By TrivBits, History Desk — Published September 8, 2026

Table of Contents

• Key Takeaways
• How Genghis Khan Killed Enough to Change the Climate
• The Science Behind the Carbon Dip
• Other Historical Die-Offs and Their Climate Impact
• What This Tells Us About Human Impact on Climate
• The Mongol Empire’s Lasting Environmental Legacy
• Frequently Asked Questions
• Sources

The Mongol conquests of the 13th century weren’t just a turning point in human history—they may have changed the planet’s climate. Genghis Khan killed an estimated 40 million people during his brutal campaigns across Asia and Europe, roughly 11 percent of the world’s population at the time. That staggering death toll didn’t just reshape civilizations. According to scientific research, it actually removed enough carbon from the atmosphere to measurably cool the Earth.

This grim environmental side effect reveals an uncomfortable truth: human activity has been influencing global climate for far longer than the industrial era. The rise and fall of ancient empires left marks not just in history books, but in ice cores and tree rings that scientists study today.

Key Takeaways

• Genghis Khan’s conquests killed approximately 40 million people, about 11% of the global population in the 13th century
• The massive depopulation caused farmland to revert to forest across vast territories, absorbing roughly 700 million tons of carbon dioxide
• This reforestation created a measurable dip in atmospheric carbon levels during the medieval period
• The Mongol Empire’s environmental impact demonstrates that pre-industrial human activity could influence global climate
• Scientists discovered this connection by analyzing carbon records from ice cores and historical population data
• The effect was temporary, reversing as populations eventually recovered and cleared land again

How Genghis Khan Killed Enough to Change the Climate

The Mongol war machine was devastatingly efficient. Between 1206 and 1368, the Mongol Empire grew to become the largest contiguous land empire in history, stretching from Korea to Hungary. Genghis Khan and his successors didn’t just conquer—they often annihilated entire populations who resisted. Cities that fought back were razed completely, their inhabitants slaughtered as warnings to others.

The numbers are almost incomprehensible. In regions like Persia and China, historical records suggest populations plummeted by half or more. Entire agricultural regions were depopulated. Irrigation systems fell into disrepair. Cultivated fields that had fed millions simply stopped being farmed.

Here’s where climate enters the picture. When farmers disappeared, their cropland didn’t stay bare. Nature reclaimed it. Across the vast Mongol Empire, forests grew where wheat and rice once stood. Trees absorbed carbon dioxide as they grew, pulling it from the atmosphere and locking it away in wood and soil.

The Science Behind the Carbon Dip

Researchers at the Carnegie Institution’s Department of Global Ecology studied this phenomenon by combining historical records with climate data. They examined periods when major population crashes occurred throughout the past millennium, looking for corresponding changes in atmospheric carbon levels.

The Mongol conquests stood out dramatically. The team estimated that about 700 million tons of carbon were scrubbed from the atmosphere as abandoned farmland across roughly 1.1 million square kilometers reverted to forest. To put that in perspective, that’s roughly equivalent to the world’s total annual gasoline consumption today.

Ice core samples from Antarctica and Greenland preserve tiny bubbles of ancient atmosphere. These samples show a small but measurable decrease in carbon dioxide concentrations during the 13th and 14th centuries. The timing matches the Mongol expansion and the depopulation it caused across two continents.

Other Historical Die-Offs and Their Climate Impact

The Mongol conquests weren’t the only population catastrophe to leave an environmental signature. The same research team identified several other events that created similar, though smaller, effects.

The conquest and disease-driven depopulation of the Americas may have had an even larger climate impact than the Mongol invasions. When European diseases killed an estimated 90 percent of indigenous populations, vast agricultural systems across two continents were abandoned. Some scientists argue this contributed to the Little Ice Age, a period of global cooling from roughly 1300 to 1850.

What This Tells Us About Human Impact on Climate

This research flips a common assumption. Most people think human influence on global climate began with coal-fired factories and automobiles. But civilizations have been altering Earth’s carbon cycle for millennia—just usually in the opposite direction.

Agriculture typically increases atmospheric carbon. When humans clear forests to plant crops, carbon stored in trees returns to the air. Rice paddies produce methane. Livestock farming adds greenhouse gases. For thousands of years before the industrial era, the slow expansion of farming across the planet was gradually warming the climate.

Population crashes temporarily reversed this process. The Mongol conquests created an accidental and involuntary carbon offset on a massive scale. Of course, this “cooling” came at the cost of unfathomable human suffering—hardly a climate solution anyone would propose.

The effect was also temporary. As populations recovered over subsequent centuries, forests were cleared again. The carbon they’d absorbed went back into the atmosphere. By the time the industrial revolution began, human activity was once again a net contributor to atmospheric carbon, but at an accelerating pace that dwarfed anything in the past.

The Mongol Empire’s Lasting Environmental Legacy

Beyond the temporary climate cooling, the Mongol conquests reshaped landscapes in ways that persisted for centuries. The destruction of irrigation systems in Persia and Mesopotamia transformed once-fertile regions into arid zones that never fully recovered. Some scholars argue that areas depopulated in the 13th century remained underpopulated well into the modern era.

The empire also inadvertently created nature reserves. Vast steppes that had been lightly grazed or cultivated were abandoned and became wild grasslands. Some of these regions remain relatively undeveloped today, preserving ecosystems that might otherwise have been transformed by agriculture.

The Mongol experience also demonstrates how tightly human population and land use are coupled to atmospheric chemistry. Every acre of forest cleared or regrown, every irrigation system built or abandoned, shifts the balance of carbon between earth and sky. Our species has been a geological force far longer than we typically acknowledge.

Frequently Asked Questions

How many people did Genghis Khan actually kill?

Estimates suggest the Mongol conquests killed approximately 40 million people, though exact numbers are debated by historians. This represented roughly 11 percent of the world’s total population during the 13th century. The death toll includes both direct casualties from warfare and indirect deaths from famine, disease, and societal collapse following Mongol invasions.

How much did the Mongol conquests cool the Earth?

The cooling effect was measurable but relatively small in absolute terms—a reduction of about 700 million tons of atmospheric carbon. This created a slight dip in global temperatures during the medieval period, detectable in ice core records but not a dramatic climate shift. The effect was significant primarily because it demonstrates pre-industrial human impact on climate.

Did the Black Death have a similar environmental impact?

Yes, the Black Death caused significant reforestation across Europe as the plague killed between one-third and one-half of the population. Abandoned farmland reverted to forest, absorbing carbon from the atmosphere. However, the environmental impact was more geographically concentrated than the Mongol conquests, which affected a larger land area across multiple continents.

Could depopulation ever be considered a climate solution?

Absolutely not. While historical population crashes did temporarily reduce atmospheric carbon, the human cost was catastrophic. Modern climate solutions focus on changing how we produce energy and use land, not reducing population through violence or disease. Sustainable technology and conservation offer ethical paths to carbon reduction that historical tragedies obviously do not.

Sources

• Carnegie Institution for Science – Global Ecology Research
• Encyclopedia Britannica – Mongol Empire History
• National Geographic – Climate and Historical Population