Introduction to The Sahara's Bleak Shore
Along a sandy track on the eastern frontier of Niger is the village of Bouri, a small farming community in this west African republic. The villagers belong to an ethnic group called the Kanuri. For centuries, the Kanuri have cultivated millet on the surrounding dunes and grown corn, wheat, sorghum, melons, and some vegetables on irrigated lands adjacent to Lake Chad. They have raised cattle, fished the waters of the lake, and shared their environment with a group of nomadic herders, who migrate through the area with their animals.
Stories of “big rains” and “great floods” are common in Bouri folklore. Some people can even remember when the shore of Lake Chad came up to the edge of the village. But in 1992, Lake Chad was more than 50 kilometers (30 miles) away and no longer a large shallow lake but two smaller lakes. And since 1968, there have been no big rains. In fact, rainfall in this region has been below average, causing periodic crop failures that have led to food shortages and, at times, even famine and death.
Dry Spell In the Sahel
The villagers live in a region called the Sahel (an Arabic word meaning coast or shore), which borders the great Sahara to the north. Like the waves of an ocean, the sands of the Sahara ebb and flow along the Sahel's shore. Because of an ongoing drought, parts of the Sahel are undergoing desertification (processes by which a region becomes arid or a desert). The sands of the Sahara have crept southward, pushing back previous boundaries of the Sahel and covering areas where people once grew crops or raised cattle. As a result of this, thousands of people have starved to death. Hunger in some places has become a way of life and is driving many people away from their villages. In some places, sand dunes have buried entire villages.
The current dry spell in the Sahel has scientists from all around the world studying the Sahara-Sahel border and asking important questions: Is the region's climate changing permanently, thus allowing the sands of the Sahara to relentlessly encroach into the Sahel? Or is this dry spell just part of a natural long-term fluctuation in rainfall that will end someday, causing the Sahara's sands to retreat?
The Immense and Arid Sahara
Stretching northward from the Sahel, the Sahara, the world's largest desert, reaches almost to the Mediterranean Sea. From the Atlantic Ocean on the west to the Red Sea on the east, the Sahara extends more than 5,630 kilometers (3,500 miles), covers an area of about 9 million square kilometers (3.5 million square miles), and includes parts of 10 countries. To place the Sahara in perspective, it is almost the size of the 48 midcontinental states of the United States. But, in contrast to the United States population of 253 million, the Sahara's population is only about 2 million to 3 million, less than one person per square mile. And most people live in scattered oases, where underground water comes to the surface to form springs or is close enough to the surface to be reached by digging.
Although seas of sand dunes, called ergs, are features of the Sahara landscape, they represent less than 15 per cent of the total area and form only an outer ring of the desert. More than half the total area of the Sahara, inside the ring of ergs, are stone-covered plains called regs. Plateaus and wind-eroded rocks protrude from these bleak but gently undulating tracts of stones and pebbles. Finally, at the center of the Sahara, amid the regs, are outcrops of volcanic mountains.
No One Definition of Desert
Scientists do not agree on a single definition of a desert, though most say a desert is a region that can support little plant life because of insufficient moisture and dry soil. Areas near the North and South poles, for example, are considered deserts because they are so cold that the moisture is frozen and cannot stimulate plant growth. The Sahara is classified as a desert because of its low rainfall, less than 25 centimeters (10 inches) each year. Low rainfall limits plant growth, and, consequently, vegetation consists of scanty tufts of short grass and scattered patches of scrub bushes and dwarf trees. In an oasis, however, there is enough water to support date palm trees.
In addition to low rainfall, deserts may have considerable differences between day and night temperatures. Temperatures in the Sahara can rise to more than 50 °C (122 °F) during a day in July, yet drop to 20 °C (68 °F) at night because the clear, cloudless skies allow the land's absorbed heat to quickly escape into the atmosphere.
The Semiarid Sahel
The Sahel is an area of semiarid steppe (vast plain) with short, grass-type vegetation, low-growing trees, and thornbushes. The Sahel region stretches southward from the Sahara to Africa's savanna (grassy plain) and extends west to east through parts of Senegal, Mauritania, Mali, Burkina Faso, Niger, Nigeria, Chad, and Sudan. Some geographers also consider the dry regions of Ethiopia, Kenya, and Somalia as belonging to the Sahel.
