Answer: Desertification
Desertification refers to land degradation which results in increasingly arid land, and the loss of bodies of water, vegetation and wildlife. It is caused by various factors, such as climate change and overexploitation of soil through human activity.
The Sumerian and the Babylonian empires are among many different ancient civilizations in the world that are thought to have declined due to desertification. It is likely that due to unwise land-use and water scarcity, their agricultural output fell, leading to their decline.