Climate Change and Society

New research published this month in the Proceedings of the National Academy of Sciences (PNAS) reveals a striking correlation between an extended period of extreme drought and large-scale settlement abandonment in North Mesopotamia at around 4,260 years ago. That period marks the decline of the Akkadian Empire, along with marked changes in other civilizations such as Egypt and Indus Valley.

Scientists have hypothesized that climate changes can cause societal changes and this study would appear to strengthen this argument. The researchers collected speleothem samples from caves on the Iranian Plateau, which is just east of Mesopotamia and, thus, down wind.

What they found were layers that showed significant increases in the accumulation of dust. These layers correlated to two separate events in time, but the most recent event, between 4,260 to 3,970 years ago correlated with “the period of abandonment of advanced urban settlements in northern Mesopotamia.” This second period was more extreme in magnitude and lasted longer than the first (290 years versus 110 years).

A speleothem is typically a stalactite or stalagmite. They’re formed as minerals precipitate out of water, often in caves, rockshelters, or cliffsides where weathering and erosion can’t outpace the chemical deposition. In caves, speleothem deposition allows for high-resolution data to be obtained from layers that are dated and analyzed in ways very similar to ice-cores. Often oxygen and carbon isotopes can reveal information about temperature and precipitation.

But in this study, the researchers were looking specifically at magnesium (Mg) and calcium (Ca) ratios in addition to the oxygen isotopes. What they found were data that showed large, abrupt changes in in the Mg/Ca ratio that indicated increased dust activity and temperature. Chemically speaking, increased Mg, which is an impurity, to Ca is an endothermic operation, meaning as Mg goes up, so does the temperature. The speleothems themselves were dated using Uranium-Thorium.

As mentioned earlier, the study revealed that there were two periods of climatic change consistent with extended periods of drought, with the second period being the more severe and longer lasting. And that second period also coincides with the abandonment of “urbanized settlements and structured economies” (in the words of the researchers), which shows signs of happening at around 4,190 years ago in the archaeological record. It wasn’t until about 3,900 years ago that we begin to see evidence of repopulation.

This abandonment is entirely consistent with the data, which shows that second period of aridity or drought beginning at around 4,260 years ago. The drought starts and within 70 years or so cities are no longer able to sustain themselves.

But it might not be the cause, however likely it might seem. It could even be one of multiple causes. Remember that earlier period of aridity and drought? It began at around 4,510 years ago and lasted for about 110 years. The latter event is described as more severe than the first, but 110 years is a long time to endure any sort of drought.

It could be that the society never fully recovered from the first event. Or perhaps the climatic rebound wasn’t enough to return things to normal. There may even be other considerations such as political, economic, or pathological factors (like disease in crops or people) that created added stressors, contributing to a wide-scale societal collapse.

SPELEOTHEM



The word “speleothem” was first described by Moore in 1952. At its root are the Greek words sp?laion, which means “cave” and théma, which means “deposit.”



Many speleothems are stalactites and stalagmites, which you might recognize as formations that either hang from the ceiling of caves like icicles or rise up from the floor like mounds. The way I’ve always remembered which is which is by remembering that “icicle” has a “c” like “stalactite,” and “mound” has an “m” like “stalagmite.”



There are other types of speleothems too. Flowstones, columns, rimstone dams, stone waterfall formations, various crystals, cave popcorn, cave pearls, calcite rafts, and snottites! Most of these are named for the way they look, and that last one, snottite, is not exception! It’s formed from bacteria that oxidize sulfur and has the consistency of–you guessed it–snot.

Further reading

Carolin, Stacy A., R. Walker, C. Day, V. Ersek, et. al (2019). Precise timing of abrupt increase in dust activity in the Middle East coincident with 4.2 ka social change. PNAS, 116(1), 67-72. https://doi.org/10.1073/pnas.1808103115

Middle East topographic map: Sémhur derivative work: Zunkir [CC BY-SA 3.0], via Wikimedia Commons