In this undated photo released by National Palace Museum, a portrait of Mongolian chieftain Genghis Khan belonging to Taiwan's National palace Museum.

London - Genghis Khan’s murderous rampage across Asia may have been triggered by unusual weather, research suggests.

Historians used to think his armies of nomadic horsemen were fleeing the bleak, cold and dry Mongolian plains for warmer regions. But academics from Columbia University in New York have discovered the rise of the Mongol Empire – from 1211 to 1225 – coincided with the warmest and wettest weather in 1 000 years.

They think this boosted grass growth that propelled his cavalry across the planet.

Researcher Amy Hessl said: “Unusual moisture creates unusual plant productivity, and that translates into horsepower.” - Daily Mail