The Convention to Combat Desertification (CCD), a United Nations treaty that went into effect in 1996, states that nations who are parties to the convention should coordinate collection and analysis of scientific data on drought and desertification to make possible "early warning and advance planning for periods of adverse climatic variation�." Because the origins of desertification are complex, the Convention also acknowledges that early warning systems must rely on data from a variety of sources -- from high-tech instruments aboard satellites orbiting the earth to the traditional knowledge of local people who over thousands of years have learned to interpret and react to changes in the world around them.

Watching the Wildlife

"Based on my own experience in developing countries, there is a lot of expertise in traditional knowledge," says Richard Ohlemacher, policy advisor at the U.S. National Oceanic and Atmospheric Administration (NOAA). "These people know the environment they live in, they know the wildlife, and they read the interactions between the landscape and the life around them. They watch behavior of the birds, where bees build their nests, and it tells them things. There are long oral traditions that tell the history over many generations."

A farmer in China's Xinjiang Uygur Autonomous Region pedals through a heavy sandstorm. (AP)

A dried-out riverbed in Morocco shows the ominous results of a three-year drought. (AP Photo/Jalil Bounhar)

Recognizing the value of such wisdom, the Convention urges signatories to "protect, integrate, enhance and validate traditional and local knowledge," as well as ensure that local populations benefit from any commercial use of their knowledge.

Thus, the Convention values traditional knowledge in the form of "grassroots indicators." Many indigenous peoples rely on observations of plant and animal life to predict the timing and amount of rainfall and the fertility of the soil. The Digo people of the Coast province of Kenya, for example, watch the migration of monkeys and butterflies to determine whether it will rain. If red ants appear and frogs are noisy, rain, they believe, is imminent, so farmers hasten their planting. If certain birds make a different cry or the local mango tree produces a lot of fruit, rains will be poor, and farmers may try to plant as much as possible while looking for alternate sources of income. The Bantu-Kambas in Kenya's eastern province believe that many shrubs in forests of few trees suggest fertile soil and land that should be cleared for cultivation, while the presence of many trees with "top roots" means that the trees have "eaten" the soil. Black soil has more "oil" and therefore is more fertile, they say, but soil that is sandy or reddish in color is poor and should not be planted until rain begins to fall.

The reliability of some grassroots indicators has been borne out by scientific study. The types and amounts of vegetation that grow naturally, for example, are now widely recognized as barometers of land status and a kind of living history of land use. Natural grazers such as antelope, giraffe, and other large animals may keep an ecosystem in balance by each eating different plants. But cattle graze exclusively on grass, and once the grass is gone, opportunistic shrubs and unpalatable bushes take over, often spreading rapidly and sucking nutrients out of the soil. In a number of areas, such as in Namibia in southwestern Africa, this increase in woody vegetation, known as "bush encroachment," is considered to be part of the process of desertification.

The Role of Science

As useful as they may be, grassroots indicators are limited to localized areas. Scientific monitoring makes it possible to see the big picture and chart trends regionally, nationally, and even globally. To track changes that could lead to desertification, scientists rely on a combination of ground-based instruments and remote sensing, primarily from orbiting satellites.

Weather observation satellites date from the beginning of the "space race" in the early 1960s. In 1972, the U.S. National Aeronautics and Space Administration (NASA) launched the first of the so-called environmental satellites capable of measuring and forecasting a number of variables about the earth's environment. These have become so sophisticated that current versions can measure almost everything about the earth's ecosystem -- winds, waves, vegetation, soil moisture, atmospheric chemistry, land and ocean temperatures, and even the thickness of the polar ice sheets.

As a result of such technology, scientists today study daily transmissions of data from a variety of instruments aboard government and commercial satellites. Thus, they can pinpoint areas at risk of desertification and predict weather phenomena such as the famous El Niño ocean current, which can cause droughts in various parts of the world. One such instrument, the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), for example, can measure how fast water leaves the earth's surface both through direct evaporation from bodies of water and through transpiration from plants. Perhaps the most valuable tool for calculating risk of desertification is the Advanced Very High Resolution Radiometer (AVHRR), which measures the infrared and near-infrared radiation reflected by plants. By doing so, it produces a sort of "greenness index" that enables scientists to map the amount and vigor of vegetation around the globe. These sensitive instruments can produce sophisticated and reliable data. One pioneer in this technology, Dr. Compton Tucker, a senior earth scientist at NASA's Goddard Space Flight Center in Maryland, has been monitoring vegetation boundaries in sub-Saharan Africa since 1980.

As a result of his work, he has been able to determine that the Sahara desert in Africa is not extending into fertile lands to the south of its boundaries, as had long been feared. Although the southern border of the Sahara advances and retreats from year to year depending on amount of rainfall, the desert's overall size has not grown.