The Sahel is a transition zone between the arid Sahara and the well-watered savanna to the south. The Sahel is also a transition area of human activity. North of the Sahel, in the Sahara, people mainly keep herds or flocks of animals. South of the Sahel, people engage mainly in agriculture. But in the Sahel, people do both. Nomadic and seminomadic herders move their livestock into the desert during the rainy season and return to the Sahel during the dry season. Over the centuries, farmers and herders maintained a balance between the Sahel's human and livestock populations and water and land resources.
The Region's Green Past
The land of the Sahara and the Sahel tells a dramatic story of past climate change. Geoligists have found that thousands of years ago the area was covered by grasslands, forests, and lakes. Ancient people hunted, fished, and farmed in a lush, green land. Geologic evidence shows that rivers once flowed across the now arid landscape of the Sahara and Sahel. For example, geologists have discovered that ancient rivers formed the regs. These rivers flowed from the central mountains into basins, picking up rocks along the way. The rivers deposited the larger, heavier rocks nearest the mountains and smaller, lighter gravel at the outer edges of the basins. These ancient rivers also cut ravines down through the rocks that form the central mountain ranges of the Sahara. The ancient river beds, called wadis, now channel water only during rare rainstorms when runoff from the surrounding area drains into them.
By studying land features, geologists also have discovered ancient natural drainage systems in the Sahara, including river courses, lakes, and deltas. This evidence has been supported by images of river channels revealed by special radar imaging equipment flown aboard the space shuttle by the U.S. National Aeronautics and Space Administration (NASA) in 1981. Geologists say that the Sahara region began to slowly dry out at the end of the last Ice Age, about 10,000 years ago. As the ice sheets then covering northern Europe retreated farther northward, the Sahara region gradually began to expand, and by about 4000 B.C., it occupied an area similar to the present desert.
Many parts of the Sahara that are now virtually without life supported human settlement in ancient times. Thousands of rock and cave paintings, particularly in the Tassili-n-Ajjer plateau in the central Sahara, depict wild animals, hunting scenes, and people herding goats, sheep, and cattle. Archaeologists have discovered that the scenes depict life in the region from before 5000 B.C. to about 1000 B.C. To trace the climate change from wet to dry in the Sahara and the expansion of this vast desert over thousands of years, climatologists have studied the Tassili paintings and the written histories of the Egyptian dynasties (Dynasty I began at about 3100 B.C., and Dynasty XX ended at about 1070 B.C.) and the Roman Empire (27 B.C to A.D. 476).
Patterns of Rainfall In the Sahel
The Sahel as a whole receives an average of 25 to 50 centimeters (10 to 20 inches) of rainfall yearly, an amount greater than in the Sahara. But the amount varies considerably from one location to another. The towns of Ouagadougou in Burkina Faso and Timbuktu in Mali illustrate the rainfall variability typical of the Sahel. Both have a dry season from November through April. In Ouagadougou, average rainfall for August is 26 centimeters (10 inches) and the annual total rainfall is 88 centimeters (35 inches). About 480 kilometers (300 miles) north in Timbuktu, the average August rainfall is only 8 centimeters (3 inches), and the total annual rainfall is only 23 centimeters (9 inches).
Scientists have found that rainfall for the region as a whole also varies considerably from year to year. Several years of above-average rainfall can follow several years of below-average precipitation. And every 25 to 35 years, drought occurs in the Sahel. A drought results when the average rainfall for an area drops far below the normal amount for a long period of time, for months or even years. Droughts of varying duration occurred in the Sahel in 1910, 1940, 1970, and 1984.
Ecological and Human Crisis In the Sahel
The years of below-average rainfall from 1968 until the 1990's have gained considerable attention for two reasons. The dry conditions have persisted over a longer period of time than in the past. And today, this dry spell affects many people, because the population in the Sahel has grown dramatically since the 1940's.
