In the rings of ancient and gnarled trees, a team of scientists has found evidence of a period of consistent warmth and wetness in Mongolia between the years 1211 and 1225 -- the exact time that Genghis Khan first rose to power.
Coincidence? They think not.
This unusual stretch of mild temperatures and unprecedented rain in an area traditionally known for its cold and arid climate would have increased the productivity of grasslands in the Mongolian steppe, the researchers say. The abundant grass would in turn increase the number of grazing animals that could live off it.
Members of Khan's army reportedly had five horses apiece, which allowed them to swiftly conquer an enormous area that stretched from eastern Asia to eastern Europe, as well as parts of northern India and the Mideast. They also traveled with a herd of livestock that provided them with food.
"I think of it as nature set the table, and Genghis Khan came to eat," said Amy Hessl, a tree-ring scientist at West Virginia University, in a video describing the research. "He didn't have to come eat, and he didn't have to eat the way he did, but the table was set and it was a matter of whether or not the culture and the people capitalized on that."
Her colleague Neil Pederson of Columbia University's Lamont Doherty Earth Observatory put it this way: "Grass was the power of the day."
The story of the unusual wet period in Mongolian history was written in scores of ancient Siberian pine trees that Pederson and Hessl first sampled on a whim during a research trip in 2010. The trees were growing on a nearly soil-less lava field in central Mongolia, dotted with horse skeletons. The researchers said that because the trees are severely water stressed, they are especially sensitive to changes in the weather.
On that first trip, the two scientists took core samples from about a dozen trees. When they got them back to the lab, they discovered that some of those samples dated back more than 1,000 years. They also found evidence of a climate narrative that they couldn't ignore. For a small stretch of time, some of the rings were visibly thicker.
"I was just excited that we had wood from the time of Chinggis Khan," said Pederson, using an Asian name for the ruler. "And it was really the trees saying 'look how wide our rings are right here.'"
Pederson and Hessl returned to the site two years later to sample more trees. Those samples confirmed the initial findings: The expansion of the Mongolian empire coincided with what the trees recorded as a warmer and wetter climate than usual.
In interviews, both Pederson and Hessl said there was still more work to be done to definitively link the trees' narrative with human events. They are working with experts from disciplines including ecology, biology, and history to find more evidence of how the fortuitous change in climate may have contributed to the making of the largest empire in history.
"There are still a lot of unknowns out there," said Pederson. "The climate information in the trees is just one little piece of the puzzle."
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