Periodically, American satellites take pictures of a strange haze over the eastern Atlantic. The haze is not water vapor but dirt - fine topsoil blown off the arid farm lands of North Africa.
At the Mauna Loa Observatory, in Hawaii, scientists can tell when spring planting starts in northern China. That's when the soil dust in their air samples increases sharply.
All over the globe, precious topsoil is blowing and washing away. Soil erosion now threatens the productivity of almost half the world's farmland, a just-released report by the Worldwatch Institute concludes.
''As demand for food climbs, the world is beginning to mine its soils, converting a renewable resource into a nonrenewable one,'' says Worldwatch president Lester Brown.
Environmentalists have long worried about the rate of topsoil erosion in the United States. The Worldwatch study, however, is one of the first attempts to measure the extent of the problem worldwide.
The report estimates that roughly 25 billion tons of the globe's topsoil is, in effect, disappearing each year. At that rate, the world will have 7 percent less topsoil in 1994 than it does today. Since the soil erodes faster in some areas than others, Worldwatch says this problem could seriously affect about half the world's farmland.
In the US, soil erosion exceeds tolerable levels on 44 percent of the land used to grow crops, the report notes. In India, the figure is 60 percent. In the Soviet Union, half a million hectares (1.2 million acres) of wind-eroded farmland are abandoned each year. One billion tons of topsoil flow out of Ethiopia's highlands annually, Worldwatch says.
The press to increase food output may be the basic cause of the problem. World farmers are struggling to feed expanding populations, says the report, and some are resorting to practices that damage their fields.
Throughout the third world, farmers are expanding onto steep land that erodes quickly. In the Andes, for instance, some new cropland is in areas so mountainous that ''fear of ... landslides has become an integral part of daily life,'' the report says.
Crop rotation, which provides soil cover and binds soil particles together, is in some areas being abandoned. Many US farmers in the lower Mississippi Valley who used to switch crops now grow corn or soybeans continuously.
In arid regions, less farmland is being allowed to lie fallow. When not allowed to rest for a season, such soil tends to dry up and blow away. The Soviets, in the '70s, reduced their fallowed land by one-third, for instance.
Although some soils in Kenya are 76 inches deep, most of the world is covered by only a thin 6 to 7 inches of topsoil. If fields begin to look like worn rugs, with the underlying layer of subsoil showing through in spots, they can't grow as many plants as they used to. The loss of 4 inches of topsoil in one West African area cut corn yields by 52 percent.
But it is possible to slow down this accelerating problem. ''One third-world country that has formulated an effective response is Kenya,'' the study notes.
Working with Swedish environmental experts, Kenya began a soil conservation program a decade ago. Today the program is in full swing: Some 3.5 million trees have been given to farmers for use in slowing wind erosion. Thousands of erosion-cutting drains and terraces have been constructed.
Today conservation is being improved on 30,000 to 35,000 farms a year, ''a level that could stabilize Kenya's soils in 25 years,'' Worldwatch says.