How did Chinggis Khan and his successors conquer much of Eurasia, and how did the Mongol conquests change the regions affected?

In the mid-twelfth century the Mongols were just one of many peoples in the eastern grasslands, neither particularly numerous nor especially advanced. Why then did the Mongols suddenly emerge as an overpowering force on the historical stage? One explanation is ecological. A drop in the mean annual temperature created a subsistence crisis. As pastures shrank, the Mongols and other nomads had to look beyond the steppe to get more of their food from the agricultural world. A second reason for their sudden rise was the appearance of a single individual, the brilliant but utterly ruthless Temujin (ca. 1162–1227), later and more commonly called Chinggis Khan (sometimes spelled Genghis or Ghengis).