How climate change contributed to the collapse of the Roman Empire and 7 other ancient civilizations

The Roman Empire has been on the mind lately. But for all its greatness, we can't help but wonder what happened to it.

The Roman Empire is lauded for its population and geographic size, with 75 million citizens at its peak and extending from northern Britain to the edges of the Sahara. But the more impressive a civilization, the more of a spectacle it becomes when it collapses.

The Roman Empire had everything going for it — interconnected cities, a universal currency, highways, libraries, and even a functioning sewage system.

A collection of things contributed to the collapse of the Roman Empire. However, researchers are now suggesting that climate change contributed to the collapse.

The Roman Empire benefitted from warm, wet, and stable weather that allowed abundant crops and economic success. When volcanic activity grew and led the world into the "Late Antique Little Ice Age," the Roman Empire began to lose its foothold.

The ice age led to low crop yields, famine, and poor health. It also made areas of the Roman Empire less hospitable and more open to invasion. Famine and poor health in the interconnected, colder areas of the Roman Empire also made it ripe for a plague to spread.

Changing weather introduced new diseases, and Rome dealt with three different plagues: smallpox, the Plague of Cyprian, and the bubonic plague.

Plagues, famine, and invasion all befell Rome as the weather shifted, contributing to its downfall.

The Vikings first settled in Greenland after Erik Thorvaldsson, known as Erik the Red, was exiled from Iceland for manslaughter in around AD 985.

Soon, a group of Vikings lived in Greenland for about 465 years, from 985 to 1450. But suddenly, they disappeared, leaving behind their homes and communities, and in the 15th century, signs of Norse habitation disappeared from the geological record.

The newest leading theory is that climate change was a major contributing factor. A study by Marisa Borreggine, a doctoral candidate in the Department of Earth and Planetary Sciences at Harvard University, found that from 1100 to 1400, rising sea levels could have flooded Viking settlements by as much as 11 feet, affecting 78 square miles, a space 1.5 times the size of San Francisco.

The reason for the rise in sea levels was not a heating period that melted glaciers and caused the sea level to rise, but the Little Ice Age.

The Little Ice Age caused the Southern Greenland Ice Sheet, the nearest to Norse settlements, to grow and weigh down the land. As a result, the land was filled with water. The ice sheet even grew so large that its gravity pulled the ocean near it.

Though rising sea levels might not have been the sole reason for leaving Greenland, it was certainly a major factor when compounded with social unrest, scarcity of resources, and other political factors.

Angkor was the capital of the Khmer Empire in Cambodia. It was labeled the "hydraulic city" for its utilization of local water sources to build a series of canals, reservoirs, dykes, and basins.

At its peak, an estimated 750,000 people lived there and covered 400 square miles. Research has begun pointing toward intense climate change as a significant contribution to the collapse of Angkor.

Because Angkor relied so heavily on water, researchers at Columbia University looked at the tree rings of evergreens in Cambodia to understand changes in precipitation.

They discovered that in the century before it collapsed, Angkor encountered two severe "megadroughts" lasting 20 to 30 years, each followed by intense monsoons. The combination of extreme dryness and intense rainfall caused catastrophic flooding.

"We're talking about dry spells the likes of which we've never seen in modern history," co-author of the study, Brendan M. Buckley, told Columbia Magazine. "And then, the skies open up and the rain won't stop."

The Indus Valley civilization, one of the largest and most populous ancient civilizations, potentially fell due in part to climate change.

The Indus civilization developed 5,600 years ago and, at its peak, is believed to have held 10% of the world's population in the culture that expanded over 386,000 square miles, more extensive than ancient Egypt and Mesopotamia.

About a century ago, archeologists began finding evidence of old cities. Evidence suggests the cities were developed with grids and evidence of advanced plumbing, not seen again until Ancient Rome. Most notably, there was a startling lack of large buildings for royalty, which was typical of civilizations at the time.

With a civilization so populous, the mystery of why so many cities and buildings were abandoned seemingly overnight has gone unsolved for decades.

