When the wind stops blowing and the dust finally settles, the impacts of airborne sediment can be felt in a surprisingly wide range of ways–often, far from where it left the earth.
Sometimes seemingly unrelated events in widely dispersed areas of the world can share a very common cause. Take, for example, a drop in numbers of silkworm cocoons in China . . . increased road maintenance costs in Kansas . . . lowered wheat production in Alberta . . . higher crop yields in Niger . . . reduced visibility in Puerto Rico . . . red sunsets in Wisconsin . . . a phytoplankton bloom in the Pacific Ocean . . . coral reefs dying off the coast of Florida . . . North African real estate moving to Bermuda . . . a 64-car pileup on a California freeway . . . and increased health risks to infants in the Southeast United States.Those are just some of the impacts of wind-blown dust. And that doesn’t count the 12% of New Zealand’s landscape affected by wind erosion; the approximately 148 million ac. in India, especially western Rajasthan, where nearly 60% of the land is covered by sand dunes and sand plains; or northern China, where soil from 579,000 mi.2 of land is blowing in the wind. In the US, the Natural Resource Inventory, conducted every five years by the US Department of Agriculture’s Natural Resources Conservation Service, reveals that, in 1992, an estimated 2.84 billion tons of soil from some 781 million ac. of range and cropland went airborne. That represented nearly half of all soil eroded from these lands that year.
A Big Problem
On an even larger scale, wind-blown dust is the most prominent airborne particle visible from space. By one estimate, anywhere from about 990 to 1,650 tons of soil dust enters the world’s atmosphere each year. In fact, dust in the air has been fingered as one possible suspect in atmospheric instability and climate change–no small feat for tiny pieces of soil with an aerodynamic diameter of 10 microns or less. That’s so small that several hundred 10-micron particles could fit into the period at the end of this sentence. (USEPA identifies such particulate matter as PM10.)
Wind erosion events can range in size from whirling dust devils twisting their way across a field of bare soil on the Texas plains to the eruption of massive dust storms in the Sahara Desert, where the wind can move from 66 to 221 million tons of fine sediment each year. In February 2000, a gigantic dust storm generated in northwest Africa covered hundreds of thousands of square miles above the eastern Atlantic Ocean with a thick cloud of Saharan sand. Rising 15,000 ft. or higher into the air, this dust can be carried west by the trade winds as far as the Caribbean Sea, where other winds can drive the airborne sediment north to the New England area of the Northeast US.
This past spring, the US National Oceanic and Atmospheric Administration reported that sediment originating from a dust storm in northern China reached the US, "blanketing areas from Canada to Arizona with a layer of dust" and clouding views of the Rocky Mountains from the eastern foothills.
Three years earlier, on April 19, a dust storm in Mongolia and north-central China spawned a cloud of dust that reached North America near the Canada-US border six days later. There, part of it swung south, covering the area from British Columbia to California. The rest blew into the upper Midwest US and Ontario.
Russian scientist Guennady Larionov of Moscow State University’s Research Laboratory of Soil Erosion notes that an early 1970 dust storm in the southeast part of the Russian Plain, which includes the Lower Don, Lower Volga, North Caucasus, and adjacent areas of Ukraine, lasted about 200 hours. At one spot, about 28 in. of soil blew away.
From 1950 to 1990, he says, soil losses in this region of the Russian Plain totaled about 11.7 billion tons. "Part of this soil mass accumulated in shelterbelts and behind them in gullies and river valleys. But about 60% of this soil mass was suspended in the atmosphere, producing environmental problems far away from the source of the dust."
Sometimes the sheer size of dust storms is out of this world. The Mars Global Surveyor satellite has captured photos of Martian dust storms, some of which have covered the entire Red Planet.
The Erosion Process
The energy for detaching and transporting sediment by wind is produced by air under higher pressure moving to an area of lower pressure. The greater the differences in air pressure, the stronger the wind and the more disturbed, unprotected soil and friable rock it can erode.
At least two-thirds of dust events are caused by the passage of warm and cold fronts and downmixing of upper level winds, points out Tom Gill, a professor with the Atmospheric Science Group at Texas Tech University, Lubbock, TX. "However, cyclogenesis [development or strengthening of counterclockwise circulation in the atmosphere] and thunderstorm outflows or haboobs produce the most dramatic dust clouds with the lowest visibilities."
The wind transports sediments in three ways. Soil aggregates and particles too heavy to be picked up by the wind, such as combinations of silt and clay typically about 0.8—2.0 mm (800-2,000 microns) in diameter, move by creep as the wind or impact from other particles pushes them along the soil surface.
