The 4.2-Kiloyear Event May Have Helped Cause The Collapse Of The Egyptian Old Kingdom And The Indus Valley Civilization

With these methods, as well as modeling, we can reconstruct what oxygen levels were like in the Jurassic, for example, or temperatures during the Pleistocene. Looking back through these records, scientists have found evidence of an anomalous event taking place in our recent past, geologically speaking. 

Known as the 4.2-kiloyear event, because it took place 4,200 years ago in our past, the event is thought to have lasted for an entire century, and may have played a role in the downfall of several ancient civilizations, including the Old Kingdom in Egypt and the Akkadian Empire in Mesopotamia.

The first evidence, discovered in the early 1990s, was found by archaeologist Harvey Weiss and his team, as they dug at a site in northeast Syria. Looking at soil samples, the team found a dusty, barren layer from around 2200 BCE, which showed no signs of any activities of worms within it. Something had altered the climate so badly during that time that a layer of inhospitable dust had built up, likely over the course of decades.

“We didn’t have an explanation for what had caused it,” Weiss told Nature in 2022, "but we knew from our local data that there was an aridification event."

This event happens to have occurred not long before the collapse of the Akkadian Empire, which dominated the region until around 2150 BCE, when cities were abandoned and the empire was no more, with Weiss suspecting that the aridification event may have been the cause.

The data since then, be it from stalagmites in a cave in the Himalayas, or a thin layer of clay on ancient Chinese ruins, has suggested that this may not have been a local event. 

"A thin layer of clay was found on the preserved ruins, which points to a possible connection between the demise of the advanced civilisation and floods of the Yangtze River or floods from the East China Sea. No evidence could be found for human causes such as warlike conflicts," Christoph Spötl, head of the Quaternary Research Group at the Department of Geology, explained in a statement following the study of the Chinese ruins. "However, no clear conclusions on the cause were possible from the mud layer itself."

Though it remains controversial, with some arguing that these were all local events, or confined to certain regions, the International Commission on Stratigraphy declared the 4.2-kiloyear event as the beginning of a new era, known as the Late Holocene Meghalayan Age.

So, what caused it? Given debate is still going on as to whether the event was global, that is still tricky to say for sure. However, some scientists have linked it to changes in activity of the Atlantic meridional overturning circulation (AMOC). 

"The cooler [Northern Hemisphere] and warmer Southern Hemisphere (SH) illustrate that this event could be related to the slowdown of the Atlantic Meridional Overturning Circulation (AMOC)," one team, who used models and experiments to come to their conclusions, explain in their 2019 study. "The comparison between the all-forcing experiment and the single-forcing experiments indicates that this event might have been caused by internal variability, while external forcings such as orbital and greenhouse gases might have modulation effects. A positive North Atlantic Oscillation (NAO)-like pattern in the atmosphere (low troposphere) triggered a negative Atlantic Multi-decadal Oscillation (AMO)-like pattern in the ocean, which then triggered a circum-global teleconnection (CGT)-like wave train pattern in the atmosphere (high troposphere)."

According to that team, it was this that led to the drought. While further study is needed to establish whether this was truly a global event, it does appear to have had some role in the downfall of perhaps several civilizations. If it is truly tied to activity in the AMOC, that may not be the best news.