Why did the Aral Sea shrink?

The Aral Sea shrank because water was taken out of the two rivers that feed it, mainly to irrigate cotton and rice. Beginning in the 1960s, diversions in Uzbekistan, Kazakhstan and neighbouring states cut the inflow sharply, and a lake with no outlet cannot survive that. A 2007 review by Philip Micklin states that the recent desiccation owes overwhelmingly to the expansion of irrigation that drained its two tributary rivers. [1] [2]

The Aral Sea is a terminal lake, meaning it has no outlet. Snow fields and glaciers in the mountains feed the Syr Darya, which flows into the northern part, and the Amu Darya, which flows into the southern part. Rain over the sea is low, under 200 millimetres a year, while 1,000 to 1,200 millimetres evaporates annually. The water level therefore depends heavily on river inflow. [3]

A 2014 satellite study estimates that before the 1960s net evaporation was nearly 56 cubic kilometres a year and was roughly balanced by river inflow of about the same amount, so the sea level and coverage were stable. Once irrigation projects were built, evaporation far exceeded inflow. The study puts the yearly deficit at over 10 cubic kilometres in the 1960s and about 30 in the 1970s and 1980s. [4]

NASA reports that the sea received about 50 cubic kilometres of fresh water per year as recently as 1965, a figure that fell to zero by the early 1980s. The 2014 study gives a somewhat different estimate of river inflow, about 43 cubic kilometres a year in the 1960s, dropping below 10 in recent years, citing Micklin. The two sources differ in the numbers but agree on the direction and cause. [1] [4]

The sea was once the fourth largest lake in the world. A 1988 paper in Science found its level had dropped nearly 13 metres between 1960 and 1987, with its area down 40 percent. Micklin's later review reports a 23 metre fall in level, a 74 percent loss of area, a 90 percent loss of volume, and salinity rising from 10 to more than 100 grams per litre. The sea split into a northern and a southern part. [5] [2] [6]

The 2014 study checked precipitation data for the region and found no obvious long-term decline, though it varied from year to year, from lows below 50 millimetres to highs over 250 millimetres. The authors conclude that climate is not the dominant driver and that falling river inflow is the main cause. Micklin notes that the lake has repeatedly filled and dried over the past 10 millennia from both natural and human forces, but attributes the current desiccation overwhelmingly to irrigation. [4] [2]

The North Aral Sea has stabilized, while the South Aral Sea kept shrinking and becoming saltier. A dam between the two parts, completed in 2005, lowered salinity in the north, helped fisheries recover and restored ecological stability there. [3] [4]