The dust moves on to invade Korea and Japan and, propelled across the Pacific Ocean by the jet stream air currents, eventually the western United States. Similarly, dust from the drylands of Africa travels across the Atlantic toward the multifarious islands of the Caribbean, where it is suspected of causing everything from an increase in hurricanes to die-offs of coral reefs.

These global dust storms are the result of desertification, which now affects 70 percent of all drylands on earth. Increasingly, overcultivation, overgrazing, and deforestation, aggravated by drought, are destroying vegetation and leaving precious topsoil exposed to wind and water erosion. The Worldwatch Institute estimates that, in just the past two decades, the planet has lost enough topsoil to cover the entire cropland of the United States. Trees, too, are disappearing at an alarming rate. According to the World Bank, almost half of the earth's original forest land has been destroyed, most of it during the past 30 years.

As bleak as it sounds, the situation is far from hopeless. In a number of African countries hard hit by desertification, farmers are controlling erosion, managing natural regeneration of trees and a rich diversity of other plants, and taking pride in rehabilitating degraded land. In short, they are creating or restoring natural capital through more intensified management.

Farmers are increasingly treating farming as a business that goes beyond subsistence. As such, they invest in measures that give the highest returns and reduce risks. While high-tech advances, such as plants genetically engineered to resist adverse conditions, play a role in combating desertification, farmers are learning to combine these advances with traditional systems of land management, where local control and an appreciation for a region's natural diversity can help guarantee sustainable farming practices. For example, farmers in the Upper Niger River Valley Zone preferentially use fertilizer and improved seed varieties on fields where they have built up soil organic matter and applied soil conservation measures. They know that the efficiencies of both the fertilizer and improved varieties will be optimized on such fields.

Battling Erosion

The first line of defense against desertification is preventing water and wind erosion. In hilly terrain, many farmers today rely on some variation of the ancient practice of terracing. In the African country of Burkina Faso, for example, farmers place lines of rock and debris on the contour, making sure the barriers are porous enough to let water seep through slowly. In areas such as Thailand and the Philippines, farmers are planting hedges of vetiver, a tall, deep-rooted perennial grass, to form easy-to-maintain living walls along slopes.

In regions so arid or so desertified that blowing sand is a constant problem, individuals and governments use sand fences often made of crop residue such as millet and sorghum stalks bound together and large boulders to keep the sand in place. Straw grids and drought-tolerant shrubs and trees also help decrease the action of wind on loose soil. In Niger's Majjia Valley, for example, windbreaks made out of tall trees reduce the drying effects of the wind to increase soil moisture for crops. By protecting the young plants and not having to replant, farmers extend the benefits of their rainy season by a week or more -- a period that sometimes separates a good season from a disastrous one.

Strip cropping, in which strips of grass or small grains are planted between other crops, can greatly reduce water erosion on cultivated fields. Another method gaining in popularity in South America and Africa is "no-till" farming, a variation of the planting-stick technique used by Native Americans and other early peoples to insert seeds into the ground. In no-till, residue from a previous crop is left on the ground, and the farmer makes a hole to plant the new seed without plowing up the ground. When the ground is undisturbed, wind and rain do not dislodge soil particles as easily. Organic matter in the soil receives less oxygen and decomposes more slowly, which increases fertility and helps the soil hold moisture.

Trees, of course, are an excellent barrier against wind erosion, whether planted along the borders of a single field or on a grand scale as in China's "Green Wall" project. Argentina and Mozambique are employing jobless young people to plant forests that will reclaim degraded land and generate income from forest products. The government of Afghanistan with the assistance of the U.S. government has launched the Afghan Conservation Corps with the same purpose: to provide work through tree-planting and soil and water conservation projects throughout Afghanistan.

Making the Most of Local Resources

In developing countries, there is increasing emphasis on planting or conserving a variety of species that offer multiple benefits. Getting renewed attention in Africa is a traditional land-use system known as agroforestry, in which trees are either retained or planted in fields and managed as resources along with crops and animal herds. In the Sahel region, devastated by severe drought and famine in the early 1970s, forests have been transformed into parklands, where carefully selected trees grow amid crops and pastures. Besides protecting the sandy soil from winds during the dry season and torrential downpours during the rainy season, the trees boost soil fertility and provide timber, firewood, fodder, fruits, and gums.

