Tags

,


Evaporation of the Aral Sea as animation

Evaporation of the Aral Sea as animation

The Shrinking Aral Sea

The Shrinking Aral Sea (May 5, 2001)

The Aral Sea is actually not a sea at all. It is an immense lake, a body of fresh water, although that particular description of its contents might now be more a figure of speech than practical fact. In the last 30 years, more than 60 percent of the lake has disappeared. The sequence of images above, acquired by Landsat satellites, shows the dramatic changes to the Aral Sea between 1973 and 2000.

Beginning in the 1960s, farmers and state offices in Uzbekistan, Kazakhstan, and Central Asian states opened significant diversions from the rivers that supply water to the lake, thus siphoning off millions of gallons to irrigate cotton fields and rice paddies. As recently as 1965, the Aral Sea received about 50 cubic kilometers of fresh water per year—a number that fell to zero by the early 1980s. Consequently, concentrations of salts and minerals began to rise in the shrinking body of water. That change in chemistry has led to staggering alterations in the lake’s ecology, causing precipitous drops in the Aral Sea’s fish population.

The Aral Sea supported a thriving commercial fishing industry employing roughly 60,000 people in the early 1960s. By 1977, the fish harvest was reduced by 75 percent, and by the early 1980s the commercial fishing industry had been eliminated. The shrinking Aral Sea has also had a noticeable affect on the region’s climate. The growing season there is now shorter, causing many farmers to switch from cotton to rice, which demands even more diverted water.

A secondary effect of the reduction in the Aral Sea’s overall size is the rapid exposure of the lake bed. Strong winds that blow across this part of Asia routinely pick up and deposit tens of thousands of tons of now exposed soil every year. This process has not only contributed to significant reduction in breathable air quality for nearby residents, but has also appreciably affected crop yields due to those heavily salt-laden particles falling on arable land.

Rebirth Island Joins the Mainland

"Rebirth" Island Joins the Mainland (August 25, 2001)

The yellow line superimposed on the scenes reveals the full extent of the Aral Sea in 1970, while the red line represents the border between Kazakhstan and Uzbekistan. The Aral (which is actually a lake) is drying rapidly because the two main rivers that fed it–the Amu Darya and the Syr Darya–had most of their water diverted in order to transform a desert area into cotton and rice plantations.

Vozrozhdeniye means “renaissance” or “rebirth.” The more detailed MODIS images (insets) show that the small water channel that separated the island from the mainland in the summer of 2000 had vanished by the summer 2001. This means that animals living on the island now have easy access to the mainland, and vice versa.

Dust Storm, Aral Sea

Dust Storm, Aral Sea (December 30, 2001)

The Aral Sea has shrunk to less than half its size since 1985. The Aral Sea receives little water (sometimes no water) from the two major rivers that empty into it—the Syr Darya and Amu Darya. Instead, the river water is diverted to support irrigation for the region’s extensive cotton fields. Recently, water scarcity has increased due to a prolonged drought in Central Asia. As the Aral Sea recedes, its former sea bed is exposed. The Aral’s sea bed is composed of fine sediments—including fertilizers and other agricultural chemicals—that are easily picked up by the region’s strong winds, creating thick dust storms. The International Space Station crew observed and recorded a large dust storm blowing eastward from the Aral Sea in late June 2001. This image illustrates the strong coupling between human activities (water diversions and irrigation), and rapidly changing land, sea and atmospheric processes—the winds blow across the Sea and pick up dust (former sea bottom sediments) as soon as the blowing air masses hit land. Note that the northern boundary of the dust storm occurs along the Syr Darya, where no bottom sediments are exposed.

Windswept Shores of the Aral Sea

Windswept Shores of the Aral Sea (March 27, 2003)

As recently as the 1960’s the Aral Sea of Kazakhstan and Uzbekistan was the fourth-largest inland sea in the world. Since then, its water volume has dropped by about 80% due to extensive irrigation systems developed during the Soviet era to produce cotton and other crops. What was once a single body of water has now separated into several smaller seas. Since the separation of the Little Aral from the Large Aral in 1987, the shores of what had once been an island in the middle of the Large Aral (Vozrozhdeniya Island) have expanded to form a land bridge that almost completely separates the eastern and western parts of the Large Aral. These views from the Multi-angle Imaging SpectroRadiometer (MISR) portray the Little Aral and the eastern Large Aral at the onset of winter, on December 3, 2002.

Of the two major rivers that once fed the Aral Sea, the freshwater contribution from the Amu Darya River is now negligible. The Syr Darya River now only feeds the Little Aral. Depletion of the Aral Sea has led to soil and water salination and agrochemical contamination. The retreating shoreline leaves the surface encrusted with salt and with agrochemicals brought in by the rivers. As the Sea’s moderating climatic influence has diminished, temperature variations in the region have altered, resulting in colder winters and hotter, drier summers. When strong westerly winds occur, large quantities of saline dust (and agrochemical toxins) can travel several hundred kilometers.

In these images, several groups of low cumulus clouds are clustered over open bodies of water and are identifiable in the stereo view by their height above the surface. A number of large white streaks extend eastward toward the Kyzylkum desert. Although their altitude cannot be ascertained from the nadir image, the stereo anaglyph shows that they are close to, or at, the surface. Several of these features originate from the eastern edge of the Large Aral, and may be associated with windblown snow and/or salt particles carried aloft.