Sediment light enough to be lifted by the wind but too heavy to travel very far moves by saltation. These particles, about 0.1—0.8 mm (100-800 microns) hop along the ground, moving just a few inches at a time and rising no higher than about 2-4 in. above the soil surface. These saltating grains cause the most damage to crops, abrading leaves and stems of young plants. They also contribute finer soil particles to the air stream.
"Saltaters leave the ground at a trajectory of about 60º to the horizon and come gliding back at about a 12º angle," states Larry Hagen, a researcher with the USDA Wind Erosion Research Unit, based at Kansas State University in Manhattan, KS. "These hard, saltating particles bombard immobile soil clods and the soil crust, cutting and breaking them down and supplying suspension materials. Typically they include the PM10 size and smaller particles that pose a health hazard.
"In drier climates, you’ll find loose PM10 and smaller particles in the soil. However, in more humid areas, saltaters contribute the bulk of these small particles in the air. The PM2.5 particles are rarely found loose by themselves and are usually created by the action of saltaters colliding with larger aggregates."
These PM10, PM2.5, and other soil particles smaller than about 20-60 microns in size travel by suspension in the wind, reducing visibility and soil quality and contributing to health problems for humans, livestock, and wildlife. These are the particles that can travel hundreds and thousands of miles, remaining airborne until washed back to earth by rain.
"The proportions of creep, saltation, and suspension in eroded soil vary with surface roughness, soil composition, and size of the eroding surface," Hagen explains. "Part of the reason is that for a given wind speed and surface condition, there is a limited transport capacity for the saltation and creep. However, the atmosphere has a much higher transport capacity for suspension-size soil than for saltation/creep particles. Moreover, abrasion of the surface by saltation often creates large amounts of suspension-size aggregates in addition to that present in the soil. Thus, on large, eroding agricultural fields or dry lakebeds, such as Owens Lake in California, the soil moved by the suspension component typically is several times the soil moved by saltation/creep."
In a paper presented in 1997 at an international symposium and workshop on wind erosion in Manhattan, KS, researcher Weinan Chen of Big Spring, TX, reported results of a study of loessial sandy loam soils in the Loess Plateau of China. About 55-70% of the wind-eroded soils moved by saltation. Creep accounted for about 5-16% of soil loss, while 8-24% of the sediment was transported in suspension.
"The erosive force of wind increases exponentially with increases in wind speed. For example, increasing wind velocity from 8 to 10 meters per second doubles the erosion capacity, while increasing wind speed from 8 to 16 meters per second generates an eightfold increase. Consequently, fast winds are capable of causing much more erosion than slow winds.
"At ground level, the roughness of the surface plays an important role in controlling the nature of wind erosion," Chen continues. "Boulders, trees, buildings, shrubs, and even small plants like grass and herbs can increase the frictional roughness of the surface and reduce wind velocity. Vegetation can also reduce the erosion effects of wind by binding soil particles to roots. Thus, as a general rule, the areas that show considerable amounts of wind erosion are open locations with little or no plant cover."
The Agricultural Toll
The impacts of wind erosion can range from the relatively minor cost and inconvenience of cleaning dust and dirt from clothes, cars, and homes to major threats to human life and the environment. In the US, awareness of the costs of wind erosion swept into the national conscience in the 1930s when clouds of airborne sediment from bare, bone-dry fields in Oklahoma and other Midwest states darkened the skies as far east as the Atlantic seaboard. Forcing many families in the wind-prone areas to abandon their farms for a new life in California, the disastrous Dust Bowl also spawned creation of the USDA’s Soil Conservation Service, now called the Natural Resources Conservation Service, to help farmers adopt soil-saving farming and ranching practices. Even today, wind accounts for more than 40% of all soil eroded each year from fields and land placed in the federal Conservation Reserve Program in the US. In 1997, the annual wind erosion rate averaged 2 tons/ac. nationwide.
Wind erosion is a major cause of soil degradation on agricultural land in arid and semiarid areas throughout the world. These areas include North America; southern South America; much of North Africa and the Near East; parts of southern, central, and eastern Asia; the Siberian Plains; and Australia. Northwest China is also prone to severe soil losses from wind. At the Manhattan conference in 1997, Xuewen Huang, a scientist with the Institute of Desert Research in Lanzhou, China, described a wind erosion study in Inner Mongolia. He found that soil losses from wind on cropland ranged as high as 44-260 tons/ha (109-642 tons/ac.) between winter and spring. On grassland plowed for crops, as much as 500-3,900 tons/ha (220-1,700 tons/ac.) of soil was lost to the wind. At this same conference, Jianyou Shen, also with the Institute of Desert Research, noted that more than 2 in. of topsoil can blow away in one year from cultivated land. In response to wind erosion, some fields may be replanted five or six times a season.