A relatively recent variation on agroforestry is a technique called "farmer-managed or farmer-assisted natural regeneration," says Mike McGahuey, a natural resource management advisor with the U.S. Agency for International Development (USAID). In this system, farmers allow several species of what are usually considered useless trees to grow to shrub size in their fields. Besides helping to control wind and water erosion, the trees contribute to soil fertility through their leaf litter. Some agroforestry systems have proved so effective that it is not necessary to leave fields fallow (unplanted) periodically in order for soil productivity to be maintained, McGahuey adds.

There are several tree species that have proved especially valuable for agroforestry in Africa. One is the Acacia albida, a leguminous tree that loses its nitrogen-rich leaves during the rainy season. Grain planted beneath these trees often gets a head- start, benefiting from the slight shading, moisture, and extra nutrients. The trees also provide high-quality browse and fruit for livestock.

Another stellar species is the shea butter tree, a popular field crop that grows throughout the semi-dry zone that stretches from West Africa to the Sudan. Women especially gather the seeds and render them into oil that is used for cooking and cosmetics and is starting to be used for chocolate production in Europe.

In the West African country of Mali, a versatile plant known as jatropha has been the centerpiece of a German government-sponsored project designed to provide renewable energy, erosion control, and income for villagers, especially women. Jatropha, a long-lived, drought-resistant perennial that is unpalatable to animals, makes an ideal living fence. It keeps animals out of fields, and its surface roots anchor the soil, reducing both wind and water erosion. Jatropha leaves, seeds, and latex traditionally have been used for various medical purposes. The seeds also yield oil that has proved to be an inexpensive fuel and lubricant for engines used to drive grain mills and water pumps.

Jatropha oil products can be used alone or in combination with other plant oils such as shea butter to produce a high-quality soap that village women can easily sell in local markets or nearby towns. The press cake that remains after oil extraction serves as an excellent organic fertilizer in an area where soil is rapidly depleted of humus and chemical fertilizers are very expensive.

The Importance of Land Tenure

Agroforestry and other conservation measures work only if people have incentives to invest in the land. "Most land stewardship investments pay off only over a period of time, perhaps seven to ten years," says Jon Anderson, a natural resource policy advisor with USAID. "So, you need some security of tenure to know that you will profit from that investment in seven or ten years. If you don't know if you'll be able to farm the same piece of land year after year, you don't make the investments in soil and water conservation. The land becomes more degraded, and you end up with the desertification."

Africa provides a textbook example of the importance of land tenure. Prior to European colonization, land was controlled by elders within a village or group of villages who allocated land and established rules for use of resources, Anderson explains. People had no individual property rights, but they had security of access to a piece of land. During the colonial period, the colonial powers decreed that land belonged to the state, partly because they wanted to extract African resources for their homelands and partly because they feared conflict among various ethnic groups. The problem was that colonial rulers were unable to enforce the rules in many areas, so land was basically controlled by whoever got there first. When many of the African countries gained independence in the early 1960s, they either tried to retain the colonial land system or adopted a socialistic approach of collective land management. Either way, there were no local land rights.

In Africa, the good news is that many countries are moving toward decentralization, "going back to a system of local responsibility and rights over resources," Jon Anderson says. Unlike the traditional system, in which women had no role, the current effort aims to be participatory and equitable, making sure that women are included in decision-making. This is a point stressed in the Convention to Combat Desertification, which repeatedly emphasizes the full participation of people most affected by desertification and specifically mentions "the important role played by women."

The new decentralization is evident in places such as Mali, where communes encompassing five or six villages manage their own resources and decide, for example, whether to set aside an area as protected forest, whether to protect a certain tree species, or when to take some land out of agriculture. The process of making and enforcing their own rules has been particularly effective for rural communities when accompanied by cooperative-training programs. In these programs, often supported by USAID and other donors, communities learn and apply democratic principles to forest management. Because these local decisions have real consequences on the livelihoods of the community, the democratic lessons are usually well learned.

