The size and location of the world's deserts are always changing.
Over millions of years, as climates change and mountains rise, new dry and wet areas develop. But within the last 100 years, deserts have been growing at a frightening speed. This is partly because of natural changes, but the greatest desert makers are humans.
In developing countries, 90 percent of the people use wood for cooking and heating. They cut down trees for firewood. But trees are important. They cool the land under them and keep the sun off the smaller plants. When leaves fall from a tree, they make the land richer. When the trees are gone, the smaller plants die, and there is nothing but sand. Yet people must use firewood, raise animals, and grow crops in order to live.
Humans can make deserts, but humans can also prevent their growth.
Algeria planted a green wall of trees across the edge of the Sahara to stop the desert sand from spreading. Mauritania planted a similar wall around Nouakchott, the capital. Iran puts a thin covering of petroleum on sandy areas and plants trees. The oil keeps the water and small in the land, and men on motorcycles keep the sheep and goats away. Other countries build long canals to bring water to desert areas.
Yet land that will probably become desert in the future equals the size of Australia, the United States, and the former Soviet Union together. Can people stop the growth of the world's deserts and save the land that is so essential to life?