British scientist Michael Fullen with the School of Applied Sciences at the University of Wolverhampton notes that China’s 12 deserts and desertified land occupy 1.52 million km2 (587,000 mi.2), or almost 16% of the country. This area is expanding at an annual average rate of about 2,100 km2 (800 mi.2). "Dust storms are major mechanisms of exporting sediment into desert margins and damaging desert-margin ecosystems," he says. "This dust activates a complex sequence of events that affects development of soil and plant communities."
D.J. Mitchell, along with Fullen and others with the University of Wolverhampton, has studied desertification near Yanchi on the southern edge of China’s Mu-Us Sandy Land. "As a result of sand encroachment and desertification at Yanchi, the once-stable steppe is now a mixture of fixed, semifixed, and mobile dunes," Mitchell explains. "During dry periods, dunes become unstable, leading to increased sand mobility and further burial of rangeland soils." The Institute of Desert Research of Academia Sinica has coordinated efforts to combat desertification. Communal planting of Salix psammophila prevents sands from shifting, he reports. Over a four-year period, improved management of rangeland reduced desertification by 10% and increased the amount of rangeland available for grazing by 40%.
In the US, wind erosion on agricultural land is most widespread in the Great Plains states. It can also be a serious problem elsewhere on cultivated organic soils, sandy coastal areas, and alluvial soils along river bottoms. Wind erosion can also be a major problem in the Prairie Provinces of Canada. During the 1980s, for example, an estimated 809,000 ha (2 million ac.) of land in Alberta were damaged by wind erosion.
Wind damages soil by removing the lighter, more fertile, and less dense soil components, such as organic matter, clays, and silts. Because soil aggregates are heavier than individual particles of sediment, they resist wind erosion. As a result, loams, clay loams, and silt loams are less susceptible to the wind than sands, loamy sands, and sandy loams. Organic matter, which helps hold aggregates together, also increases a soil’s resistance to wind erosion.
Edward Skidmore, a scientist with the USDA Wind Erosion Research Unit, has examined how wind erosion reduces soil quality and productivity. In one project, he and his colleagues sampled saltation drifts from several recently eroded fields. "We found that sandy loams became loamy sands and loamy sands became sands," he reports. "Sand in the saltation drift increased 10% to 30% with a corresponding decrease in organic matter and cation exchange capacity [a measure of the soil’s ability to provide nutrients to plants]. However, the texture, organic matter, and cation exchange capacity of the silt loams and silty clay loams changed only slightly."
Lost soil productivity isn’t the only agricultural impact of wind erosion. Blowing sediment cuts and abrades plants, reducing seedling survival and growth, lowering crop yields, and damaging quality of vegetables. Such stress can increase susceptibility to diseases and help spread some plant pathogens. A coating of dust has been found to increase leaf temperatures and water loss while decreasing intake of carbon dioxide by plants. It’s conceivable that the various impacts of wind erosion in a given area could contribute to a die-off of soil-protecting plants, leading to further wind erosion.
Chinese environmental economists, including Xia Guang with the Policy Research Center of the National Environmental Protection Agency in Beijing, have investigated the impacts of air pollution, including wind-blown dust, in China. In 1992, their estimates of air-pollution costs included $636 million (5.3 billion yuan) in lost grain production, $122 million (1.02 billion yuan) in vegetable losses, $67.2 million (560 million yuan) in reduced fruit yields, and a $5.5 million (46.4 million yuan) loss of silkworm cocoons.
The Threat to Human Health
Wind-blown dust, especially PM10, and other equally small particulates of smoke and various chemicals have been linked to increased respiratory illnesses, lung damage, and premature deaths. The 1990 US federal Clean Water Air Act requires states to control PM10. In fact, an analysis by USEPA 10 years ago revealed that an estimated 60,000 people in the US die each year from federally allowed levels of airborne dust. As a result of this and other studies, USEPA added new air-quality standards in 1997 that will require states to control particles as small as PM2.5. USEPA estimates that these more stringent requirements, which are being implemented now, will reduce premature deaths in the US by about 15,000 a year and serious respiratory problems in children by about 250,000 cases annually.
The threat to health from airborne dust depends on the size of the particulate matter, the velocity of the surrounding air, and an individual’s breathing rate. Once inhaled, these particles are drawn through the respiratory tract to different regions of the lungs. Relatively large particles, say about 30 microns, are usually deposited in the nose or upper airways, according to the World Health Organization. Finer particles, typically smaller than 10 microns, might reach the gas-exchange region deep in the lungs where they can interfere with breathing and lead to increased chronic respiratory illnesses.
