Genghis Khan may have found the most brutal solution to global warming.

The Mongols conquered one of the largest empires ever starting in the 13th century AD. In doing so they also slaughtered the populations of many entire cities, and even whole civilizations, like the Khwarzm in what is now Kazakhstan.

Without people to farm the land, much of it reverted to forests. Those forests inhaled large amounts of carbon dioxide from the atmosphere, according to a paper published in the journal The Holocene. Those forests may have stockpiled 700 million tons of carbon dioxide, as much as the world's current use of gasoline produces in a year. 

Lead author, Julia Pongratz, and Ken Caldeira of the Carnegie Institution and colleagues at the Max Planck Institute for Meteorology in Germany analyzed historical records of land use and compared those with a global climate/ carbon cycle model to track how land use and historical events altered carbon dioxide levels.

The longer the duration of the human catastrophe, the more carbon was absorbed by new forests, Pongrantz found. The depopulation of the Americas after invasion by the Spanish, English and other European powers had a similar atmospheric impact to the Mongol conquests. But other shorter-term mass deaths didn't slow agriculture down enough to affect the atmosphere.

"We found that during the short events such as the Black Death and the Ming Dynasty collapse, the forest re-growth wasn't enough to overcome the emissions from decaying material in the soil," says Pongratz. "But during the longer-lasting ones like the Mongol invasion and the conquest of the Americas there was enough time for the forests to re-grow and absorb significant amounts of carbon."

"Today about a quarter of the net primary production on the Earth's land surface is used by humans in some way, mostly through agriculture," Pograntz said. "So there is a large potential for our land-use choices to alter the global carbon cycle.”

“In the past we have had a substantial impact on global climate and the carbon cycle, but it was all unintentional,” Pograntz said. “Based on the knowledge we have gained from the past, we are now in a position to make land-use decisions that will diminish our impact on climate and the carbon cycle. We cannot ignore the knowledge we have gained."

The current rate and scale of climate change are unprecedented in human history, and another study point out that if humanity does not heed the fate of the Greenland Vikings and the Roman Empire, civilization may be doomed to repeat the chaos of collapse.

Rome rose and prospered during a stable, warm, moist period in climate history, but collapsed during colder, drier, more variable times, according to research by a team of researchers led by Ulf Buntgen of the Swiss Federal Research Institute for Forest, Snow and Landscape, which Emily Sohn of Discovery News wrote about recently.

Climate Changes Linked to Fall of Roman Empire

An increase in climate variability from 250 AD to 600 AD, coincided with the spread of the Huns and migrations of Germanic tribes, the researchers said in a recent issue of Science. They studied preserved tree rings from that time to determine climate, a science called dendrochronology. During that period, trees had smaller rings, meaning less yearly growth and harsher conditions.

Historians corroborate the researcher's work. Edward Gibbon wrote in The History of the Decline and Fall of the Roman Empire that German tribes crossed a frozen Rhine River on December 31, 406. Others have noted that the freezing of the Rhine was speculation, but the research of Buntgen and his team at least show that the German migrations and invasions coincided with cold weather.

A warm period also coincided with the re-development of organized civilization in Europe, but another cold period in the 14th century may have aided in the spread of the bubonic plague, which thrives in cooler conditions. At the same time, Viking settlements in Greenland disappeared. Cold weather may have made agriculture and rearing livestock impossible on that giant, ice-covered island.

Atlantic Circulation On the Fasttrack for Change

But wait, isn't the problem now that the climate is getting warmer? Shouldn't Europe be happy that the climate is getting balmier? The problem is that warmer average temperatures don't mean warmer temperatures everywhere.

The Sensitive Seasons of Europe

Europe's pleasant climate is caused by an ocean current that brings warm water from the tropics north. But as the waters in the Arctic get warmer, the temperature difference that drives that current is breaking down. Other research has shown a significant decrease just since the 1970's in the strength of one of the currents involved.

“Those who cannot remember the past are condemned to repeat it,” said the Spanish-American writer George Santayana. These two studies show that the rise and fall of human civilizations are often linked to the climate, and that human activities can affect the climate.

IMAGE 1: Vasily Maksimov (1844-1911): Mongols at the Walls of Vladimir; Wikimedia Commons.

IMAGE 2: Mongol troops; Wikimedia Commons.

IMAGE 3: Joseph-Noël Sylvestre: The Sack of Rome by the Barbarians in 410; Wikimedia Commons.