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Even though the people on the planet have some influence over their ecology (by protecting their Kosmema gods and by killing us (both prevent excessive flooding in some way), they would need VERY good archives, or outside visitor prophetic warnings, to figure out the connection.

Needed:
One world-spanning empire/ trade network, lasting at least one full cycle (280 years)
Lots of enquiring minds in that empire / network to study the effects of the changing water levels in various parts of the world - as well as study what happens to the gods before and after the levels change.
Probably another half-cycle (at least) to affirm that the hypothesis was correct, and yet another cycle to start preparing for it the right way.

They´d be 700 years down the line before they figure out a protocol to protect their world.
By which times the empire has probably be overrun by barbarians seeking refuge from a part of the world that has been flooded and invaded by our species (which would interrupt trade networks over water in SO MANY ways).

This is going to be so much fun!