One summary/review article among many: https://www.annualreviews.org/doi/10.1146/annurev-earth-060115-012512 https://www.annualreviews.org/doi/10.1146/annurev-

One summary/review article among many: https://www.annualreviews.org/doi/10.1146/annurev-earth-060115-012512 https://www.annualreviews.org/doi/10.1146/annurev-earth-0601... Basically political and economic dysfunction combined with drought and soil degradation over a couple centuries.