It was local climate change and poor resource management -- two problems which fed on each other. Desertification isn't new to the 20th century... Iran was ori

It was local climate change and poor resource management -- two problems which fed on each other. Desertification isn't new to the 20th century... Iran was originally wooded, but the forests were marginal -- not very fertile, easy to lose, mostly valuable for keeping topsoil in place. These were cut at rates faster than they could regenerate, and the country desertified. The ancient Persians were superb engineers (their air-conditioning and water-transport methods, in particular, get a lot of attention in sustainability circles today), but poor foresters. Of course, regenerative forestry was only invented in the 19th century. Persia isn't the only part of the world where marginal forests were over-harvested and lost; the forests that survived were those in very tree-friendly regions, or in uninhabited regions, or both. In Persia, the temperate-rainforest zone north of the Elburz Mountains (on the south shore of the Caspian Sea) was forested much longer, although these days it's more urbanized than anything else. The problem in Iraq -- Mesopotania -- was a different one: the Babylonians put in irrigation canals, but, over time, irrigation canals render the soil salty and toxic to plants. This takes thousands of years; but by the time the Mongols arrived, parts of Iraq were already a salt marsh, and after Hugalu Khan destroyed the irrigation systems, no one could get much value from rebuilding them. The climate is still mild and pleasant; the soil, much less so.