Genghis Khan was a great military leader. The Mongol Empire that he built conquered large parts of Asia. The empire he started 800 years ago became the largest one in history. Now, a team of scientists believe they've found a reason for his military success.

The weather was just right during the period he rose to power. It was particularly wet and warm between 1211 and 1225.

The scientists discovered the weather of Khan's period through trees in Mongolia. By cutting into ancient trees, scientists can find out about the past. At the center of a tree is its core. Starting there are small rings. The rings get bigger from there. By looking at the rings, scientists can tell how old a tree was. The rings can also tell them about the weather during certain time periods.

Mongolia is known for its cold and dry climate. Yet, the temperatures in this period were warmer and rain was heavy. These conditions would have allowed for grasslands to grow better, the scientists say. And more animals could graze and live off the land.

Eastern Asia To Eastern Europe

Members of Genghis Khan's army reportedly had five horses each. Having so many horses allowed them to swiftly conquer a huge area. When one horse got tired, they could ride another. Khan's army also traveled with a herd of animals. These animals provided them with food.

The Mongol Empire stretched from eastern Asia to eastern Europe when it was at its peak. It even included parts of northern India and the Mideast.

"I think of it as nature set the table, and Genghis Khan came to eat," said Amy Hessl. She's a tree-ring scientist at West Virginia University. "He didn't have to come eat, and he didn't have to eat the way he did, but the table was set." She said what mattered is that Khan took advantage of the lucky weather.

Hessl studied the trees with Neil Pederson of Columbia University. He put it this way: "Grass was the power of the day."

The story of the unusual wet period in Mongolian history was found in ancient Siberian pine trees. Pederson and Hessl first discovered them during a trip in 2010. The trees were growing on a lava field in central Mongolia. The field had nearly no soil. Horse skeletons were scattered around. The trees in this field are in severe need of water. That causes them to be especially affected by changes in the weather, the scientists said.

Warmer And Wetter Weather

On that first trip, Pederson and Hessl took samples from the centers of about a dozen trees. They got them back to the lab. There they discovered that some of those samples dated back more than 1,000 years. They also found evidence of a climate history. 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 (Genghis) Khan," Pederson said. "And it was really the trees saying 'look how wide our rings are right here.'"

Pederson and Hessl returned to the same field two years later. They sampled more trees. Those samples agreed with the original findings: The spread of the Mongol Empire happened along with what the trees recorded as a warmer and wetter climate than usual.

Pederson and Hessl still plan to do more work. They need to link the trees' story with human events. They are working with experts in science and history. They're hoping to find more evidence. They want to be sure about how the lucky change in climate may have helped Genghis Khan build his empire.

"There are still a lot of unknowns out there," Pederson said. "The climate information in the trees is just one little piece of the puzzle."]]> experts conditions severe conquer graze advantage ancient affected climate evidence