We can divide the environmental history of the medieval Islamic Empire into three different phases. The early phase, from the 600s to about 800 AD, is the time of the Umayyad and the Abbasid Caliphs. The climate was probably pretty similar to what it had been for a thousand years before that. You can get a good idea of what that was like by checking out the earlier environment of West Asia and Ma’rib dam had broken in the late 500s AD.
But by the Middle Ages, people in West Asia were pretty good at changing their environment to make it more useful to them. They dug canals leading away from all the major rivers – the Nile, the Tigris, the Euphrates – to irrigate their fields. Even in Iran, where there aren’t any really big rivers, people dug many irrigation canals like the Haffar, reservoirs, aqueducts, and cisterns. They made the best use of their water. Under the Islamic caliphs, with good irrigation systems, West Asia grew plenty of food. People thought of the Islamic Empire as one big garden.
But the second phase brought the Medieval Warm Period, between about 800-1200 AD. That probably affected West Asia just as it affected people in Europe, in Central America, and in North America. Nobody knows much about it yet. But probably West Asia and North Africa became even warmer and drier than they had been before. Possibly the climate change encouraged West Asian farmers to grow Indian crops like cotton, sugar, rice, and oranges that they had not grown before.
The third phase of the environmental history of the Islamic Empire is a colder period known as the Little Ice Age. The Little Ice Age started about 1200-1300 AD. It may have encouraged the Mongols to leave the cold of Siberia to invade warmer West Asia and China. In any case, when they invaded, the Mongols totally destroyed the complicated irrigation systems of Afghanistan, Iran and Iraq. That left Central Asia and West Asia much drier and much poorer than before.