What can we learn from this complex history of unfolding polycrises?
We can start by observing how climate change (regardless of its source), pandemics, mass migrations, the hollowing out of the money supply, over-extended military commitments, the rise of new threats, declines in harvests and grain supplies, the hubris of ruling elites and extremes of wealth-power inequality all feed off of and reinforce each other.
Put another way, polycrisis is endemic to complex, interconnected systems. If the problems were limited to 1+1+1+1+1=5, the empire could maintain its coherence and adapt in ways to resolve the multiple overlapping crises.
But emergent systems--that is, complex, interconnected systems--are not just a collection of dynamics; the resulting polycrisis has its own dynamics and unique features that are distinct from the features of the five sub-crises. In other words, 1+1+1+1+1=15, and the system / empire is overwhelmed and collapses.
This is why polycrises are different from existential crises: the system could handle one, two or even three crises with its existing resources and structures, but a fourth anf fifth crises changes the nature of the threat.