People abandoned thriving cities in Mesopotamia, the Indus Valley and farther afield at about the same time as a decades-long drought gripped parts of the planet.
. They described a 20-centimetre-thick layer of grey sand-like pellets, mixed with other fine powders — a stark contrast to the thick loams from around 2300. “We didn’t have an explanation for what had caused it, but we knew from our local data that there was an aridification event,” says Weiss.
There are a number of difficulties, including how to interpret the inherently indirect data from climate proxies. And it is not always possible to precisely date a climatic shift or event. One of the best methods is uranium–thorium dating, which relies on the radioactive decay of uranium to thorium, and can give dates with an uncertainty as small as 1%. But if a stalagmite never had much uranium in the first place, or has been heavily contaminated with other substances, the dates become fuzzy.
, Scroxton and his colleagues studied a stalagmite from Madagascar and found evidence of drying for much of the period between 4,320 and 3,830 yearsfalls in that period, but the drought seems to have been drawn out — although not entirely continuous — rather than abrupt.These types of uncertainty regarding exactly how and when the climate changed in each region make it harder to draw links to changes in human societies.
It is also crucial to consider what aspects of a society have collapsed, says Alan Greaves, an archaeologist at the University of Liverpool, UK. “The palace system or that kind of thing, for certain, may collapse.” Societies such as the Maya stopped building monumental structures. But Greaves says we should not read too much into that.
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