Did weather defeat the Mongol Empire?

Genghis Khan is perhaps one of the most famed leaders ever to have trod the globe, with the Mongol dynasty that he forged ruling over an empire that still rampages through the history books.

The gradual expansion of that empire, with its setbacks, motives, and evolving ambitions, has long fascinated historians. Now researchers have dug into an event that seemed to defy satisfactory explanation: the sudden withdrawal of the Mongol hordes from Hungary in 1242, following their blistering advance through eastern Europe.

A multitude of factors is thought to have catalyzed this retreat, but according to fresh research, published Thursday in Scientific Reports, environmental fluctuations were central to it all.

Using a single environmental factor to explain a historical event is "always tricky," Aaron Putnam, a professor at The University of Maine's School of Earth and Climate Sciences who was not involved with this study, tells The Christian Science Monitor. However, "the authors do a good job of making a solid chronological case that climatic deterioration played an important role in [the Mongols'] abandonment of the Hungarian Plains."

The Mongol thirst for conquest began in the early 13th century under Genghis Khan, continued by his son, Ögödei Khan, and grandson, Batu. By 1279, they had overrun huge swathes of Eurasia, including China, central Asia, Russia and Iran.

In the late 1230s, they were establishing their rule over Russia, creating what would be known as the Golden Horde, and in 1240 the Mongol armies exploded into Eastern Europe. Using a sophistication of military coordination unknown to the European natives, the Mongols crushed two armies: one Polish, one Hungarian.

In early 1242, the Mongols crossed the Danube into western Hungary, wreaked havoc for two more months, and then engaged in an abrupt withdrawal through Serbia and Bulgaria, back to Russia.

Why?

A plethora of explanations has been proposed, but, according to the authors, "none of these concepts – political, military and environmental – is fully satisfactory, and each has undergone extensive scrutiny and critique."

To search for new answers, the authors delved into documentary evidence and paleoclimatic analysis, particularly tree-ring chronologies. 

Written evidence indicates that the spring of 1241 was warm, with plentiful food and fodder for both troops and horses as the Mongols swept through Hungary and Poland. But winter struck early, with vigor: the Danube froze over, providing a bridge for the Mongol armies.

As the following year melted into spring, the accumulation of snow and ice caused flooding, creating marshland where before there had been solid ground. This not only played havoc on the movement of the Mongols and their horses, but it contributed to the onset of famine.

"The single most important element to the Mongol expansion was their reliance on horses," Dr. Putnam, who has undertaken similar research, tells the Monitor in an email exchange. "When they ventured into landscapes/climates not suitable for maintaining large herds of horses, their efforts began to falter." 

But as the authors of this paper note, evidence indicates there was sufficient pasture to support the Mongols' vast herds. The invaders left crops and agricultural land intact as they invaded Hungary and set up administrative systems, for example. 

Much of the Hungarian plains' soil is "particularly prone to stagnant moisture and ponding," according to the paper. Combine that with evidence of the brutal winter, the early onset of spring, and the subsequent thaw and snowmelt, and conditions would have dramatically reduced land for grazing. 

Combined with the ravages of war, these climatic conditions likely contributed to the failed harvest and Hungary's ensuing "great famine," reducing food for civilians and soldiers alike.

Other factors may have also influenced the Mongols' withdrawal, some academics warn, including political conflicts after the death of Ögödei Khan in December 1241. And literary sources "may have exaggerated the severity of the cold, as well as the lack of pastures," Morris Rossabi, a professor of Inner Asian and East Asian history at Columbia University, tells the Monitor. He describes tree ring evidence in Hungary as "unclear."

Yet Dr. Rossabi agrees that "the combination of depleted grasslands, due to the appearance of numerous Mongol horses, and perhaps the low temperature may have contributed to the Mongols' exodus." 

As for the authors themselves, they stress that perhaps the most important aspect of their work lies not with the specifics of this Mongol retreat, but rather the principle it highlights, one that should perhaps be given greater consideration when looking at major historical events.

"The 'environmental hypothesis' that we propose," write the authors, "for the first time suggests that the [Mongol withdrawal] may be the result of a general syndrome in which the effectiveness of nomadic armies was constrained by a short-term, regional-scale climate fluctuation."