It is calculated that, during the past 50 years, the Sahara has engulfed some 650,000 square kilometres of arable land, vast areas of India have been overrun by the Thar Desert, and the Atacama has swallowed thousands of square kilometres of northern Chile. Similar disasters have befallen large parts of the Middle East, Argentina, southern Africa, and parts of Siberia and China.

Of course, climatic vagaries or alterations may have been responsible for some of the adverse changes that have occurred — there is some dispute on this point. But it has been calculated that between 60 and 200 million tonnes of soil are being lost annually from the Sahara and blown into the Atlantic Ocean.

Does drought feed upon drought by setting up positive feedback loops? Does Man accelerate such feedbacks when he overstocks or overcultivates the desert margin? We must conclude that the undeniabletendency for deserts to expand is largely due to the unwise response of human societies to the strains imposed by naturally occurring drought; and there is no convincing evidence that a lasting decrease in rainfall is in progress around the desert areas, although such desiccations have often occurred in the past.