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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.

Overview

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.

Proposal

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.

  1. 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.

  2. 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.

Question

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?

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  • $\begingroup$ Get "Q" to change the planck constant ofthe universe? $\endgroup$ – pojo-guy Aug 11 '17 at 14:21
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Take a look at this physics stack exchange question. The freezing point of water depends on pressure. If the atmospheric pressure was greatly decreased, then the melting point of water will change as well. If we lost atmosphere to a cataclysmic event, or if the force of gravity was somehow reduced, using technology, magic, or by losing mass of the earth, then water would actually transition directly from a solid to a gas at -40 degrees given the reduced pressure.

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  • $\begingroup$ You'd need an intense pressure drop at both poles for this to occur, but I like the theory. $\endgroup$ – Twelfth Oct 10 '14 at 20:52
  • $\begingroup$ This is pretty much the obvious way to do it - the only other case I can think of, but haven't got the chops to make an answer out of, is inserting trace amounts of various elements and simple chemicals into seawater over time, gradually shifting the melting point lower. Trace amounts to prevent detection - I don't know if this is chemically possible however. $\endgroup$ – mechalynx Oct 10 '14 at 21:03
  • $\begingroup$ @ivy_lynx: The problem is that anything that happens gradually would not have the desired sudden impact. $\endgroup$ – Wrzlprmft Oct 10 '14 at 21:05
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    $\begingroup$ For reference, -40C is 233.15K. At that temperature, there's nowhere on that phase diagram where water will be liquid, but, as mentioned, you cold potentially have ice sublimate directly from a solid to a gas. $\endgroup$ – Caleb Hines Oct 10 '14 at 22:16
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    $\begingroup$ However, it looks as if the atmospheric pressure would have to be approximately .00001 (one hundred thousandth) of its current value. Obviously, breathing would be a major issue. $\endgroup$ – Caleb Hines Oct 10 '14 at 22:20
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This is a really interesting solution to the "deep freeze" conundrum. As you say the state change from solid to liquid of that much water would suck massive amounts of warmth out of the planet. I know you didn't ask this but I want to quickly talk about the results, as you may be disappointed.

The effects it would have

It would also have interesting effects on sea levels, sea life, and lots of other cases.

There are a few glitches in the plan though, firstly that a lot of Antarctica is already below those temperatures so would not melt. Secondly the state change of some of the ice would cool the other ice around it, slowing down the conversion. Essentially you have a negative feedback loop that will put the brakes on the process (although it would still continue).

The other catch is that you would get most of the cooling effects at the poles - which are already cold. The tropics would not be affected at all (until sea levels started rising). Ocean currents would be heavily disturbed by the sudden influx of fresh water at the poles though.

How it could happen

As to how it could happen - that's basically impossible with current knowledge of physics.

One possible "plausible if you don't poke too closely" explanation is something like the reverse of Ice-nine. Ice nine is a (fictional) polymorph of water that melts at 42 degrees Celsius rather than at 0 degrees. In contact with water it acts as a seed crystal and converts all the rest of the water into also being Ice-nine, causing it to instantly freeze.

You could hypothesize something similar here. A comet containing some special polymorph of water goes past and we go through its tail. Some of the water polymorph reaches the ground scattered all over the globe and immediately starts a chain reaction converting all other water it finds into the same less stable form as itself that only melts at -40 degrees.

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  • $\begingroup$ I suppose you wouldn't want to be catching falling ice-nine flakes on your tongue! $\endgroup$ – user2338816 Oct 11 '14 at 6:59
  • $\begingroup$ @user2338816 Yep, that would be lethal. $\endgroup$ – Tim B Oct 11 '14 at 15:26
  • $\begingroup$ @TimB - I can see the immidiate affect being a dramatic cooling event at the poles, which should greatly lower pressure bringing about 2 massive lows on either end of the globe. I'll have to go lookup if there are any previous examples of this in our recorded weather history, but it should result in a pretty massive storm at each pole...I might have to lookup what happened when the Icelandic low was at it's most massive. $\endgroup$ – Twelfth Oct 15 '14 at 18:07
  • $\begingroup$ Alas, I would have liked to write the polymorph answer. The thing is, every polymorph of water would show this reaction of converting normal water on contact. For the interested reader: During the 1970s polywater was a suggested polymorph of water which builds in capillaries with a much lower freezing point and a much higher boiling point. The story has one obvious flaw: While a new polymorph of water may even spontanously form, the effect will transform the Earth to a lifeless planet, so climate change is the least of the problems. $\endgroup$ – Thorsten S. Aug 11 '17 at 10:09
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As others noted, there's no real physical effect that does this. However let's look at how a fictional effect might work.

Water can be supercooled to quite low temperatures, that means, there does exist a liquid state of water; indeed that liquid state still exists at -40°C. That supercooled state is, however, very unstable, that is, the water will freeze with the slightest disturbance.

So a fictional cause for the desired effect would have to make the supercooled water state the preferable state at that temperature, that is, reduce the energy of that state, or increase the energy of the ice state.

The trick to do this should be the weakening of the hydrogen bonds, which are responsible for the water anomalies (including the high melting point).

However to do that, you'd need to leave the field of hard science.

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  • $\begingroup$ The melting point is per definition the point where the preferable state switches from solid to liquid. So if the supercooled state is the preferable state at some temperature, this temperature is per definition above the boiling point. Also said supercooled state is not supercooled anymore but a regular liquid state, as a supercooled liquid is per definition a liquid at a temperature below the melting point. So, you are de facto proposing to lower the melting temperature of water, which is where we started. $\endgroup$ – Wrzlprmft Oct 11 '14 at 21:31
  • $\begingroup$ Moreover, supercooled liquids do not freeze, because they require a cristallisation seed for this. One very good cristallisation seed is ice. $\endgroup$ – Wrzlprmft Oct 11 '14 at 21:32
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The equations for the energy needed to melt ice assume that we must raise the temperature of ice above the melting point. You are proposing to lower the melting point below the current temperature of the ice. It therefore already has the energy it needs to melt. So the glaciers and all but the coldest part of the poles will turn to water, not instantly but fairly quickly.

Short-term effects will be sea level rise, massive flooding anywhere downstream of mountains with glaciers (search for "Calgary 2013 floods" for an example) and an upheaval of nearly every important ocean process.

How you could do it without also making major changes to the earth like removing the atmosphere (which would disrupt the experiment) will require a change to some fundamental laws of nature. Your idea of deuterium won't work - the physical properties of deuterium are close enough that you can drink heavy water for a week with no ill effects (2 weeks is a problem). You're going to need the assistance of an omnipotent deity for this. One with a mischievous streak - Q immediately comes to mind.

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  • $\begingroup$ Which he will have unless the melting point is lowered to the ice temperature. -20 ice, -40 melting point. SHould be enough latent thermal energy sitting there. $\endgroup$ – paul Oct 11 '14 at 13:16
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I know this is old, but it's surprising no one mentioned salt saturation. Water containing the maximum soluble amount of NaCl freezes at -21 C.The maximum salinity that can be dissolved is 27%.

The oceans are around 3.5% salt. The dead sea is more salty than this level, at 34% salinity.

By Wilson44691 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=18790421

So you can get halfway there by using conditions that actually exist on earth.

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