I'm trying to put together a plot of the number of human slaves, worldwide, as far back as I can go. The number 30M for 2010- is easy to find, and I can find plenty of numbers for the US pre-civil war, but is there a reference that might have cumulative global numbers dating back any significant length of time?
Worst case, I'll have to look up populations for individual countries or regions at more or less random points in time and interpolate.