2. how did climate change force the mongols south?

It became dry and the warm climatic conditions damaged the steppes and green pasture lands which made the Mongols to move towards China.

Explanation:

The tree ring data provide the necessary historic information that the Mongolian steppes were in the grip of intense drought and the mongol tribes competed for survival in those dry climatic conditions which prevailed thousand years ago. But during 1211 -25 it is observed that Mongolia had experienced an unusual rainfall  which filled the atmosphere with humidity and moisture.

This climatic conditions had made the dry steppes to fro into grasslands which fed the mongol' vital herds of  horses. It is observed that every mongol had around five horses which paved way for creation of huge cavalry force of Mongol Army. this change in climatic conditions gave rise to Genghis khan to expand his first ever huge land empire.  

the authority to tax.