Groan.

Via WaPo:

Was climate change responsible for the Mongol hordes?

First scientists told us that a “distinct drying” during the third century might have encouraged the fall of the Roman Empire, prompting critics to decry the idea as the“latest global warming scare tactic.” Now the journal Science is highlighting similar speculations that wet and warm conditions in the central Asian steppe in the 13th century help explain the exceptionally rapid expansion of the imfamously cruel Mongols under Genghis Khan.

Mongol horsemen relied on domesticated animals; Amy Hessl, one of the scientists involved in the research, explained to LiveScience in July that a single Mongol fighter required 10 horses, plus livestock that could keep up with the horde and provide food. Wetter conditions in the steppe would have encouraged grass and other plants to grow, providing plentiful grazing opportunities for all sorts of animals. When the climate became colder and dryer in the middle of the 13th century, the Mongol Empire splintered, and its rulers moved their capital away from the steppe to modern-day Beijing.

It is, in fact, because scientists deduced that plantlife flourished and then failed in Mongolia during this period that they can make these connections. Both the Mongol and the Roman hypotheses rely on the information researchers such as Hessl gathered from tree rings. Wider rings indicate more favorable growing conditions. Thinner ones indicate leaner times. Matching up tree rings with historical dates produces fascinating correlations.