A major man-made environmental disaster 1,800 years ago

Satellite picture of the Basin of the formerly sea Lop Nur in the Desert of Lop. (Credit: NASA. Mission: STS047, Roll-Frame 151 – 26. Color adjusting: Michael Gaebler; Source: Wikimedia Commons)

Water withdrawal for irrigation led to low levels or even desiccation for lake Lop Nur in northwestern China. It was probably the cause for the decline of Loulan Kingdom.

14.03.2017: The Aral Sea is considered a striking example for the impact of mankind to nature: Since water from its inflows has been used for widespread irrigation, the Sea is declining over time. The coastline moved for several tens of kilometres during the last decades. Such tremendous impacts due to human activity have happened in earlier times as well, according to a study by international authors including the GFZ German Research Center for Geosciences.

The team examined sediments from the Tarim Basin in order to figure out the decline of the Loulan Kingdom in northwestern China between the 3rd and 7th century CE. Previous studies suggested that deteriorating climatic conditions led to the abandonment of ancient desert cities. However, the likely cause is water withdrawal for irrigation in the middle reaches of rivers. This in turn caused water shortage downstream and the eventual decline of the lake Lop Nur, write the authors, among others Birgit Plessen from the GFZ’s section Climate Dynamics and Landscape Evolution, in Nature Scientific Reports. They state: “The decline of Loulan Civilization was not a consequence of climate change but probably the result of a major man-made environmental disaster comparable to the recent Aral Sea crisis.”

Flourishing civilization 2,000 years ago

Lop Nur was a salty closed basin-lake located in the eastern part of the Tarim Basin. Two millennia ago, when the climate conditions were generally relative wet, its area was an estimated 17,000 to 50,000 square kilometres. For comparison: The state of Brandenburg has nearly 30,000 square kilometres which ranges roughly in the middle of the estimated size of Lop Nur. Northwest of the lake, at the ancient Silk Road, the city of Loulan was founded lending its name to the Kingdom. Ruins of ancient cities, remnants of fields, and irrigation channels have been found in the desert region referring to a flourishing civilization during the Han Dynasty 2,000 years ago. But in the 3rd century CE the decline was emerging. Historical documents describe diminishing river discharge and subsequent reductions in soldiers’ grain rations after ca. 270 CE. More and more cities were abandoned at that time.

Intensified irrigation farming 

Scientists have suggested a regional climate change. However, the new study comes to another conclusion. The team analyzed sediments, eg. stable oxygen isotope analyses of bulk carbonates and ostracod shells, in order to describe the evolution of the lake Lop Nur. They found a dramatic drop of the water level. For the same period, other studies have shown relatively wet conditions adjacent to the site. Thus, declining levels of lake Lop Nur and other lakes most likely resulted from water withdrawal during the Han Empire through intensified irrigation farming along the mountain forelands, write the authors. Lake Lop Nor may even have desiccated in this time. Today’s climate in the Tarim Basin is dryer, the lake does not exist anymore. In 1921, it was filled for the last time and subsequently dried out in the early 1960ies. Later efforts to prevent further deterioration of the ecosystem by the help of water from Lake Bosten had no success.

The area of the Aral Sea has dwindeled to an eighth

The Aral Sea has experienced similar dynamics. The main inflows, Amudarja and Syrdaria, were used for widespread irrigation of cotton fields. The water level of the lake has fallen by about 25 meters, its area has dwindeled to an eighth. The remaining water gets more and more salty, the new land on the banks is a desert named after the vanishing lake: Aralkum. (rn)

Original study: Mischke, S., Liu, C., Zhang, J., Zhang, C., Zhang, H., Jiao, P., Plessen, B., 2017. The world´s earliest Aral-Sea type disaster: the decline of the Loulan Kingdom in the Tarim Basin. Scientific Reports, DOI: 10.1038/srep43102