Human history is full of stories of collapsed civilizations, whether they were settlements that existed for a few centuries and then vanished or prosperous empires that achieved incredible feats of science and technology.

While there are often political or social reasons civilizations fail, sometimes the cause isn’t anybody’s fault. Sometimes, thriving civilizations collapsed simply because their local climate changed. [intro song] The Akkadian Empire once stretched across modern-day Syria and Iraq.

It was the first ancient empire of Mesopotamia. and it lasted for nearly two hundred years. But sometime around 2150 BCE., it collapsed. The reason for this collapse was mostly unknown until researchers in the 1970s began studying an archeological site called Tell Leilan.

The site is located in northern Syria, and used to be under the control of the Akkadian Empire. The Tell Leilan site holds the ancient remains of a city that was inhabited between 2700 to 2200 BCE, then uninhabited for about 300 years, and then inhabited again beginning around 1900 BCE. And since those 300 years where the city was deserted coincide with the collapse of the Akkadian Empire, it piqued a lot of people’s interest.

When researchers dug into the soil around the city, they found a 20 centimeter deep layer of gray sand-like pellets, bracketed by layers of thick, loamy soil. They dated that layer of sand to 2200 BCE, around the time the settlement became abandoned. This was confirmed when another study looked at deep sea cores from the nearby Gulf of Oman.

The sediment that makes up deep sea cores originally comes from land, and ends up in the ocean either by floating along rivers or landing on the surface as wind-blown dust. Since sediment builds up  really slowly in the ocean, deep sea cores can act as a physical timeline  that lets us study huge time periods. And the cores from the Gulf of  Oman showed an abrupt increase   in land-based dust from around that time,  which was likely from the Akkadian Empire.

Wind-blown dust tends to  increase when areas dry out,   leading scientists to suspect that a  severe drought took down the Empire. Really puts new meaning into the phrase “dust in the wind,” huh? While drought is a common cause for the collapse of many ancient civilizations, sometimes the problem isn’t the drought itself, but what happens afterwards.

That’s likely what caused the collapse of Angkor, the ancient capital of the Khmer Empire, located in what is now Cambodia. In its prime, Angkor was an architectural wonder, with a population of nearly a million people and a land area roughly the size of Los Angeles. You can still visit the city’s temple complex, Angkor Wat, today, and the city itself is an active archaeological site.

Unfortunately, the city collapsed in 1431, which historians thought was due to an invasion by outsiders. But the real story may have been buried underground for centuries, hiding in ancient pollen. Researchers looked at drill cores taken from two meters below the Earth’s surface at Angkor Wat, and they were able to look at preserved pollen grains, which let them reconstruct how the environment of the city had changed across centuries.

See, looking at the pollen tells you what kinds of plants were growing in the area, and that’ll change based on whether an area is being actively farmed or whether nature is taking its course. These pollen grains suggested that the collapse of the city didn’t happen all at once, so an invasion no longer made sense. It looked more like the civilization gradually collapsed over time.

And there was even more evidence of this slow decline hiding in tree rings from trees that were alive during Angkor’s collapse. Trees grow rings every year. During dry years, trees grow more slowly, so the rings tend to be narrow.

And during wet years, trees grow faster, so the rings tend to be wider. The Angkor tree rings showed evidence of a long multi-year drought in the middle of the 14th century, but it wasn’t the drought alone that brought about the city’s collapse. After the drought, changes in the strength of the seasonal monsoons brought about a lot of rain.

This region of the world is already pretty rainy, so we’re talking huge amounts of rain, enough to cause major floods. To make matters worse, the floods were followed by more drought. Angkor had a pretty robust water capture system, but researchers believe that the shifts between drought and flood conditions may have overwhelmed it, making people desert the city for a more stable climate.

Another civilization likely collapsed during the 14th century, although it’s hard to tell exactly when they vanished, because they were located so remotely that no one could contact them for centuries. These were the Greenland Norse, a group of Vikings that settled in Greenland sometime around 1000 CE. The settlements were thriving for centuries, before they lost contact with the rest of the world.

In 1721, after more than 200 years of not hearing from the Greenland settlers, missionaries tried to reestablish contact with them, only to find crumbling villages and ruins. Theories about this collapse included everything from the Black Death to piracy, but today researchers think the collapse was caused by climate change. See, researchers were able to reconstruct the climate of Greenland from the oxygen isotopes found in Greenland lake sediments.

The two most common isotopes of oxygen are 18O and 16O , which is much more common  in nature than its sibling. Because 18O is heavier than 16O, it requires more energy to evaporate and circulate within the water cycle. So it moves through that cycle more in warmer, higher energy periods of time, and during warmer periods, there’s more 18O in the rainfall.

And that mean that if we look at the ratio of oxygen isotopes present in things that were around in a given time period, we get an indication of what the temperature was at that time But how do you check up on those old isotopes?

Simple: Bugs! See, insect larva get most of their water from rainwater, so the 18O in their bodies roughly correlates to the 18O of the rainwater during their lifetimes. Researchers were able to calculate the oxygen isotope ratio from fossilized insects, giving them a proxy for the temperature at the time the bugs had lived. This reconstruction showed that between 900 and 1400 CE, roughly when the Norse settled Greenland, Greenland was 1.5 degrees Celsius warmer than normal.

The warmth would have made Greenland look like a great place to settle and build a civilization, luring the Norse in. But this was a trap, because after 1400 CE, temperatures got a lot colder. Now this cooldown isn’t isolated to Greenland.

It happened all over the Northern Hemisphere, and is known as the Little Ice Age. During the Little Ice Age, the Northern Hemisphere got about 0.6 degrees Celsius colder. And Greenland was hit by the cold particularly bad.

The average temperature dropped by 1.5 degrees C, and stayed there for about 400 years. Unfortunately, the Greenland Norse weren’t really equipped to deal with that kind of cold. Although evidence shows they did their best they ultimately weren’t resilient enough to handle the changing climate.

None of us were around to witness the collapse of these three civilizations, which means that figuring out what happened to them isn’t exactly straightforward. To fill in these knowledge gaps, scientists have to become detectives, looking through the clues left behind to figure out what happened to people hundreds to thousands of years ago. From deep sea dust to ancient trees to fossilized bugs, chemical and biological clues can tell us so much about the people of the past.

And sometimes, those clues can even tell us about the forces that brought down empires. Thanks to our patrons over on Patreon for making videos like this possible. Our community of patrons helps us keep the lights on and the cameras rolling, and we couldn’t make our videos without you all.

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