Aral Sea

Aral Sea (August 25, 2003)

In July 2003, the Kazakhstan government, with funding from the World Bank, began a massive restoration project for the Aral Sea. Once the fourth largest lake on Earth, the Aral Sea has shrunk dramatically over the past few decades as the primary rivers that fed the Sea have been diverted and tapped nearly dry for cotton farming and other agriculture. The southern part of the Sea was fed by the Amurdar’ya and the northern part was fed by the Syrdar’ya, forming a large inland lake that moderated the region’s continental climate and supported a productive fishing industry.

July - September, 1989 Landsat mosaic at 250 m resolution (364 k JPEG)

July - September, 1989 Landsat mosaic at 250 m resolution (364 k JPEG)

The diversion process began in the 1960s, and by 1989, Landsat satellite imagery (left image) showed that the northern and southern half of the sea had already become virtually separated. The drying out of the Sea’s southern part exposed the salty seabed. Dust storms increased, spreading the salty soil right into the agricultural lands. As the agricultural land becomes contaminated by the salt, the farmers try to combat it by flushing the soil with huge volumes of water. What water makes its way back to the Sea is increasingly saline and polluted by pesticides and fertilizer.

August 12, 2003 Aqua MODIS scene at 250 m resolution (276 k JPEG)

August 12, 2003 Aqua MODIS scene at 250 m resolution (276 k JPEG)

Recent hydrographic surveys have revised downward the lifespan of the dying lake; complete desiccation could happen in as few as 15 years.

The southern Aral Sea has been deemed beyond salvaging, and the restoration effort will instead focus on the much smaller, but less polluted and saline, northern sea (top right). The World Bank effort will construct a permanent dike (a temporary one made of sand has been in place since about 1995) between the two portions of the Sea, sealing the southern half’s fate. The northern Small Aral Sea will be allowed to refill from the inflow of the Syrdar’ya, and though it is never expected to regain its former extent, planners think that it will refill enough to support a robust fishing again. It should also help to stabilize the continental climate—increasing rainfall, smoothing out winter-summer temperature extremes, and suppressing dust storms.

Unusual Wave Clouds over the Aral Sea

Unusual Wave Clouds over the Aral Sea (March 25, 2009)

Wave clouds typically form when a mountain, island, or even another mass of air forces an air mass to rise. The air cools as it rises, and if there is moisture in the air, the water condenses into clouds. Once the air has gone over the obstacle, it sinks again. The air warms as it drops, and the cloud dissipates. Like ripples on a pond, the wave motion continues, and the air rises and drops again and again until eventually the wave dissipates. The result is a line of clouds marking the crests of the wave, separated by clear areas in the troughs of the wave.

In this case, the western shore of the Aral Sea clearly created the disturbance needed to generate the initial wave motion. But what was the disturbance? There are several possibilities. First, the waves could be caused by a change in wind speed as the air moved eastward over the sea. Air moves slowly over land, slowed by plants, changes in elevation, and other rough features. When the air reached the sea, the surface it encountered was smooth, and the air near the surface would have moved faster. The air above it, however, would have continued to move at the previous pace. Imagine a jogger who unexpectedly steps on a patch of ice: his feet move forward faster than his head, and he falls back as his legs slip forward under him. The air experiences much the same motion. The lower layer of air (in contact with the smooth surface of the water) moves more quickly than the air above it, pushing the lower layer of air up. This motion could have created the wave that generated these clouds.

Another possibility is that the shoreline is elevated above the water. The Aral Sea has been steadily shrinking. What remains is the deepest portions of the sea. It is possible that the western shore is distinctly defined, and that there is a drop to the surface of the water. This change in elevation would cause the surface air to drop suddenly, which could also initiate wave motion. Both factors, the air speed and the shoreline, may in fact be working together to influence the shape of the clouds. Regardless of how they formed, the result is a beautiful fan of white over the emerald water of the Aral Sea.

Aral Sea Expansion

Aral Sea Expansion

Once the fourth largest lake on Earth, the Aral Sea has shrunk dramatically over the past few decades as the primary rivers that fed the Sea were almost completely diverted for cotton farming and other agriculture. The decline of the sea has produced severe environmental, social, and public health problems. As part of a last-hope restoration effort, the World Bank funded the construction of a dam to separate the smaller, but less polluted and salty northern Aral Sea from the southern part.

According to a recent article on the New York Times Website, since the dam’s completion in the summer of 2005, the northern Aral Sea has been filling up more rapidly than planners expected.

The most obvious expansion of the lake’s area is in the northeast, where a narrow channel from the main body of the lake connects to a widening pool of shallow, sediment-clouded water. The former shorelines of the lake appear paler brown than the surrounding landscape. A more subtle change is in the width of the channel that connects the main body of the lake to the lake’s most western part; the tip of the peninsula jutting down into the lake appears farther from the southern shore in 2006 than it did in 2005. Some ice remains at the western side of the lake in both images. A thinner strip of ice hugs the southeastern shoreline in the 2005 scene. At lower right in each image is the Syrdar’ya, the northern Aral Sea’s only tributary.