World's Land Slipping in Quality

THE GIST:

• The study measured the extent of human-induced global land degradation.
• Land degradation can have severe economic and environmental consequences.
• The only previous attempt to assess the scope of global land degradation was in 1991.

Nearly 25 percent of land around the world is in bad shape and getting worse, according to a new study, and human activities are to blame.

It's the first study to directly measure the extent of human-induced global land degradation. The phenomenon describes a decline in the quality of soil and vegetation that the land can't recover from on its own.

Land degradation can have severe economic and environmental consequences, said David Dent, a Netherlands-based environmental scientist with ISRIC-World Soil Information, a soil research and education organization.

"Once it's gone, it's awfully hard to get it back again," Dent said. "It's bad news for water. It's bad news for food production. And it's bad news for forests. It's bad news."

"We wanted to see how that impacted their performance," she said.

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The only previous attempt to assess the scope of global land degradation was in 1991, when researchers from ISRIC compiled the knowledge of experts from around the world to produce a somewhat subjective map of where land was in decline. That study, called the Global Assessment of Human-Induced Soil Degradation (GLASOD), suggested that 15 percent of the planet's land was degraded. But Dent wanted a more accurate -- and more recent -- count.

"The United Nations Convention to Combat Desertification has used this as its raison d'etre to tell the world what a terrible state the world was in," Dent told Discovery News. He thought, "We can do better."

In search of cold, hard numbers, Dent and colleagues tapped into a trove of NASA satellite data. Since 1981, a succession of satellites has taken regular measurements of solar radiation that is reflected into space from the Earth's surface.

Chlorophyll, the green pigment in plants, absorbs near infrared wavelengths but it reflects red ones. So, by looking in the satellite data at the ratio of red to infrared wavelengths emitted in a given region, scientists can calculate how much vegetation is there.

The team broke the world's landmasses into millions of 8-kilometer (4.9-mile) squares. For each square, they plotted what the value of the red-to-infrared ratio was every two weeks over the last 25 years. To make sure they were looking only at human-caused land degradation, the researchers made adjustments for other factors that might affect the numbers, such as global warming, volcanic eruptions and droughts.