Before the 1940's, a lack of water had helped prevent significant increases in the numbers of people and animals in the Sahel. But beginning in the 1940's, Western aid and technology enabled people to drill hundreds of deep wells that tapped ground water far below the surface. Beneath the Sahara and Sahel, a vast amount of ground water has accumulated over thousands of years. The water in most places is at a depth of more than 2,000 meters (1.25 miles). Also, during the 1950's and early 1960's there was above-average rainfall. With abundant water available, human and livestock populations in the Sahel increased dramatically. Natural vegetation was cleared to grow more crops to feed more people, and more cattle were raised to support greater numbers of nomadic herders. In many areas, the increased water supplies helped support twice the number of people and animals that could have survived before the wells were drilled.
Then in 1968, the long dry period began. The pressing needs of the growing population resulted in overcultivation of crops and overcutting of trees for fuel. The increased number of cattle caused overgrazing. Livestock herds began to congregate regularly around the wells, trampling and destroying virtually all vegetation for a radius of more than 32 kilometers (20 miles) around each well. This depletion of vegetation caused soil erosion. Topsoil was blown away during the long, dry, windy seasons, and experts say that in some places of the Sahel, damage to the land is virtually irreversible.
The situation became critical with the drought that began in 1970 and persisted until 1974. Crops failed and people starved. The people of the Sahel and Western relief agencies were unprepared for such a catastrophe. Perhaps as many as 300,000 people died, despite food relief efforts by the United States, France, Britain, the European Community, and the former Soviet Union. Livestock perished in every country of the Sahel. Estimates of animals lost ranged as high as 20 million cattle and 12 million sheep.
Between 1974 and 1983, though the dry spell persisted, average annual rainfall was sufficient once again to produce crops. Then in 1984 and 1985, drought occurred once again. It was not as devastating as in the 1970's, and Western relief was better prepared to meet this emergency—but not before thousands more people and cattle died.
Climate Change or Land Abuse?
Beginning in the 1970's, the cause of the Sahel's persistent dryness spurred considerable speculation worldwide. Some experts suggested that permanent climatic change was occurring. A number of reports on the Sahel announced that the Sahara was advancing southward at rates of 3 to 10 kilometers (2 to 6 miles) per year. Indeed, weather data suggested that the Sahara-Sahel border, which receives 20 centimeters (8 inches) of rain, had shifted about 150 kilometers (93 miles) to the south. Such revelations seemed to support theories that the Sahara was continuing the expansion begun thousands of years ago.
Concerned over desertification in the Sahel and other parts of the world, the United Nations held a conference in 1977 in Nairobi, Kenya, to define and discuss the complex processes involved. The principal causes of desertification in Africa, the experts concluded, appeared related to land use, especially through overcultivation, overgrazing, and the cutting down of trees. The conference developed a plan of action to halt desertification by improving land-use practices. The plan called for an investment of $2.4 billion each year for 20 years. But by mid-1992, little of lasting benefit had been done. Reversing land degradation is a low priority for most African governments and is not attractive to many foreign investors, who are interested in more visible economic development programs.
Need For Closer Scientific Scrutiny
Nevertheless, at least one positive outcome arose from the Nairobi conference. Scientists wanted to monitor the Sahara in more precise and detailed ways than ever before. Prior to the 1970's, scientists studying the Sahel had used a variety of methods. Climatologists had observed annual weather conditions, read historical accounts of floods and droughts, and analyzed general records of climatic trends. After the 1970's, scientists intensified their studies. But in many cases, researchers concentrated only on relatively small areas of the region.
For example, a study by ecologist Hugh Lamprey, commissioned by the UN and published in 1988, based its conclusions on aircraft observations of sand dunes in northern Sudan. The study compared the vegetation boundary in 1958 with the boundary in 1975 and concluded that the desert in that region of Sudan had moved 97 kilometers (60 miles) southward.
But contrary evidence also emerged in the 1980's, when Swedish scientists from Lund University published the results of a detailed study conducted in northern Sudan. Using data from satellites and extensive ground investigation, they found no evidence of a current expansion of the Sahara in Sudan. Instead, the studies suggest that long-term drought and famine in the Sahel are caused by natural fluctuations of rainfall. This research stresses the Sahel environment's resiliency and ability to recover rather than its fragility.