The return to agroforestry is made possible by a change in national control over forests, which prior to the 1980s allowed people in many African countries to get permits to go onto any land, regardless of who farmed it, and cut all the trees. "There was no incentive for anybody to plant a tree or not to take a tree when no one else was looking," says Mike McGahuey. But when Niger experimented with giving a local community some control over a forest, it found that trees regenerated within five years to the point where some harvesting could resume. Species that had not been seen in a generation sprang up naturally. "There is a lesson here that much of the developing world could learn from Niger," McGahuey says. "You don't put up a one-time technical fix. You create enabling conditions so that people who are the source of the problem find it in their interest to do something to solve it. It is almost always linked to finding ways to link better livelihoods with better management."

Loss of Traditional Knowledge

Farmers have been struggling with threats such as erosion and drought for as long as humans have tilled the earth. The ancient Chinese, the Mayans of Central America, and the Incas of the Andes independently devised methods of terracing -- planting crops on a staircase of flat steps to slow the flow of water down a hillside and prevent fields from eroding. Traditionally, farmers in dryland areas learned to meet their needs by maintaining a diversity of crop varieties, including ones that perform well under varying levels of rainfall, ones that mature at different times, and ones suited for various food and other uses. Farmers in the Andes, for example, cultivated as many as 3,000 different varieties of potatoes. Such diversity provided a minimum level of productivity, even under the most unfavorable conditions.

In many cases, social and political changes have compelled people to abandon traditional wisdom. Poverty and population pressures force many in Africa, for example, to eke as much as possible from exhausted soil and to cut forests for fuelwood or for additional farming space. Such misuse of the land has led to loss of the diversity that sustained people in the past and has left previously productive land desertified and useless. In many countries, farmers have lost touch with natural methods of planting, fertilizing, and pest control, turning instead to ever more efficient chemicals that unfortunately rob the soil of fertility-enhancing qualities in the long term, and sometimes to careless use of machines that leave soil exposed to wind and water.

The good news is that in all corners of the globe efforts are under way to reclaim desertified areas, prevent further degradation, and preserve traditional knowledge about how to care for the land. Many of these efforts are encouraged by the U.N. Convention to Combat Desertification. Among other things the Convention calls on nations to protect "traditional and local knowledge, know-how, and practices."

A Marriage of Science and Tradition

The answer to preventing desertification lies not only in traditional kinds of land management. There is clearly a role for the beneficial side of modern science, which can vastly improve agricultural efficiency by developing crops to resist drought, pests, or even salts left by irrigation water. Israel, for example, is developing saline-resistant varieties of tomatoes, melons, and grapes that also happen to be sweeter and firmer, making them ideal for export. They are also helping West African countries to use salt-tolerant forage species to reclaim lands lost to erosion and reduced volumes of fresh-water rivers. To preserve biodiversity and save endangered plant species, the Consultative Group on International Agricultural Research (CGIAR), which is sponsored by the World Bank and several U.N. agencies, maintains collections of germplasm -- the genetic resources -- of more than 3,000 crop, forage, and agroforestry species. The collection includes farmers' varieties and improved varieties, as well as the wild species from which those varieties were created.

Although scientists can assist and sometimes improve upon nature, in the end people must work with nature in order to prevent environmental catastrophes. "Desertification is by and large an unnatural process," says Jeri Berc, a soil scientist with the U.S. Department of Agriculture's Natural Resources Conservation Service. "So we should be going back and mimicking nature and getting our production within the boundaries of ecologically resilient systems." It is important, she adds, to "appreciate the sophistication of ecological agriculture" -- the traditional knowledge about the many different kinds of species that can be planted in the same terrain in rotation depending on different conditions.

But knowledge alone -- whether traditional or high-tech -- is not enough. To be truly successful, many common methods of land management to prevent desertification require economic incentives and the security of land tenure that enable people to live and farm sustainably on the land