April 20, 2009 -- 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."
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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.
Their findings, published in the journal Soil Use and Management, revealed that 24 percent of land worldwide is degraded because of things people are doing. Worst off were African countries south of the equator, Southeast Asia and south China.
About 20 percent of the degraded regions were on crop land, and about 40 percent were in forests. Surprisingly, whether a forest was designated as protected or not made no difference. The scientists were also surprised to find that the Amazon didn't make much of a dent on the map.
Especially concerning, Dent said, was that the areas highlighted in the new study had almost no overlap with the areas highlighted in the 1991 study. That suggests that degradation is cumulative, and that it's getting progressively worse.
One and a half billion people currently live in degraded areas. And as soils decline, people reach a point where they can't grow enough food to feed themselves. They move on, leaving the dead land behind.
"Once it has happened, it becomes very expensive to correct that situation," said Hari Eswaran, a soil scientist with the United States Department of Agriculture's Natural Resources Conservation Service in Washington, D.C. "But we can prevent it by helping these people to practice more sustainable forms of agriculture."
Choosing appropriate crops, for example, maintaining them correctly, and reducing erosion are all strategies that can help people survive, Eswaran said.
Those strategies can also help the environment. The scientists calculated that all of the vegetation that has been lost from the world's degraded land would have removed an extra billion tons of carbon out of the atmosphere if it were still healthy and green.
Related Links:
Report: Global Assessment of Human-Induced Soil Degradation (GLASOD)
Journal: Soil Use and Management
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