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Dustiest Places on Earth–Dead and Dying Seas

Introduction

In arid lands, changing climate or the diversion of water for use by cities and agriculture leads to dead and dying inland seas and lakes. Winds blowing over exposed lake beds produce the dustiest places on earth. Three dead or dying bodies of water are especially important:

  1. The Bodélé Depression in the Sahel is the dustiest area on earth.
  2. The area around the nearly dry Owens Lake is the dustiest area in North America.
  3. The Aral Sea was almost completely destroyed in the 20th century, making it one of the great ecological disasters. It is now an important source of dust and aerosol pollution.

The Dustiest Place on Earth: the Bodélé Depression in the Sahel

The dustiest place on earth is the Bodélé Depression in northern Chad in the Sahel. It is all that remains of the much-larger, freshwater Lake Megachad that formed when the Sahel and the Sahara were much wetter thousands of years ago (read Desertification in the Sahel for more on the wetter Sahara). The depression is a major source of dust for the Atlantic, depositing calcium, iron, potassium, and phosphorus needed by phytoplankton. It is also the major source of dust that fertilizes the Amazon basin in South America.

map of chad showing location of mountains and depression
Map of the geography of Chad showing the Bodélé´Depression (Djurab Desert) and the mountains to the east and north with a gap to the northeast. The boundary of Chad is marked with crosses. The area of Lake Megachad is the hatched area.
From School of Geography And The Environment, University of Oxford, BodEx Experiment.

The area is dusty for several reasons:

  1. It is windy. A narrow gap between the the Tibesti Mountains of northern Chad and the Ennedi Mountains of eastern Chad and western Sudan funnels and accelerates winds into the depression.
  2. The area is very dry. Some areas get less than 25- 50 mm or rain per year (1-2 inches/year).
  3. The dry lake bed is covered with the remains of diatoms, the microscopic phytoplankton with silica shells. Such deposits are called diatomaceous earth or diatomite. It easily crumbles into a fine white powder.
  4. Wind blowing pellets of diatomite and sand particles along dunes causes the particles to break into dust. The wind then carries dust from the dunes into the atmosphere.

white dust from the bodele depression begins to blow toward the southwest in this space image
White dust begins to blow from the Bodélé Depression toward Lake Chad region (green area) in this Moderate Resolution Imaging Spectroradiometer (MODIS) image acquired by the Terra satellite on March 21, 2004.
From NASA Earth Observatory Dust in the Bodélé Depression.

average atmospheric dust over northern Africa
Thirteen-year-average concentration of aerosols in the atmosphere over northern Africa measured by NASA satellites, including the Total Ozone Mapping Spectrometer (TOMS). The Bodélé Depression is the bright red spot in the middle of the image.
From Giles (2005).

Here's what it looks like on the ground.

BodEx team in the Bodele depression during a storm.
The BodEx Team from the University of Oxford in the Bodélé Depression during a weak storm. They are standing on diatomite. Click on the image for a zoom.
From Department of Earth Sciences, University College London News Release

The Dustiest Place in North America: The Owens Valley

The dustiest place in North America is the area around Owens Lake in Owens Valley, California. It is the single largest source of PM-10 dust in the United States. Unlike the Bodélé, which is due to natural causes, the Owens Valley problem is due to human causes.

The story begins with the Los Angeles Department of Water and Power obtaining water rights to almost all water in the Owens Valley during the early 20th century. Los angeles in 1905 had a rapidly growing population of around 100,000. Yet it was located in a semi-arid area with little rain, around 300–400 mm/year (15 inches per year). To continue growing, it needed to import water from a secure source in a wetter region. The water in the Owens Valley 250 miles to the northeast was selected.

  1. Water in the Owens Valley comes from streams flowing eastward out of the Sierra Nevada. Much of the water sinks into the porous soil of the valley, creating a large pool of shallow groundwater, mostly in the northern end of the valley.
  2. In the early years of the 20th century, Los Angeles secretly bought land and all associated surface and groundwater rights from large land owners in the valley. Later the purchases were made openly. By 1935 Los Angeles owned most of the land in the valley and the city had captured most of the water flowing eastward out of the Sierra Nevada into the Owens River.
  3. The water in the Owens River, and much groundwater were diverted into the California Aquaduct and sent to Los Angeles.
  4. With no inflow of water, Owens Lake dried up. By 1926 all that remained was a dry lake bed.

map of california showing owens valley and its relation to los angeles
Map of Owens Valley (center of map) and its relation to Los Angeles. Owens Lake is the blue area in the center (east of Visalia and north of Ridgecrest). The Sierra Nevada mountains are west of the valley and the Inyo Mountains are east. Dashed line is boundary of California.
From Google Maps.

The dry lake bed is the dust source.

