China has some 5000 or so years of history, most of which have very detailed records. The current government took censuses, recorded astrological events, described major issues, and stored a massive library of literature that grew over time. And periodically, it collapsed and was replaced with a different one, the details of the rebellion also being added to the records. A historian reports that the records all had something in common, namely, they all occured during periods of local climate cooling.
Now, correlation isn't causation. Ice cream sales are linked to riots, but to claim that ice cream causes riots is insane. There's a common factor in those, namely the hot weather. (People buy ice cream during hot weather to enjoy the cool sensation, but the hot weather also makes them way crankier.) The historian believes that the colder weather caused more crop failures, which lead to social instability ("I'm starving! Why's that jackass eating?!?!") which lead to revolution. And conquest, as China's neighbors became more interested in conquest when their own positions were undermined.
If the historian's findings were correct, then threatened dynasties could have saved themselves with some sort of food relief program. Tragically for them, this never occured to them, and they now reside in the dustbin of history.