I'm totally adding Environmental Problems of the Greeks and Romans to my reading stack.

Another interesting aspect of the need to forever move west to continue cotton cultivation is that there's a trend of increasing automation from East to West. Essentially, better cotton harvesting methods had a high initial capital expenditure, and it was much more economically feasible for new plantations to adopt them. Older, established farms had too much sunk costs in old technologies. At least, that's an interpretation I read in The Travels of a T-shirt in the Global Economy.