Dustiest Places on Earth–Dead and Dying Seas
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
- The Bodélé Depression in the
Sahel is the dustiest area on earth.
- The area around the nearly dry Owens
Lake is the dustiest area in North America.
- 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 the geography of Chad showing the
Desert) and the mountains to the east and north with a gap to the
Chad is marked with crosses. The area of Lake Megachad is the hatched
From School of Geography And The Environment, University of Oxford,
The area is dusty for several reasons:
- 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.
- The area is very dry. Some areas get less than 25- 50 mm or rain
per year (1-2 inches/year).
- 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.
- Wind blowing pellets of diatomite and sand particles along dunes
causes the particles to break into dust. The wind then carries dust
the dunes into the atmosphere.
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.
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.
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
The Dustiest Place in North America: The
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.
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
- Water in the Owens Valley comes from streams flowing eastward out of the
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.
- 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.
- The water in the Owens River, and much groundwater were diverted into the
California Aquaduct and sent to Los Angeles.
- With no inflow of water, Owens
Lake dried up. By 1926 all that remained was a dry lake bed.
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.
- 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).
- 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.
- 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).
- 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
(Dry) Lake, California: A Human-Induced Dust Problem by Marith
C. Reheis of the U.S. Geological Survey.
- High wind can blow dust from the valley over large areas of the Mojave
Desert, impairing the health of 40,000 people.
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
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
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.
From the World
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
From Stone (1999).
The demand for water was exacerbated several factors:
- 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.
- Poor irrigation practice. This led to farmlands saturated with salt and
poorly suited for continued use, which led to even more use of irrigation
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.
- The lake is in a very dry area. Average rainfall is 150–200 mm/year with
large variability typical of semi-arid regions.
- 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.
- High winds, little rainfall, and hot summers led to high evaporation.
- 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.
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
of the Aral Sea.
The Aral Sea is a major source of toxic dust that is a public
health hazard for millions of people.
- 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.
- 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
From O'Hara (2000).
- 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
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
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 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.
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.
Sea - To save a Vanishing Sea, a Science article by Pala
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
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):
Pala, C. (2006). ECOLOGY: Once a Terminal Case, the
North Aral Sea Shows New Signs of Life. Science 312
Stone, R. (1999). ARAL SEA: Coming to Grips With
the Aral Sea's Grim Legacy. Science 284
3 August, 2009