What is the relationship between dynasty change, climate change and population change in China history?

The climate change in ancient China occurred alternately in cold period and warm period. That is to say, a cold period is followed by a warm period.

When the climate enters the cold period, the northern boundary of crop growth gradually moves south, the arable land area of farmers in the north decreases sharply, and the multiple cropping index becomes extremely low, which easily leads to the contradiction between man and land. As the northern boundary of crops moves south, farmers must find more suitable land for farming, so a large-scale migration tide inevitably appears.

When the historical climate entered a cold period, social contradictions became more serious and dynasties changed frequently. On the contrary, when the historical climate enters a warm period, it is easier to have a prosperous time and manage the world. The most typical example is that the prosperous times of Han and Tang Dynasties in China all appeared in the warm period, while the troubled times such as Sanjin, Wuhu Sixteen Countries and Song and Jin Dynasties all appeared in the cold period.