That’s not all. Last fall, the Caribbean Radiation Early Warning System, an air-sampling network funded by the US Department of Defense to monitor radioactivity, detected bacteria in dust blown from Africa to the US southern coastline. These bacteria have been linked to human respiratory and environmental health problems. The dust also carried mercury, lead, and the radioisotope beryllium-7.
More Costs
By reducing visibility, airborne dust can threaten life in other ways too. In 1991, for example, a windstorm on Interstate 5 in California contributed to a 64-vehicle pileup. Three years ago, dust blown by high winds across Interstate 84 in eastern Oregon led to three separate multiple-vehicle accidents, which killed at least six people and injured 27 more.
Airborne dust can work its way inside engines, bearings, and other sensitive mechanical and electrical components, adding to the costs of maintaining cars, buses, trucks, and farm and construction equipment and aircraft. This dust also increases road maintenance costs. A report from the USDA Wind Erosion Research Unit notes that in Seward County, KS, the state highway department spent more than $15,000 in 1996 to remove 965 tons of sand from 500 ft. of highway and ditches. Of course, sediment deposited in ditches and behind fences, trees, and other structures can also wash away in the next rain to pollute streams and lakes.
Dust is also a nuisance, drifting across residential driveways and landscapes, covering clothing, furniture, and homes with a gritty coating. In the spring of 1998, residents of eastern China and Korea even had to contend with mud falling from the dust-laden skies in raindrops.
Environmental Impacts
Eolian processes can also have detrimental consequences for the environment, such as disrupting rainfall. For example, dust and smoke particles in the air provide the nuclei around which water vapor condenses into cloud droplets. Daniel Rosenfeld and his colleagues at The Hebrew University in Jerusalem have compared the size of droplets in dusty clouds with those in nearby clouds where no dust was present. The dusty clouds produced less rain because they contained so many nuclei that the cloud droplets remained too small to form heavier drops that fall as rain. By reducing rainfall, these dusty clouds could lead to more drought, which would reduce vegetative growth, leading to more erosion, more dust, and even less rainfall. At the same time, Rosenfeld notes, global rainfall must balance global evaporation. As a result, wind erosion in one region could cause excessive rainfall thousands of miles away.
Increased airborne sediment from land-use practices that contribute to accelerated wind erosion might be a factor in climate change, believes Joseph Prospero, director of the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami. "Clearly it has some impact. Dust in the lower atmosphere reflects solar energy that would normally reach the earth’s surface. This leads to increased heating and instability of the upper atmosphere. But whether it affects the net radiation flux of the earth is another matter."
Sediment kicked up by sandstorms in the Sahara Desert and carried by trade winds might explain why coral reefs, sea fans, and sea urchins in the Caribbean are threatened by a variety of new diseases, such as white pox, white plague, and black band. Right now scientists don’t know if the diseases reflect natural or human causes and whether the agents are viruses, bacteria, fungi, or protozoa. Researchers note, however, that the decline of these corral reefs coincides with increased aridity and desertification in North Africa, which began in the mid-1960s and increased over the next two decades before beginning to subside in the 1990s.
Peak dust events in Barbados and elsewhere in the western Atlantic Ocean coincide with damaging events on coral reefs throughout the Caribbean. Many of these sick reefs lie in areas with relatively few people and where there’s minimal threat from such suspects as sewage, excess nutrients in runoff from land, and dredging. What’s more, a fungus, Aspergillus sp., that affects sea fans throughout the Caribbean is found on land and does not reproduce in seawater. One hypothesis is that wind-blown sediment brings in the disease-causing agents.
Those agents may include iron or other nutrients attached to the soil particles. As Prospero points out, a lack of iron limits biological activity, even when such nutrients as phosphorus and nitrogen are present. That can limit growth microorganisms, algae, and free-floating plants, including phytoplankton. Several years ago, in fact, scientists dumped 1,000 lb. of iron in seawater to fertilize production of 4 million lb. of plants.
"Because it’s highly insoluble, iron isn’t carried by river or ocean currents," Prospero states. "The only way it can be carried out into the ocean is by wind-blown dust."
Whatever the ultimate explanation for the die-off of coral reefs and plankton blooms, one thing is certain: Dust in the wind can affect the world and life on it in unexpected ways big and small. If nothing else, it highlights the nature and scope of the problems facing erosion control professionals in protecting earth’s air and water quality.