Thanks for the suggestions!
I just want to figure it out enough that it can be there, the event most distant in the past that is dealt with on this planet. It's what happens after it sets the stage that is most important.
I'll likely work on the idea of a massive belt of ice intersecting the planet's orbit long enough for this volume of water to accumulate in the atmosphere, but brief enough to overwhelm the people, killing them all. Muahhaha. Or right, inside voice.
Astrographer - My blog.
-How to Fit a Map to a Globe
-Regina, Jewel of the Spinward Main(uvmapping to apply icosahedral projection worldmaps to 3d globes)
-Building a Ridge Heightmap in PS
-Faking Morphological Dilate and Contract with PS
-Editing Noise Into Terrain the Burpwallow Way
-Wilbur is Waldronate's. I'm just a fan.
It just takes a lot more energy to warm up water. So depending on how much you dump in there, how fast, etc, etc, etc you could cause a nice dip in the temperature. I doubt the energy of the asteroids would compensate for the heat capacity of the water.
You say the world is a cold world. This could be the apex of a long growing ice age of the world. Most of the north and South hemispheres are under massive ice sheets, This will bring the sea level down a lot. Our last ice age reduced the sea level well over 300 m. Now the population are gathered mainly along the equatorial belt which has a cold temperate climate at best. This will explain the use of the sea for heat and power. Now you need some thing to happen that is quick but not so sudden, like an impact, that will destroy all things. I propose that the ice is crushing the continental plates together and now one of the mid sea faults fails and splits wide open causing a massive earthquake, tsunamis and volcanic action that cascades along all the major faults spewing forth magma, hot gases and a lot of other nastiness. This quickly heats up the seas and air melting the glaciers very quickly raising the sea level at an alarming rate. As the pressures dissipate the volcanic activity subsides over time but the big melt is unstoppable as the air and sea are warmer and greenhouse gasses are heating up the air. This disaster will take a few years to complete and devastate crops technology and displace and kill lots of people. All the water that was trapped in the ice is now flooding all the lowlands that were once coastline and if the land was low to begin with... Man that was long but fun.
More to think about - I like it, keep the suggestions coming!
OK...so even if they average r. was 20m across, I'd still need over a trillion of these things to connect.
BUT...I revisited my spreadsheet where I worked this stuff out....there's actually only a difference of 10% between the volume of ice in Antarctica. etc, and the amount I need. So an event that causes my more substantial ice caps (fitting in with my living-near-the-ocean plan) to form would work.
The trick is to have it happen in a catastrophic (i.e., number of hours) manner. But that much ice, no matter what, isn't going to melt that quickly.
Then again any flare of a magnitude that would melt the polar caps in a matter of hours is likely to incinerate the rest of the surface to a depth of several dozen miles, I imagine
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Assuming an impact speed of roughly 25 miles/sec (roughly 40 000 meters/sec) then the kinetic energy of that beastie will be about 800 000 000 joules / kg (KE = MVV). Assuming the ice starts out at -270C then it takes 1 130 000 joules to heat the ice to 0C, ballpark 330 000 joules of energy to melt it, 418 000 to raise it to boiling, and another 330 000 to turn the ice into steam. That's a grand total of 2 208 000 joules to take a kg of ice from interstellar temperatures to just over boiling. That only leaves about 798 million joules of energy to heat that kg of ice.
It's why ice meteors make an incredibly bright light and a huge shock wave as they are decelerated rapidly. No real worry about cooling things off by dropping rocks from orbit. The big problem is to keep from frying the planet.
A 10cmx10cm column of water weighs 1kg per 10cm. An 800 meter column over that area weighs 8000kg. 8000 * 800MJ (ballpark) gives a delightful stellar-grade plasma if you drop it all at once.