  1. The lake is in a very dry area, the rain shadow of the Sierra Nevada. Rainfall varies from 6 inches/year (150 mm/yr) in wet years to 3 inches/year (75 mm/yr) in dry years. Thus the sediments are dry most of the year, especially in summer (75%–98% of the rain falls in the winter).
  2. High winds occur frequently in the valley, tending to blow along the length of the valley, especially during spring and early fall when the high valley walls funnel and accelerate winds aloft into the valley. The very strongest winds, associated with rotors in the lee of the Sierras, blow from west to east across the valley as strong storms come ashore from the Pacific.
  3. The lake bed is covered with alkaline salts composed of sodium sulfates and sodium carbonates that are easily eroded. The lake has no natural outlet, and all salts contained in the water flowing into the lake over at least the last 800,000 years stayed in the lake. As the lake dried out, they precipitated onto the bottom (the large white area in the photo below).
  4. The dust is a fine mixture of salt, clay, and sulfates. Dust is up to 30% salt, and it includes important concentrations of arsenic, cadmium, nickel, and chromium. Lake deposits contain 50–150 parts per million of arsenic. For more information read Owens (Dry) Lake, California: A Human-Induced Dust Problem by Marith C. Reheis of the U.S. Geological Survey.
  5. High wind can blow dust from the valley over large areas of the Mojave Desert, impairing the health of 40,000 people.

owens valley seen from space
All that is left of the original Owens Lake in Owens Valley, California east of the Sierra Nevada mountains. North is to the left. The Sierra Nevada is at the top. The Owens Valley is the brown region in the center. The Inyo Mountains are at lower right. Click on image for a zoom.
From NASA Earth Observatory.

Deposits on the dry Owens Lake bed
Deposits on the lake bed of the former Lake Owens and monitoring station. Click on the image for a zoom.
From Sensit Corporation.

The US Environmental Protection Agency classified the Owens Valley as a "serious nonattainment area" in 1993, and in 1999 they approved a plan to mitigate the hazard. The plan requires Los Angeles to take steps to mitigate the problem by allowing water to flood the lower parts of the lake keeping it moist, by covering large areas with gravel to reduce wind erosion, and to plant vegetation in some areas. Implementation of the plan was finally started in 2001, but the area was still a nonattainment area in 2007. See A Century Later, Los Angeles Atones for Water Sins in the New York Times.

Photograph of the Owens Lake bed and areas being mitigated by Los Angeles
Photograph of the former Lake Owens showing deposits on the bottom of the lake (white areas) and the areas being mitigated to reduce dust (blue and green areas). The view is to the north with the Sierra Nevada on the left. Click on image for a zoom.
Photograph taken on 5 April 2009 by Charles W. Hull and posted on DVInfo.

Dying Seas: The Aral Sea

The Aral Sea was once the 4th largest lake in the world, with a salinity 1/3 that of seawater. As with the Lake Owens, society's demand for water led to the shrinking of the sea, exposing salty lake bed deposits to wind erosion. The demand for water came from the former Soviet Union hoping to expand irrigated agriculture into the arid and semi-arid land along the two large rivers draining into the sea, the the Syr Darya and Aru Darya. The demand for irrigation water led to large-scale diversion of water from rivers, leading to perhaps the most notorious ecological catastrophe of human making.

map of Aral Sea area in central Asia.
Map of Aral Sea area in central Asia.
From the World Bank.

Demand for Water

The Aral region's plight traces its roots to the early days of the Soviet Union, when communist authorities hatched a plan to grow all the cotton the budding superpower would need by irrigating vast plains in central Asia. The Soviets revved up cotton production in the mid-1920s, then 30 years later began carving hundreds of kilometers of unlined canals from the Aral's two tributaries--the Amu Darya and the Syr Darya – into the surrounding desert to nourish new cotton fields. The strategy paid dividends: The Soviet Union soon joined China and the United States as the world's leading cotton exporters. But by the early 1960s, the first signs of trouble began to appear: The Aral Sea was unmistakably shrinking as irrigation projects sucked billions of liters of water from its feeder rivers.
From Stone (1999).

The demand for water was exacerbated several factors:

  1. An increase in the population working in agriculture. Population in the region increased from 7–8 million people at the beginning of the 20th century to more than 50 million people in 1997, many of whom engaged in farming irrigated lands.
  2. Poor irrigation practice. This led to farmlands saturated with salt and poorly suited for continued use, which led to even more use of irrigation water— to flushing salt from the soil.

    The region is steeped in plant-stunting calcium sulfate, which is why very little grows, even near rivers. This salt leaches to the surface when land is excessively irrigated and requires increasing amounts of water to wash it away.
    From Pala (2005).

Evaporation of the Aral Sea

The loss of flow into the sea eventually caused it to dry up and become a source of toxic dust.

