Page 243 - Environment: The Science Behind the Stories

40% of Earth’s land surface. Precipitation in these regions
is meager, so drylands are prone to desertification. Deserti-
fication describes a form of land degradation in which more
than 10% of productivity is lost as a result of erosion, soil
compaction, forest removal, overgrazing, drought, salini-
zation, climate change, water depletion, and other factors.
Most such degradation results from wind and water erosion
(Figure 9.11). Severe desertification can expand existing
desert areas and create new ones (Figure 9.12). This process
has occurred most dramatically in areas of the Middle East
that have been inhabited, farmed, and grazed for long peri-
ods—including the Fertile Crescent region, where agricul-
ture first originated (p. 235).
By some estimates, desertification endangers the food
supply or well-being of more than 1 billion people in over
100 countries and costs tens of billions of dollars in income
each year. China alone loses $6.5 billion annually from deser-
tification. In its western reaches, desert areas are expanding
because of overgrazing from over 400 million goats, sheep,
and cattle. In the Sistan Basin along the border of Iran and
Afghanistan, an oasis that supported a million livestock
recently turned barren in just 5 years, and windblown sand Figure 9.12 Severe desertification can cause desert areas
buried more than 100 villages. In Kenya, overgrazing and to expand. Here, immense sand dunes moving in from the Gobi
deforestation fueled by rapid population growth has left 80% Desert are burying farm fields in northwestern China.
of its land vulnerable to desertification. Everywhere, soil
degradation forces ranchers to crowd onto poorer land and
farmers to reduce the fallow periods during which land lies
unplanted and can regain nutrients. In a positive feedback As a result of desertification, gigantic dust storms from
cycle (pp. 124–125), both of these actions worsen soil deg- denuded land in China are now blowing across the Pacific
radation further. Ocean to North America, and dust storms from Africa’s
Desertification is expected to grow worse as climate Sahara Desert blow across the Atlantic Ocean to the Caribbean
change alters rainfall patterns, making some areas drier. Sea (see Figure 17.11c, p. 475). Such massive dust storms
A 2007 United Nations report estimated that 50 million occurred in the United States during the early 20th century,
people could soon be displaced. The report also suggested when desertification shook American agriculture and society
that industrialized nations fund reforestation projects in to their very roots.
drylands of the developing world to slow desertification
while gaining carbon credits in emissions trading pro-
grams (pp. 201–202, 530–531). The Dust Bowl shook the United States
Prior to large-scale cultivation of North America’s Great
Plains, native prairie grasses of this temperate grassland
Soil structure problems region held soils in place. In the late 19th and early 20th cen-
(3.4%) Chemical turies, many settlers arrived in Oklahoma, Texas, Kansas,
problems New Mexico, and Colorado hoping to make a living there as
(9.7%) farmers. Between 1879 and 1929, cultivated area in the region
soared from 5 million ha (12 million acres) to 40 million ha
(100 million acres). Farmers grew abundant wheat and ranch-
ers grazed many thousands of cattle, sometimes expanding
Wind onto unsuitable land and causing erosion by removing native
erosion grasses and altering soil structure.
(41.8%)
Water In the early 1930s, a drought worsened the ongoing
erosion human impacts, and the region’s strong winds began to erode
(45.2%) millions of tons of topsoil (Figure 9.13). Massive dust storms
traveled up to 2000 km (1250 mi), blackening rain and snow
as far away as New York and Washington, D.C. Some areas
lost as much as 10 cm (4 in.) of topsoil in a few years. The
most-affected region in the southern Great Plains became
Figure 9.11 Soil degradation on drylands is due primarily to
erosion by wind and water. Data from U.N. Environment Programme. known as the Dust Bowl, a term now also used for the histori-
2002. Tackling land degradation and desertification. Washington and Rome: cal event itself. The “black blizzards” of the Dust Bowl forced
242 Global Environment Facility and International Fund for Agricultural Development. thousands of farmers off their land.

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