Environmental problems such as siltation, soil depletion and deforestation, saltation, and deforestation, while less dramatic than the impact of diseases, were subtle factors in the collapse of many societies and empires.
As settled agriculture spread throughout Africa and Eurasia, extensive irrigation systems and slash-and-burn agriculture took a toll on the land. By 600 C.E. people had been farming the same lands for thousands of years. Crops such as wheat and barley depleted the soil of nutrients and, in some areas, led to desertification.
During this era, wood constituted a vital natural resource. It was used as the primary fuel source for fire and as the main building material. Over time, forests around urban and agricultural areas became denuded of large trees. The loss of ground cover resulted in loss of topsoil, mudslides in hilly areas, and challenges to local economies as the need for wood forced urban areas to import timber from further away. Although these challenges were often noted in ancient accounts, there was little that ancient and classical societies could do to avoid them.
Agricultural societies, including pastoralists, introduced new diseases into human civilizations. Agriculture upsets the soil and allows for more standing water, spreading the breeding ground for the Anopheles mosquito, which carries malaria. Close contact with domesticated animals such as cattle and pigs allowed for the spread of viruses such as measles and smallpox that plagued mankind until the modern era. Other diseases also became more common. For example, the bubonic plague bacteria was transmitted via rats, a pest common worldwide.