  1. The lake is in a very dry area. Average rainfall is 150–200 mm/year with large variability typical of semi-arid regions.
  2. High winds occur frequently in the region, especially in the western regions of the sea, with more than 50 days of storms per year and winds to 20–25 m/s— State of Environment of the Aral Sea Basin.
  3. High winds, little rainfall, and hot summers led to high evaporation.
  4. High evaporation caused the area of the sea to shrink by more than 90%, and the sea has split into several smaller parts leaving large expanse of exposed seabed, light areas in the image below. 33 000 km2 of former seabed is now exposed. This is an area the size of Maryland.

    Map showing the reduction in the area of thhe Aral Sea from 1960
    The change in area of the Aral Sea from 1960 (black border) to 2009. By the end of the period, the lake had shrunk to four disconnected small lakes, green areas plus very shallow gray area in center. The image also shows dust blowing across the center of the old lake.
    From NASA Earth Observatory: Evaporation of the Aral Sea.

Consequences

The Aral Sea is a major source of toxic dust that is a public health hazard for millions of people.

  1. The lake has no natural outlet, and all material carried into the lake has stayed in the lake. The material included pesticides, herbicides, toxic industrial waste, and sewage from agricultural and urban areas. All were deposited on the seabed when the water evaporated.

  2. Dust from the exposed, dry sea floor is a toxic mixture of salt, clay, oil hydrocarbons, phenols, heavy metals and minerals, synthetic surface-active substances, and chlororganic pesticides such as metaphos, corotan, butiphos, hexachloran, lindan, and DDT. (State of Environment of the Aral Sea Basin) High winds blow 100 million tons of toxic dust over large areas of Asia. High concentrations of dust have been measured in southern Turkmenistan, hundreds of kilometers from the sea. Traces of dust have been found as far away as Greenland and Antarctica.

    The Aral Sea region is one of the world's foremost ecological disaster zones and there is increasing local concern for the health of millions of people living in this region. We have found that dust deposition rates across eastern Turkmenistan are among the highest in the world and that the dust is contaminated with pesticide.
    From O'Hara (2000).

  3. The dry lake bed provides easy access to the former Vozrozhdeniye (Renaissance) Island, a remote, hard-to-reach island that housed Soviet bioweapons testing facility used for field tests of "weaponized anthrax, tularemia, brucellosis, plague, typhus, Q fever, smallpox, botulinum toxin, and Venezuelan equine encephalitis"–Pala (2005).

The loss of the sea has changed the regional climate.

During the the last 5-10 years [since 1990] the drying off of the Aral Sea, brought about noticeable changes in climate conditions. In the past the Aral was considered a regulator mitigating cold winds from Siberia and reducing the summer heat. Climate changes have led to a dryer and shorter summer in the region, and longer and colder winters. The vegetative season has been reduced to 170 days. The pasture productivity has decreased by a half, and meadow vegetation destruction has decreased meadow productivity 10 times. On the shores of the Aral Sea precipitation was reduced several times. Average precipitation magnitude is 150-200 mm with considerable seasonal ununiformity.
From State of Environment of the Aral Sea Basin.

The Aral Sea Fishery is gone. At one time, more than 50,000 tons of fish were caught each year. All that remains now are rusting fishing boats stranded miles from the nearest water.

abandoned fishing boats, Aral Sea
Abandoned ships resting on the seabed of the now-evaporated Aral Sea. Photographed by Paul Thomas in May 2008 near the town of Moynaq from a small cliff that used to overlook the sea. Click on the image for a zoom.
From Paul Thomas Trekearth.

More Information

Read Coming to Grips With the Aral Sea's Grim Legacy, a Science article by Stone (1999). There's no undoing this sea's demise, but scientists are hoping to soften the impact.

Aral Sea - To save a Vanishing Sea, a Science article by Pala (2005).
A project backed by the World Bank aims to reverse the Aral Sea's rapid decline, but it could also increase traffic to an abandoned bioweapons testing site.

The northern portion of the Aral Sea is slowly being brought back to life. A dike supported by the World Bank and repairs along the banks of the Syr Darya River have increased the water level dramatically. An article in Science by Pala (2007).

References

Giles, Jim (2005) Climate science: The dustiest place on Earth, Nature 434: 816–819. Clicking on the link downloads a 904-KB pdf file.

O'Hara, S. L., G. F. S. Wiggs, et al. (2000). Exposure to airborne dust contaminated with pesticide in the Aral Sea region. The Lancet 355 (9204): 627–628.

Pala, C. (2005). ENVIRONMENTAL RESTORATION: To Save a Vanishing Sea. Science 307 (5712): 1032–1034.

Pala, C. (2006). ECOLOGY: Once a Terminal Case, the North Aral Sea Shows New Signs of Life. Science 312 (5771): 183.

Stone, R. (1999). ARAL SEA: Coming to Grips With the Aral Sea's Grim Legacy. Science 284 (5411): 30–33.

Revised on: 3 August, 2009

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