I'm trying to figure out what sort of mechanism might <within reason> increase water sea levels on a fictional planet by 800m around the globe. In the end, the planet ends up a typical mishmash of climate zones. Most of humanity is destroyed.

That works out to 2.8E+19L of water for my planet. (I'm not worried about salinity levels - though it's something I should think about)

I'm ok with the water slowly,over thousands of years, receding back to "normal" levels, but it's not necessary.


I've thought about meteors (I think those are the ones made out of ice) hitting, etc, but the size involved would destroy the planet. Even if thousands smaller did so, they're still too large.

I'd like the solution to at least be in the realm of plausibility...not something "magical", if that makes any sense.