Nomads are communities without fixed habitation who regularly move to and from areas. Such groups include hunter-gatherers, pastoral nomads, tinkers and trader nomads. In the twentieth century, the population of nomadic pastoral tribes slowly decreased, reaching an estimated 30–40 million nomads in the world as of 1995.

A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the land surface of the Earth is arid or semi-arid. This includes much of the polar regions, where little precipitation occurs, and which are sometimes called polar deserts or "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location.