But the tree rings also showed a sudden lurch into much colder, drier conditions around 1258, when the trees hardly grew. This was around the time the Mongol empire began to fall apart and the Mongols moved their capital into what is now Beijing. It was part of a global climate event, and a recent archaeological dig in London revealed that a catastrophic famine struck England at the same time, leading to thousands of deaths. The downturn in climate was caused by a massive volcanic eruption that blanketed the globe in ash and cut down sunlight across the world.
Plester, Jeremy. 2012. “Weatherwatch: Climate helped Genghis Khan create the Mongol empire”. The Guardian. Posted: September 23, 2012. Available online: http://www.guardian.co.uk/news/2012/sep/23/weatherwatch-climate-ghengis-khan-mongolia