By reconstructing the landscapes of plains and rivers where these ancient civilizations once settled by analyzing satellite images of the area and collecting sediment samples.

Through these methods, the research teams could determine what crops were planted and when, how landscapes changed, and how that affected settlements.

Many of these cities were built on rivers fed by monsoon rains. As the climate changed and the monsoons became less frequent, crops and water sources were less reliable, facilitating a move east towards a reliable water source.

This shift devastated the Indus Civilizations, which relied on significant surplus years. As a result, larger cities collapsed as more minor, localized communities and economies started performing better.

Tikal is believed to have been founded in 600 BC, with some of its first buildings dating from AD 250 to 900. At its peak, Tikal had over 60,000 citizens, was the economic hub of Mayan civilization, and occupied a land mass about the size of Texas.

Recent research points to the catastrophic effect of drought on the Mayans. Researchers used four detailed records of past climate change obtained from three nearby lakes and a stalagmite on a cave floor, developing a model of the balance between evaporation and rainfall.

The results point to intense droughts lasting for a decade, mainly from decreased summer rain activity. The Mayans had bet on summer rains to aid their farming practices, so the effects were catastrophic when rainfall was reduced by even a modest amount.

Layered on top of the drought, research published in 2020 suggests that Mayan water sources were contaminated with toxic algae and mercury. The Mayans built their city in a way that aimed to capture as much rainwater as possible in centralized reservoirs in the city. However, their frequent use of cinnabar, a mercury-based ore, mixed with the accumulated rainwater and found its way into the reservoirs, turning them poisonous.

The toxic algae, which was resistant to high temperatures from boiling, resulted from smoky fires and washing ceramic plates in the reservoirs, imbuing the water with the organic material needed for the algae to bloom. According to co-author of the study, Kenneth Tankersley, "The water would have looked nasty. It would have tasted nasty. Nobody would have wanted to drink that water."

Compounded with persistent drought, lack of drinking water, diminishing crops, and proliferating disease, the Mayans met the perfect storm for the collapse of a civilization.

The remote island of Easter Island, also known as Rapa Nui, is 2,000 miles west of South America and has long been a cautionary tale of resource management.

For many years, the leading hypothesis for the collapse of the Rapa Nui civilization was attributed to the deforestation by the people, which led to cannibalization, warfare, and societal decline, as Jared Diamond detailed in his 2011 book "Collapse."

However, with technological advances and methodology changes in the last 20 years, different hypotheses have come to light. Instead of the Rapa Nui causing deforestation, studies believe rats brought over by European settlers might have caused it.

Deforestation by rats and the introduction of diseases from European settlers in 1722 created a combination of events that contributed to the demise of the Rapa Nui. The population of Rapanui dropped to 111 in 1877, not because of cannibalism but because of slave traders, Sapiens Anthropology Magazine reported.

The traditional narrative of "ecocide" done by the Rapa Nui has continually been contested in the past two decades as newer research has pointed to these other factors.

Cahokia was a city in what is now Illinois, with about 20,000 people. It was one of the largest cities and the cultural and political center of Mississippi River cities.

Cahokia was established around AD 700 and thrived for three centuries before its collapse. The collapse of Cahokia is attributed to changing weather periods. Cahokia began during the warmer "Medieval Climatic Anomaly," and its collapse was during the Little Ice Age.

Previous theories surrounding the collapse of the Cahokia suggest that the indigenous members overharvested wood, causing erosion and flooding. However, research conducted on minerals found in lake sediments offers insight into the changing temperatures and water levels.

The patterns are similar to other ancient civilizations that grow communities on river beds — intense periods of drought, followed by rain, causing flooding.

Before the Inca Empire took control and spread along South America, the Tiwanaku civilization lived and prospered in what is now Lake Titicaca, Bolivia, from AD 550 to 950.

The Tiwanaku were estimated to have 50,000 agricultural fields and irrigation technology, allowing them to maintain production despite the weather.

Despite their fields and irrigation, research published in 2020 suggests that a period of drought from AD 915 to 1025 and arid conditions contributed to the collapse of the Tiwanaku around the last century of their civilization.