Drought and desertification in the Great Plains states in the 1920s forced many farmers to move from the Dust Bowl to more fertile ground.

Alfred Eisenstaedt/Time & Life Pictures/­Getty Images

In the 1920s, the United States en­tered an economic recession. Farmers in Western states tried to raise profits by plowing and planting more acreage with new mechanized farming methods.

Within a decade, a massive drought hit the entire country. Strong winds swept across the Great Plains, stirring up loose topsoil that had been displaced by overplowing and overgrazing of cattle. The results were dozens of epic dust storms that swallowed whole cities in blinding black clouds. The semiarid soil of the plains, which had fed generations with its fertile soil, was now a lifeless desert known as the Dust Bowl.

­The Dust Bowl is a perfect example of desertification, the degradation of dryland ecosystems through a combination of natural and human causes. Droughts are an unavoidable occurrence in semi-arid regions like the Western United States, large portions of sub-Saharan Africa, Central Asia and Latin America. But over the millennia, these fragile ecosystems have discovered ways to survive.

The real problem is when human beings try to take too many resources from land that can sustain very little human life. When we talk about desertification, we're not only talking about the slow spread of existing deserts, but the creation of entirely new ones. When too many people try to plant crops, graze cattle and harvest firewood in a fragile dryland ecosystem, they tip the balance of sustainability.

The result is that new deserts are growing at a rate of 20,000 square miles (51,800 square kilometers) a year [source: Steele]. Nearly half of the world's total land mass is composed of dryland ecosystems, areas defined by low annual rainfall and high temperatures. It's estimated that 10 to 20 percent of these regions are already degraded -- unsuitable for human, animal or plant life [source: GreenFacts.org].

Dryland regions are also home to billions of the world's poorest, most marginalized populations. Desertification leads to famine, mass starvation and unprecedented human migration. As people are displaced by new deserts, they are forced into even more unstable regions, where the desertification process continues.

­Desertification is one of the world's most pressing environmental issues, but it's not irreversible. Decades after the Dust Bowl, federal conservation programs were able to restore the Great Plains to fertility. Keep reading to learn more about the causes and effects of desertification as well as the best methods for bringing it to a halt.