About one billion people live in arid or semiarid desert lands that occupy about one third of the world's land surface. In these drier parts of the world, deserts are increasing rapidly from a combination of natural processes and human activities, a process known as desertification or land degradation. An annual rainfall of less than 10 in (25 cm) will produce a desert anywhere in the world. In the semiarid areas along the desert margins, where the annual rainfall is around 16 in (40 cm), the ecosystem is inherently fragile with seasonal rains supporting the temporary growth of plants. Recent changes in the climate of these regions have meant that the rains are now unreliable and the lands that were once semiarid are now becoming desert. The process of desertification is precipitated by prolonged droughts, causing the top layers of the soil to dry out and blow away. The eroded soils become...