How Climate Change Helped Decimate a 4,000 Year Old Megacity

More than 4,000 years ago, three civilizations dominated South Asia and North Africa. Ancient Egypt and Mesopotamia are names you'll surely recognize, but the lesser-known Indus Valley Civilization was actually the largest of the three. During its height, at around 2600 BCE, the Indus spread across what is now India and Pakistan, and built large cities like Mohenjo-Daro, whose population is estimated to have been well into five figures.

Around 1800 BCE, the Indus civilization began to decline, and all but disappeared by 1300 BCE. The reason has been the source of controversy for decades, but new research adds evidence to the theory that climate change led to a sharp weakening of the key summer monsoon season, which left the Indus river valley drier and inhospitable.

Tracking weather patterns from millennia ago isn't easy. The University of Cambridge research team first started by finding an ancient lake, called Kotla Dahar, that still existed in the Indus' time. The dirt at the bottom of an ancient lake doesn't offer many clues, but what it holds does: By identifying the species and chemical makeup of ancient snails buried in the former lake, the Cambridge team was able to calculate how much rainfall the region received thousands of years ago. The results are published in Geology.

They found that the paleolake in Haryana, India was a deep body of water between 6,500 and 5,800 years ago, which corresponded with a time of heavy monsoon action. But, in snail shells dating to around 4,100 years ago—right before the time the Indus when into decline—the researchers found an increase of an oxygen isotope, which suggests the lake was drying up due to a weakening of the summer monsoon.

"We think that we now have a really strong indication that a major climate event occurred in the area where a large number of Indus settlements were situated," study co-author Professor David Hodell said in a release. "Taken together with other evidence from Meghalaya in northeast India, Oman and the Arabian Sea, our results provide strong evidence for a widespread weakening of the Indian summer monsoon across large parts of India 4,100 years ago."

At the time, drought was spreading throughout much of Asia. "The 4.2 ka aridification event is regarded as one of the most severe climatic changes in the Holocene, and affected several Early Bronze Age populations from the Aegean to the ancient Near East," the authors write.

Such drought would certainly have had a destabilizing effect. And even given some wiggle room within the dates—again, dating isotopes of snail shells in ancient lake beds is a tall task—the authors argue such monsoon weakening corresponds with known times for Indus decline. "The resultant age of drying at Kotla Dahar is consistent with the suggested archeological dates for the onset of Indus de-urbanization within dating uncertainties," the authors write.

As you might expect, drought wreaks havoc on agriculture. Feeding a megacity, even an ancient one like Mohenjo-Daro, takes a strong farm sector, and without one, people will disappear. "Our paleoclimate record also provides indirect evidence for the suggestion that the ISM weakening at ca. 4.1 ka in northwestern India likely led to severe decline in summer overbank flooding that adversely affected monsoon-supported agriculture in this region," the authors write.

The Indus civilization collapse has remained a mystery for at least a century of archeological investigation, but the climate angle has been batted around for nearly that long. As V.N. Misra notes in a deep look at the subject, British archeologists Sir Aurel Stein and Sir John Marshall both posited in 1931 that the Indus lived in a far wetter climate, which was held as fact until the 60s, when an American team poked holes in previous evidence.

Since then, the evidence has largely been on the side of drought coinciding with the Indus collapse, although there have also been arguments to the contrary. Isotopic studies have provided more conclusive evidence. A 2003 study in Geophysical Research Letters also found evidence of drought occurring around 4,200 years ago. Combined with the most recent study, it's becoming more clear that while drought alone may not have caused the Indus collapse, it does appear to have helped push things along.

"We know that there was a clear shift away from large populations living in megacities," co-author Dr. Cameron Petrie said. "But precisely what happened to the Indus civilization has remained a mystery. It is unlikely that there was a single cause, but a climate change event would have induced a whole host of knock-on effects."

And guess what? Research in the last few years has shown that the current warming climate will likely lead to a decrease in India's monsoon season. A 2012 paper in Environmental Research Letters put it rather simply: "Indian monsoon rainfall is vital for a large share of the world's population," the authors write in their abstract, before noting that "monsoon failure is possible but very rare under pre-industrial conditions, while under future warming it becomes much more frequent."

Compounding the problem, Pakistani media reported last fall that researchers have modeled a decline in Himalayan glaciers, which means that rivers already feeling the effects of decreasing monsoon intensity could also have less snow melt to rely on. For the hundreds of millions of people in the region, the coming drought may feel a bit too reminiscent of the Indus' collapse for comfort. But there is one major difference: This time, the climate change is man-made.