Spoiler alert: energy.
Ok, ok, a little more than that. Smil makes a compelling case that, at the end of the day, civilization’s growth is bound by the amount of energy they can consume. This goes beyond forms of energy like electricty or manual work; some of Smil’s most interesting explorations go into how we put energy into the food we consume. Nitrogen fertilizer, for example, is maybe the most consequential invention of all time, simply because it increase the energy produced by agriculture by orders of magnitude.
Smil’s discussions of globalization are interesting, too — that globalization was a choice, and was not inevitable. He suggests that we may soon see a reverse in the trend, away from globalization, and I’m inclined to believe him.