Before the development of artificial fertilizers and like the the Haber-Bolshe process The earth had reached its limit pretty much. food to sustain the population the land wasn't good enough This is actually a problem that really going into the 19th century before the development of Artificial Fertilizer. Also the fact that the climate shifted in the mid 1200s So for anyone that may be confused about when they say oh global warming or any of the stuff. It's not accurate.