This question comes up from Is there a man-made or natural event that can cause an abrupt climate change within hours/a day? – trying to get KaguraRap a functional answer to an abrupt temperature drop that would catch humans off guard. I’ve got a functional outcome I think, but I need reasons for why this would happen.
The Earth’s climate heavily depends on the formation of ice to regulate the climate's temperature. You have a little too much energy (too hot), and some ice melts. You don’t have enough energy and ice forms, releasing energy which turns into heat. This relation works so well because the amount of energy required to melt ice is extremely high. End result is an exceedingly efficient buffer that regulates the amount of energy in our climate system (and ultimately how hot the globe is). Key point here is the melting of ice takes a large amount of energy out of the climate.
Ignore the reason as to how or why this would happen for a moment, but lets say an event occurred that changed the melting point for water from 0 °C down to −40 °C. The specific heat capacity of water is 4.181 J/g/K while the enthalpy of fusion is 334 J/g. Quick version (point out math errors if they exist please), but this means the same amount of energy required to melt ice would warm the same mass of water by around 80 degrees.
So the event happens, water now freezes at −40 °C and starts a massive meltoff as if it were ice in 40 °C + weather.
The amount of energy taken from the environment to melt the ice is really large. Basically −334 J per gram melted. This should have an incredibly quick cooling effect over the globe, and one that is felt worldwide.
The melting ice on land should have an immediate effect on global sea levels, creating the opportunity for some coastal flooding. The mass drop in temperature would kick up storms as the temperature rapidly drops and the energy redistributes, so a “hurricane” (or rather a nor’easter as it wouldn’t have the warm core a tropical storm does) would create a freeze/flood event for a city such as New York.
KaguraRap – I think I have a scenario that will create the rapid temperature decrease and give a Day-After-Tomorrow-style flood to your story.
And to everyone else I’ll pose the question – what could possibly affect water to drop its melting point like this? I know salt and other solvents would do it, but not quite to the scale required here. A degradation of deuterium (the hydrogen isotope in heavy water) maybe? I’d take presence of dark matter traveling through it as well. Alien technology that could inject something into ice worldwide?
In particular, how could the reduction of its melting point be done abruptly, instead of over a long period of time?