While reading this book I was continually puzzled as to why the author did not use the concepts developed by "Project Orion". This was a government study involving Freeman Dyson. It proposed to use atomic bombs to propel gigantic ships into space. Imagine a ship 40m in diameter powered by .14kt bombs and capable of lifting 1600 tons. Full description here:
Insane? Yes under normal circumstances, but we are talking about the end of the world right? Who cares if the atmosphere was polluted with fallout. Couldn't a crash program build at least dozens of these ships and save many thousands instead of 1500? 1600 tons of lift means they could put structures into orbit that would dwarf ISS. What is especially puzzling is the fact that Stephenson mentions ships of this kind in his book Anathem!
Maybe 2 years wasn't enough time? No, the original research was done in the '60s and they were ready to start prototyping when it was canceled.
Great book though.