Tucker also is using high-resolution satellite data to count individual trees and bushes at two sites in Senegal and one site in Mauritania in the transition zone between the Sahara and the Sahel region to the south. A lower density of trees and bushes could signify desertification, either because the area has become more arid or because people have cut them for firewood, exposing the land to wind and water erosion. Satellite data also could determine if vegetation palatable to livestock has been replaced by unpalatable woody plants -- a sign that overgrazing is destroying the productivity of the land.

But even satellite imaging has its limits. Most scientists agree that it will not completely replace ground-based observation. Tucker's satellite counts of African trees and shrubs have been supplemented by the work of another scientist -- forest ecologist Dr. Patrick Gonzalez of the Nature Conservancy who on foot surveyed forest species over several hundred kilometers in Senegal and consulted old maps and village elders to determine how various types of vegetation had shifted over time. Gonzalez found that a decline in species richness suggestive of desertification has significantly limited options for local people. Useful firewood is limited to two botanical species, and 25 species of plants used for traditional medicines have diminished significantly. Eight species that provided fruit and leaves in past droughts have disappeared from more than half of their range, decreasing chances that people could find the emergency foods that have sustained them during past famines.

Such field surveys can corroborate information from satellites or reveal gaps in the data, Tucker says. From field surveys, scientists also learn how big a bush has to be in order to count it from a satellite or whether a satellite might do something like count two big trees as one. "If you don't do field work, you can easily draw wrong conclusions and extrapolate them over huge areas," Tucker emphasizes.

Another important tool, known as Geographic Information Systems (GIS), allows scientists to work with data from both ground-based and satellite sources to get a picture of how forces work together to influence desertification. Using sophisticated computer software, experts can overlay a basic terrain map, for instance, with maps showing changes in rainfall, vegetation, population density, or agricultural patterns over a period of many years. "Data from a number of sources is processed and turned into pictures, graphs, charts that give us information to understand land degradation, desertification, climate, natural disasters -- anything about the earth," says Ohlemacher.

Geographers with the U.S. Department of Agriculture's Natural Resources Conservation Service have used GIS in conjunction with soil maps and data on soil temperature and moisture from 25,000 ground-based monitoring stations around the globe to map "tension zones" vulnerable to desertification and the social conflicts it might help trigger. They concluded that more than 75 percent of the world's population lives in regions that do not have a high capacity for grain and feed production. The most high-risk tension zones, which include the countries of West Africa, the northeast corner of Brazil, and large areas of Central and southern Asia, cover nearly 12 million square kilometers and are home to more than 1.4 billion people.

Making Use of the Data

Scientific information obtained in recent years has led to intriguing findings about how desertification affects the environment around the globe. Research indicates that dust generated by land degradation may actually accelerate the desertification process. Israeli scientists studying satellite images of dust clouds off the Atlantic coast of Africa and over the eastern Mediterranean found that dust and other particles in the air, such as from burning of forests, cause water droplets in clouds to be smaller, leading to decreased rainfall and worse droughts. In short, they concluded, dust begets dust.

Other research suggests that desertification in Africa may contribute to algae infestations, coral diseases, and die-offs of sea urchins in the Caribbean. It is well documented by satellite imaging that dust from Africa crosses the Atlantic Ocean. Components of soil on islands throughout the Caribbean, as well as in the Amazon rain forest, are known to originate in Africa. Scientists suspect some of that dust carries nutrients that fertilize pernicious algal growth in the Caribbean and bacterial, viral, and fungal diseases that attack marine life.

The challenge is to put all of this research data to practical use. An ad-hoc panel on early warning systems created under the Convention to Combat Desertification has concluded that early warning information on drought and desertification is not yet being used adequately in long-term national planning. The ability to predict droughts and weather phenomena such as El Niño up to six months in advance is, however, making it possible for nations and regions to prepare for acute droughts and food shortages that could result. The Famine Early Warning Systems Network (FEWS NET), funded by the U.S. Agency for International Development, collects satellite and ground-based data on weather, crop, and rangeland conditions to monitor the impact of desertification and variations in climate. FEWS NET in turn uses this information to warn 17 African nations and several regional centers in Africa of potential threats to their food security and works to strengthen their ability to deal with food crises.

A broader challenge is for the international community to coordinate their long-term efforts to monitor the earth's environment. To that end, ministerial-level representatives of more than 30 nations met in 2003 for the first Earth Observation Summit, hosted by the U.S. Department of State. The participants agreed to a 10-year plan to develop a comprehensive, coordinated earth observation system that will further improve global decision-making on climate and environmental issues.