| World's land slipping in quality|
|Nearly 25% of land around the world is in bad shape and getting worse, according to a new study, and human activities are to blame.|
The study, which appears in the journal Soil Use and Management, is the first 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, says David Dent, a Netherlands-based environmental scientist with ISRIC-World Soil Information, a soil research and education organisation.
"Once it's gone, it's awfully hard to get it back again," says Dent. "It's bad news for water. It's bad news for food production. And it's bad news for forests. It's bad news."
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% 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," says Dent.
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. 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 divided the world's landmasses into millions of 8 kilometre 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 reveal that 24% 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% of the degraded regions were on crop land, and about 40% 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, says Dent, 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. 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," says Dr Hari Eswaran, a soil scientist with the US Department of Agriculture's Natural Resources Conservation Service.
"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, says Eswaran.
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 tonnes of carbon out of the atmosphere if it were still healthy and green.