The Mongol empire in the 13th century conquered great swaths of Asia, the
Middle East and even parts of Europe at staggering speed, but how did Genghis
Khan and his armies manage to conquer so much and so fast? The answer may lie
in some ancient dead trees found recently in an old volcanic lava flow in Mongolia. The trees were so
well preserved that their annual growth rings were still visible and gave an
astonishing insight into the climate of the 1200s. The wood rings were spaced
wide apart showing that the trees grew well, thanks to plenty of rain. And
because the trees did well, the chances are that the grasslands of the vast
Mongolian plains also grew lush in the wet climate. Those rich grasslands would
have fuelled the Mongol armies, giving plenty of grazing land for the thousands
of horses that the troops relied on, and livestock to feed the soldiers.

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.

http://www.guardian.co.uk