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David Putnam David Putnam takes samples to analyze wood and a climate of fertility in the Chinese desert.
David Putnam takes samples to analyze wood and a climate of fertility in the Chinese desert. (courtesy of David Putnam)

by Anna Bisaro
Oct 28, 2013

In a period of less than 100 years, the Mongol Empire spread across Eurasia and encompassed the greatest contiguous land empire in history under the great Khan dynasty.

Historians generally attribute rising power and domination to leadership, new inventions and military strategy. But, some climate scientists credit the Little Ice Age for the Mongol expansion and take a new look at the impact climate can deliver on the rise and fall of civilizations.

“Human societies are quite sensitive to climate change,” said David Putnam, whose multiple fields of glacial geology, anthropology and archeology professor at the University of Maine give him a unique perspective. He argues that the Little Ice Age started it all for the Mongols some 800 years ago.


While touring the desert of northern China, Putnam and his son Aaron, now a climate scientist at Columbia University, found sand dunes once clearly deposited by water. But there is no water in sight now and hasn’t been for centuries. Yet they also found mud cracks and mollusk burrows. The father and son partners also stumbled upon wood samples – an odd occurrence for the middle of the desert. 

Anna Bisaro/MEDILL David Putnam shows how climate change impacts culture at Comer Conference in October.
Anna Bisaro/MEDILL
David Putnam shows how climate change impacts culture at Comer Conference in October.

More curious about when the trees germinated than when they died, the Putnams took samples of the wood to be dated back in the U.S. At what time did the climate of that region allow for such growth? 

The trip to the desert actually began as a trip to the Tarim Basin in northwest China where father and son were tracking when glaciers receded from that area. The pair had wanted to collect beryllium-10 samples from boulders in moraines. These samples could be processed back in the states and used to determine at what time glaciers had last covered those moraines. Beryllium-10 is an isotope that acts like carbon-14, except that it is created when  cosmic rays strike the atmosphere, releasing showers of secondary particles that strike the rock and then start to generate the isotope. Beryllium-10 collects once ice frees the rock, a way of clocking when the glaciers of the last ice age lumbered north. 

The boulders where they tried to collect samples had been eroded by sand storms and political disputes in the region made working there difficult, though. The team decided to give up on the field mission and travel south to the desert instead. 


The Little Ice Age was the last period of extreme cooling prior to the climatic conditions we know today. There is some dispute among scientists as to when the Little Ice Age began – somewhere between 1100 and 1300 A.D. – but it is more widely believed to have ended by 1850 A.D. Putnam supports the earlier start, related to the Mongol expansion, while Joerg Schaefer of Columbia University offers findings to support the later onset. 

“The Little Ice Age is notoriously hard to date,” Schaefer said in a presentation at the Comer Abrupt Climate Change Conference in October. Scientists can use “historical records, in the areas where you have them, but apart from that, moraines and other Little Ice Age features are hard to date,” he said.

A famine that spread death and crisis across Europe is widely recorded in historical records of the early 1300s and linked to the start of the Little Ice Age.

The isotope beryllium-10, Schaefer argues, has helped in the dating process. The isotope can’t help date when rock is exposed to air, meaning any ice has retreated. But it is still difficult for scientists to determine exactly when the period ended. 

But Schaefer is more confident about why glaciers started retreating between 1850 and 1900: the introduction of more carbon dioxide in the atmosphere due of industrialization. Levels of CO2, a greenhouse gas released with fossil fuel emissions, is linked to rising temperatures on Earth.     

Whether the Little Ice Age was a global phenomenon is still hard to determine. Schaefer’s fieldwork suggests that while the northern and southern hemispheres appear to have responded somewhat differently, glacial retreat occurred in New Zealand, Patagonia and the Swiss Alps all around the same time. The Tropics appear to show a pattern in sync with the northern hemisphere, but more research needs to be done to confirm those findings, Schaefer said. 

Schaefer said wants to expand his research on the Little Ice Age and explore how the advancement of glaciers starting in the 14th century affected various cultures. He said he wants to compare his climate change findings with historical accounts of famine and societal struggle to see if there is any link. Climate change, Schaefer said, has always affected humans. 


Putnam talked at the Climate Conference about the need to factor climate into the narratives of social and political change. Owen Lattimore, an American scholar of China and central Asia, hypothesized that changes in climate could account for the spread of nomadic cultures, for instance. Lattimore met resistance and his ideas were rejected by the academic world at the time. 

“People like the idea of a military innovation, a great man or other reasons,” Putnam said. “They didn’t go for this.” 

Putnam admitted during his conference talk and in an interview that many other factors contributed to the rise of the Mongol Empire. Genghis Khan, the man that unified the Mongols and led the first conquests in the 13th century, was a powerful leader, in Putnam’s eyes. 

With the rise of the Mongol Empire starting in 1206, the earth, as many scientists now believe, was experiencing a period of cooling before carbon dioxide levels began to bring global temperatures up to where they are today.  CO2 emissions, related to human use of fossil fuels, have risen steadily with the start of Industrial Revolution.

That cooling period, according to Putnam, lowered the snow line in the mountains of Asia and created a colder and wetter climate. The Mongols were oasis farmers and herders of livestock. Their crop production depended completely on irrigation systems and the change to shorter and colder growing seasons did not bode well for them. 

But, the wetter climate produced more grass where livestock could graze, particularly the Mongol’s prized horses, Putnam said. So while farming suffered, the livestock thrived, giving the Mongols a perfect storm for expanding the empire. 

Putnam argues, “The bottom line: if they didn’t’ have fuel for horses, they couldn’t go” colonize anywhere else.

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