And ironically, it’s XKCD What-if that is wrong. Well, not wrong by commission, but wrong by omission.
First off, I’m a huge fan of both XKCD (well the 98% of their comic strips that are SFJ) and XKCD-What If. If I was 1/20th as funny as Randall Munroe, I’d probably be writing witty webcomics instead of trying to build spacecraft robot arms and rocket ships…
But in this particular XKCD-What If, he was trying to answer the question “Is there enough energy to move the entire current human population off-planet?” The answer started off reasonably enough, talking about the energy needed to lift a mass to escape velocity, Tsiolkovsky’s rocket equation and such. The problem is that XKCD in effect only compared three options for transporting people to escape velocity: LOX/Gasoline Single-Stage-to-Escape-Velocity rockets, Space Elevators, and Orion Nuclear Pulse Propulsion.
No discussion of staging, of higher energy upper stages (like say LOX/LH2), of high Isp in-space propulsion (like Solar Electric Propulsion), of rotating tethers made using existing materials, and most offensively of all, no mention of propellant depots or tapping offworld resources for that last several km/s of delta-V!
If humanity had enough time to reconstruct an Orion, it could certainly afford to finish proving out propellant depots and tapping propellants from the Moon, NEOs, or even Phobos/Deimos if necessary. Which would be far less environmentally damaging than Orion or launching billions of LOX/Gasoline SSTEVs!
So, while it is still true that moving all of humanity off planet would be a very challenging enterprise, it’s not quite as bad as XKCD makes it sound.
[As a friend pointed out, I didn't exactly disprove XKCD, because I didn't use math. Ok, I'm lazy. Would people be interested in me doing the math to show my work?]