Presumably it would have happened more slowly, as the world population was less then. Though, maybe that wouldn't have actually made that much difference, as the rapid population growth of the last few hundred years is probably largely the result of technological progress (specifically, increases in farm output and modern medical technology). If there had been a population boom during the Roman empire, they might have been able to consume the world's fossil fuel resources pretty quickly, and with predictable results.

I would read that book.