Determining the causes of drought in the Sahel would enable international organizations, such as the United Nations, to develop better programs for aid to the region. The West has donated billions of dollars in food, equipment, and technology, but with little long-lasting benefit. The development programs have included planting and irrigating trees to stabilize sand dunes—a very expensive undertaking that has shown virtually no success in halting the encroachment of the desert sands. Partly as a result of new studies, some aid organizations have changed their focus from one of attempting to hold back the desert to one of addressing land abuse.
Tracking the Sahara's Shifting Border
One of the most important new studies reported in 1991, contradicting the idea that the Sahara is relentlessly moving southward, was reported by physicist Compton J. Tucker and his colleagues at the Goddard Space Flight Center in Greenbelt, Maryland, and at Texas Tech University in Lubbock. Tucker wanted to determine the extent of the Sahara by determining its entire southern boundary with the Sahel.
These scientists pointed out that reports of constant southerly expansion into the Sahel were based on study data from only a few locations, and the studies did not take into account the droughts of the 1970's and 1980's. Given the vastness of the Sahara and Sahel and their characteristic rainfall variability, these scientists held that data from a few locations do not indicate trends for the entire region.
Satellite Studies
The scientists began by analyzing data obtained by weather satellites over Africa from 1980 through 1990. Weather satellites were designed to gather data showing the location and movement of cloud masses. However, there are many days with no cloud cover over desert regions, and consequently the satellites provided views of the ground. These images could be analyzed for indications of growing vegetation.
The scientists theorized that the variation in rainfall over the Sahara and Sahel should correspond with the variation in vegetation greenness in the region. To measure the greenness of vegetation, the scientists looked at satellite images of visible light and reflected infrared rays, both of which reveal plant growth on the ground. The researchers used a ratio between visible light and reflected infrared rays to create a "vegetation index" for each month of the 11-year study.
A Shifting Sahara-Sahel Border
The satellite images showed that the Sahara border actually shifted back and forth, somewhat like the ebb and flow of ocean tides. The vegetation shifted with the changes of precipitation during normal seasonal cycles in the Sahel. For example, in July—which is summer in the Northern Hemisphere—the scientists saw an area of greenness north of the equator and a reduction in the apparent size of the Sahara. By January, when it was summer in the Southern Hemisphere, the Sahel was experiencing its long dry season, and there was little plant growth. At this time, the Sahara appeared larger, and the vegetation greenness index showed that there was maximum plant growth between the equator and the southern tip of Africa.
Tucker calculated that from 1983 to 1984, the average southward movement of the border between the Sahara and the Sahel was 99 kilometers (62 miles). But from 1984 to 1985, the border moved to the north an average of 110 kilometers (68 miles). And from 1985 to 1986, it moved northward again, this time an average of 33 kilometers (21 miles). Although the Sahara's southern boundary in 1990 was an average of 130 kilometers (80 miles) south of where it was in 1980, its annual fluctuations during the study period were so great, the scientists said, that no long-term trend could be established.
Tucker's study also showed a close relationship between greenness (according to the vegetation index) and rainfall data collected from 42 locations in the Sahara. The study defined the southern boundary of the Sahara as the area with a rainfall total of 20 centimeters (8 inches) per year. By this definition, the scientists found that the Sahel fluctuated north or south within a 233-kilometer (145-mile) zone during the study years. Within that zone, it was sometimes possible to grow food and at other times it was not.
Local Border Fluctuations
The movement of the Sahara as measured by the vegetation index or by the amount of annual rainfall was not uniform along the southern boundary of the Sahel. In many areas, the desert's boundary fluctuated greatly. But in some parts of southern Mauritania and from central Niger through western Chad, the variations in rainfall and in the location of the desert boundary were very small. These areas are of particular significance for future studies of desertification, according to the scientists, because it should be easier to detect changes to vegetation from other causes without having to deal with such a variable factor as rainfall.
Scientific evidence seems to point to a bewildering future pattern of rainfall and drought for the people of the Sahel. Climatologists say that at least 10 more years of study will be required before they can say whether the shifting boundary of the Sahara signals long-term expansion or contraction. In the short term, drought and intense human activities continue to threaten the land.