greenhouse gases might even be able to protect us better from that, in which case we're in a catch-22 until we develop some kind of huge scattering laser able to not only smash an asteroid or comet, but make sure the smaller pieces are too small to still really affect Terra's atmosphere.
The thing is, if humans are only a tiny factor in global warming, then that means it's mostly natural. And unless you're some kind of hyperintelligent extraterrestial, or one of the humans near the end of "The Last Question", you ain't gonna stop Mother Earth's natural cycles, no more than you can stop winter. And if the human factor is small, then spending hundreds of billions of dollars on an extremely small factor is just flushing money down the toilet and screwing the economy severely.
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