Temps de lecture :5 minutes
The thin layer of topsoil that covers the planet’s land surface is the foundation of civilization. This soil, measured in inches over much of the earth, was formed over long stretches of geological time as new soil formation exceeded the natural rate of erosion. As soil accumulated over the eons, it provided a medium in which plants could grow. In turn, plants protect the soil from erosion. Human activity is disrupting this relationship.
Sometime within the last century, soil erosion began to exceed new soil formation in large areas. Perhaps a third or more of all cropland is losing topsoil faster than new soil is forming, thereby reducing the land’s inherent productivity. Today the foundation of civilization is crumbling. The seeds of collapse of some early civilizations, such as the Mayans, may have originated in soil erosion that undermined the food supply. (1)
The accelerating soil erosion over the last century can be seen in the dust bowls that form as vegetation is destroyed and wind erosion soars out of control. Among those that stand out are the Dust Bowl in the U.S. Great Plains during the 1930s, the dust bowls in the Soviet Virgin Lands in the 1960s, the huge one that is forming today in northwest China, and the one taking shape in the Sahelian region of Africa. Each of these is associated with a familiar pattern of overgrazing, deforestation, and agricultural expansion onto marginal land, followed by retrenchment as the soil begins to disappear. (2)
Twentieth-century population growth pushed agriculture onto highly vulnerable land in many countries. The overplowing of the U.S. Great Plains during the late nineteenth and early twentieth centuries, for example, led to the 1930s Dust Bowl. This was a tragic era in U.S. history, one that forced hundreds of thousands of farm families to leave the Great Plains. Many migrated to California in search of a new life, a move immortalized in John Steinbeck’s The Grapes of Wrath. (3)
Three decades later, history repeated itself in the Soviet Union. The Virgin Lands Project between 1954 and 1960 centered on plowing an area of grassland for wheat that was larger than the wheatland in Canada and Australia combined. Initially this resulted in an impressive expansion in Soviet grain production, but the success was short-lived as a dust bowl developed there as well. (4)
Kazakhstan, at the center of this Virgin Lands Project, saw its grainland area peak at just over 25 million hectares around 1980, then shrink to 14 million hectares today. Even on the remaining land, however, the average wheat yield is scarcely 1 ton per hectare, a far cry from the nearly 8 tons per hectare that farmers get in France, Western Europe’s leading wheat producer. (5)
A similar situation exists in Mongolia, where over the last 20 years half the wheatland has been abandoned and wheat yields have also fallen by half, shrinking the harvest by three fourths. Mongolia—a country almost three times the size of France with a population of 2.6 million—is now forced to import nearly 60 percent of its wheat. (6)
Dust storms originating in the new dust bowls are now faithfully recorded in satellite images. On January 9, 2005, the National Aeronautics and Space Administration (NASA) released images of a vast dust storm moving westward out of central Africa. This vast cloud of tan-colored dust stretched over some 5,300 kilometers (roughly 3,300 miles). NASA noted that if the storm were relocated to the United States, it would cover the country and extend into the oceans on both coasts. (7)
Andrew Goudie, Professor of Geography at Oxford University, reports that Saharan dust storms—once rare—are now commonplace. He estimates they have increased 10-fold during the last half-century. Among the countries in the region most affected by topsoil loss from wind erosion are Niger, Chad, Mauritania, northern Nigeria, and Burkino Faso. In Mauritania, in Africa’s far west, the number of dust storms jumped from 2 a year in the early 1960s to 80 a year today. (8)
The Bodélé Depression in Chad is the source of an estimated 1.3 billion tons of wind-borne soil a year, up 10-fold from 1947 when measurements began. The 2–3 billion tons of fine soil particles that leave Africa each year in dust storms are slowly draining the continent of its fertility and, hence, its biological productivity. In addition, dust storms leaving Africa travel westward across the Atlantic, depositing so much dust in the Caribbean that they cloud the water and damage coral reefs there. (9)
In China, plowing excesses became common in several provinces as agriculture pushed northward and westward into the pastoral zone between 1987 and 1996. In Inner Mongolia (Nei Monggol), for example, the cultivated area increased by 1.1 million hectares, or 22 percent, during this period. Other provinces that expanded their cultivated area by 3 percent or more during this nine-year span include Heilongjiang, Hunan, Tibet (Xizang), Qinghai, and Xinjiang. Severe wind erosion of soil on this newly plowed land made it clear that its only sustainable use was controlled grazing. As a result, Chinese agriculture is now engaged in a strategic withdrawal in these provinces, pulling back to land that can sustain crop production. (10)
Water erosion also takes a toll on soils. This can be seen in the silting of reservoirs and in muddy, silt-laden rivers flowing into the sea. Pakistan’s two large reservoirs, Mangla and Tarbela, which store Indus River water for the country’s vast irrigation network, are losing roughly 1 percent of their storage capacity each year as they fill with silt from deforested watersheds. (11)
Ethiopia, a mountainous country with highly erodible soils on steeply sloping land, is losing an estimated 1 billion tons of topsoil a year, washed away by rain. This is one reason Ethiopia always seems to be on the verge of famine, never able to accumulate enough grain reserves to provide a meaningful measure of food security. (12)
1. One third is author’s estimate; Lester R. Brown, Building a Sustainable Society (New York: W.W. Norton & Company, 1981), p. 3.
2. Yang Youlin, Victor Squires, and Lu Qi, eds., Global Alarm: Dust and Sandstorms from the World’s Drylands (Bangkok: Secretariat of the U.N. Convention to Combat Desertification, 2002), pp. 15–28.
3. John Steinbeck, The Grapes of Wrath (New York: Viking Penguin, Inc., 1939).
4. FAO, The State of Food and Agriculture 1995 (Rome: 1995), p. 175.
5. Ibid.; USDA, Production, Supply, & Distribution, electronic database, at , updated 13 September 2005; FAO, op. cit. note 10, updated 14 July 2005.
6. U.N. Environment Programme (UNEP), Mongolia: State of the Environment 2002 (Pathumthani, Thailand: Regional Resource Centre for Asia and the Pacific, 2001), pp. 3–7; USDA, op. cit. note 27; population from United Nations, World Population Prospects: The 2004 Revision (New York: February 2005).
7. National Aeronautics and Space Administration (NASA) Earth Observatory, “Dust Storm off Western Sahara Coast,” at viewed 9 January 2005.
8. Paul Brown, “4x4s Replace the Desert Camel and Whip Up a Worldwide Dust Storm,” Guardian (London), 20 August 2004.
10. Hong Yang and Xiubin Li, “Cultivated Land and Food Supply in China,” Land Use Policy, vol. 17, no. 2 (2000), p. 5.
11. Asif Farrukh, Pakistan Grain and Feed Annual Report 2002 (Islamabad, Pakistan: USDA Foreign Agricultural Service (FAS), 2003).
12. Lester R. Brown and Edward C. Wolf, Soil Erosion: Quiet Crisis in the World Economy, Worldwatch Paper 60 (Washington, DC: Worldwatch Institute, 1984), p. 20.
Lester R. BROWN
Chapitre 5. Natural Systems Under Stress
Plan B 2.0 Rescuing a Planet Under Stress and a Civilization in Trouble (NY: W.W. Norton & Co., 2006).
Copyright © 2006 Earth Policy Institute