At https://phys.org/news/2021-11-collapse-ancient-liangzhu-culture-climate…. … Liangzhu was located in eastern China in what was then the Neolithic in that part of the world. Thye didn't use metal. A bit like Britain at the same point in time. The city thrived for a thousand years and came to an end in the 3rd millennium BC. It had an elaborate water management system but suddenly collapse according to research published in Science Advances. Liangzhu is located 160 km from modern Shanghai, both on the Yangtze river, a major artery in China, then and now. Archaeologists have discovered evidence of hydraulic engineering. This includes a complex web of navigable canals, dams, and water reservoirs. The system allowed agriculture to thrive on the flood plain. Liangzhu came to an abrupt end. It was a bit of a mystery until the recent investigations. It had been noted a thin layer of clay covers the final phase of occupation. This suggested flooding from the Yangtze river or perhaps, even the East China Sea. Dates from stalagmites in caves show a period of high precipitation = between 4345 and 4324 years ago. A low growth tree ring event is located at 2345BC, we may note.
Gary sent in a link to www.msn.com/en-us/weather/topstories/ancient-cities-had-toxic-pollution-… … or www.popsci.com/environment/maya-city-pollution-algal-bloom … the ancient city was on Lake Amatitlan and new research published in PNAS says this early Maya city produced algae blooms much like modern Guatemala City does in the modern world. This, in turn, may lead to an explanation for the abandonment of the city, around one thousand years ago. On the other hand, as it coincided with the medieval warm period and a different track of the monsoon track others may beg to differ. Going with the flow, the evidence comes from the sediments on the bottom of the lake. Basically, they don't really know why Mayan cities were periodically abandoned. The algae bloom, may in part explain the abandonment of one of them, but hardly all of them.