The hot sand of the Sahara Desert crept southward by 100 to 200 yards across the Sahel region of West Africa in just the 12 months to November 1983.
* Erosion is so bad that satellite pictures have picked up an estimated 400 million tons of dust a year being blown from West Africa across the Atlantic Ocean.
These two facts, the first reported by United Nations Food and Agriculture Organization (FAO) experts in Rome, and the second by noted environmentalist Lester Brown in Washington, add up to an epic challenge to the future of the 35 million people of the seven main Sahel countries.
Little by little, land in West (and in East and Central) Africa that has been grazed, farmed, and roamed for millennia is turning to dust and blowing away.
Why? Mainly because populations are growing so fast and using ancient but disruptive ways of farming, cooking, and livestock tending, argues a preliminary but detailed working paper now circulating in World Bank headquarters in Washington.
The population of the seven Sahel countries will hit 54.2 million by the year 2000, the UN estimates. Recurrent drought makes everything worse. More people not only means more animals grazing, but more trees cut down for firewood. Traditional Sahel foods - stews, sauces and grain porridges - require a lot of fuel for lengthy simmering.
Can the desert be stopped?
Yes, the World Bank paper as well as US and FAO experts say, by:
* Expanded, urgent research into drought-resistant, fast-growing trees, sorghum, and millet.
* Fencing off areas from livestock so that saplings and grasses can grow.
* Requiring tribes to pay for wood instead of collecting it free - and making alternative fuels such as kerosene more competitive.
* Encouraging migration from northern to more fertile southern regions.
* Using various methods such as rock terracing or skillful managing of trees to improve the carrying capacity of land.
* Creosoting wooden building poles against termites.
* Fighting erosion by planting mango, tamarind, baobab, and other trees around compounds, in fields, and along the edges of vegetable gardens to increase organic content. Also rotating crops.
* Using windbreaks and special seeds on sand dunes, building small dams and infiltration ditches, reducing the habit of burning off stubble to destroy pests , and using the ash as fertilizer.
Scientists have found many ways of making the best use of near-arid land. Jordanians use extensive terracing to grow olives, peaches, almonds, pomegranates, and grapes. In Morocco, crops are planted inside palm leaves laid in large squares to cut down wind erosion. In Australia work has been done on wrapping seeds in moisture-retaining packs, and on techniques to inject water to stimulate growth.
In Ibadan, Nigeria, researchers at the International Institute of Tropical Agriculture have worked hard on an old but newly-intriguing legume called leucaena. It grows quickly to a height of 12 ft. or more, providing firewood, forage, windbreaks, firebreaks, and shade.
Leucaena leaves have a high nitrogen content. When they drop they greatly enrich the soil below. One idea, called ''alley cropping,'' is planting food crops between rows of leucaena, using them for shelter and for fertilizer.
But. . . . In the Sahel, little progress has been made so far, FAO and World Bank officials in Washington concede. The World Bank has spent millions of dollars to foster irrigation projects along the Niger River, only to see them fail through poor local management.
''In Mali alone,'' says a senior bank official in Washington, ''there's potential to feed all of the Sahel. The problem isn't money. That's available. The problem is management. Projects are simply being abandoned.''