5 Ancient Societies Brought Down by Environmental Shifts

While climate change dominates modern headlines, fluctuating temperatures and weather trends have existed for thousands of years, impacting human civilizations throughout history.

Our current awareness has encouraged experts to re-examine the collapse of ancient cultures, identifying climate change as a key factor in their decline. Below are five civilizations that were either obliterated or significantly hastened toward their end by environmental transformations.

The Khmer Empire // Cambodia

Each year, more than 2 million visitors explore the remains of the Angkor Wat temple complex in Cambodia. Constructed in the early 12th century CE, this temple was the spiritual heart of the Khmer Empire.

Historically, it was thought that the Khmer Empire fell after the Ayutthaya Empire (present-day Thailand) invaded its capital in 1431. However, researchers from the Greater Angkor Project at the University of Sydney argue that climate change was the primary cause of its downfall.

Similar to other parts of the Khmer Empire, Angkor functioned as a 'hydraulic city,' relying on an intricate system of reservoirs and canals to sustain its population of 1 million. Around 1300, global temperatures declined, marking the start of the 'Little Ice Age,' which persisted until the 1800s. Tree ring data indicates that the Khmer Empire endured droughts interspersed with severe monsoons, which clogged its water systems with silt. As the city's population declined, maintaining the irrigation network became unfeasible.

Social unrest erupted due to crop failures and water scarcity. The population turned to Buddhism, power struggles erupted among princes, and the Ayutthaya Empire seized the chance to invade. The city's fall in 1431 marked its final collapse.

The Mississippian Culture // United States

Cahokia, once the largest city of the Mississippian culture, is now a UNESCO World Heritage Site preserved by Illinois as the Cahokia Mounds State Historic Site. It lies just a 15-minute drive from present-day St. Louis.

From approximately 800 to 1500, the Mississippian culture thrived across the southeastern and midwestern United States. This civilization was renowned for constructing intricate earthwork mounds, establishing extensive trade networks, and developing corn-based agriculture, which prospered during the Medieval Climate Anomaly—a period of unusually warm and wet conditions. Cahokia evolved from a small farming village into a significant political and religious hub, housing tens of thousands of inhabitants.

However, the onset of the Little Ice Age brought its downfall. Core samples from nearby lakes reveal that rainfall patterns shifted westward, disrupting corn cultivation and causing the Mississippi River to overflow. Cahokia experienced severe flooding around 1150 CE.

After the flood, archaeological evidence points to heightened civil unrest, marked by the construction of defensive palisades and the burning of villages. By the 1500s, when Europeans arrived, the once-thriving Mississippian settlements had been reduced to ruins.

The Maya Civilization // Central America

Spanning modern-day southeastern Mexico, Guatemala, and Belize, the Maya Civilization is celebrated for its architectural achievements, hieroglyphic scripts, and astronomical knowledge. While some remnants endured into modern times, the Maya experienced a political collapse around 900 CE.

The Maya Civilization consisted of approximately 60 city-states, each governed by a religious and political leader known as a k’uhul ajaw. Depending on the area, inhabitants cleared forests and employed wetland or dry farming methods, alongside developing advanced irrigation systems. This efficient system allowed them to flourish across Central America—until a severe drought struck.

Evidence of this climatic shift comes from stalagmites in Belize's Yok Balum Cave. These formations grow with water, and their size reflects rainfall levels, offering precise data on precipitation over the past 2000 years. Stalagmite records indicate that the Maya thrived during an exceptionally wet era, and their decline coincided with one of the region's most devastating droughts.

Relations among Maya city-states were often tense, and the drought escalated conflicts into warfare. Civilians fled, and trade routes shifted from land to sea to avoid the chaos. Within a few generations, the once-thriving cities were nearly abandoned and forgotten.

The Indus Valley Civilization // Pakistan and India

The Harappan culture established itself in the Indus River Valley, now part of Pakistan and India, around 3300 BCE, giving rise to the Indus Valley Civilization. They flourished in the region, thanks to exceptional urban planning that utilized the river's annual floods through advanced water management systems, while also implementing drainage to protect urban areas from flooding.

Approximately 4200 years ago, the Indus Valley Civilization faced one of the most significant climatic events of the modern era—the 4.2-kiloyear event. A shift in ocean currents weakened monsoon rains, preventing them from reaching far enough inland to flood the Indus River. This forced the civilization to abandon their cities and relocate closer to the coast. Archaeologists suggest this disruption also affected trade networks between Ancient Egypt and the Middle East.

Proof of this climatic event comes from a stalagmite discovered in India’s Caves of Meghalaya. The 4.2-kiloyear event was so impactful that in 2018, the International Union of Geological Sciences formally designated the current geologic era as the Meghalayan Age.

The Akkadian Empire // Iraq

The Akkadians, often regarded as the world’s first empire, governed from 2300 to 2150 BCE. Centered around the lost city of Akkad, their civilization extended across Mesopotamia, now part of modern-day Iraq [PDF].

The 4.2-kiloyear event, which severely affected the Indus Valley Civilization, also struck the Akkadian Empire. Iraq experiences a northwesterly wind called a shamal, which carries sand from Jordan and Syria, often resulting in sandstorms. These winds can reach speeds of 40 miles per hour, depositing large amounts of sand on roads. For the Akkadians, the colder and drier climate led to longer and more frequent shamals. Marine sediment cores from the Gulf of Omar reveal significant silt deposits, which harmed agricultural productivity. Archaeological evidence shows northern cities were abandoned, and clay tablets document a mass migration to the south.

However, the Akkadians attributed their downfall to divine punishment. They believed their leader angered the gods, who then sent the Gutian people to destroy them as retribution. Yet, like the Khmer Empire, the invaders were not the primary cause of their collapse. Climate change had already sealed the empire’s fate.