It didn’t really take 250 years though, early emissions were almost negligible. Most of it started like 60 years ago. You’re right that we’re not stopping it anytime soon, but the effective timelines are shorter than centuries.

Also, what’s your reasoning/source on a 10 bn “absolute” cap? It might be a cap while using modern farming, technologies and logistics, but it’s not absolute by any means. You mention beaming energy from space, then why not mention Eucomenopolis concepts that allow for trillions of people to inhabit Earth? :) Or simply once you have fusion, you can have vertical farms and Arcologies that can sustain a much larger population.

The issue isn’t that it’s impossible, rather that we’re not gonna develop any of this tech before humanity faces existential problems in many parts of the world.

Also, it’s weird that you got from “this temperature variance is minimal” and “this average is on the low side of comfortable” to “let’s get rid of 30% of population then”. o_O