Deserts are created when ground cover and ground water diminish for any reason. A desert area my grow in the direction of loss. In the 20th century the United States has experienced about a 10% growth in the deserts.

This has been due to over grazing of livestock, over pumping of the water table, and other unsound use of water practices. Increased population and the need for more agricultural products has also added to the strain.

Countermeasures: we need to eliminate the incentives to exploit arid land. We need to limit human activity in those areas not specifically designed to handle it.