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Can the oceans freeze (at least several feet below sea level)? I know that the temperature for salt water to freeze is lower/colder than fresh water, and when it does freeze, the ice is mostly salt-free, but I'm wondering if it's possible for the entire surface of the ocean to freeze over. Yes, I know about the land-bridge during our last ice age which allowed humans to cross from Asia into North America. So it is at least partly possible, but I'm wondering about ocean wide.

Also, if this is possible, what kinds of impacts would that have on the entire Earth's ecosystem? For instance, would all rivers/streams freeze over at this point?

Additionally, if the oceans could freeze, would that affect tectonic plate movement at all?

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  • $\begingroup$ Sorry @HDE 226868. I was going to edit my tags to include oceans and ice, but I see that you've covered my bases. Thank you $\endgroup$ – Matt Woodspirit Oct 13 '15 at 22:17
  • $\begingroup$ No problem. We don't have a tag for oceans, for some reason; I hope water-bodies is good enough. $\endgroup$ – HDE 226868 Oct 13 '15 at 22:19
  • $\begingroup$ If it gets cold enough, yes. $\endgroup$ – DA. Oct 13 '15 at 23:16
  • $\begingroup$ You're not expecting higher life on the planet to survive that, right? $\endgroup$ – Monica Cellio Oct 14 '15 at 19:40
  • $\begingroup$ @MonicaCellio in the case I was thinking of, such an instance would occur for a year or so and only happens once a decade $\endgroup$ – Matt Woodspirit Oct 14 '15 at 19:58
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Yes, the entire planet can freeze. It has happened at least once in Earth's history. Life at that time was all (maybe almost all) mono-cellular. We don't know how it survived - around volcanic vents is a decent guess, or perhaps the global freeze was short enough that the oceans diddn't freeze all the way down. We do know that this event marked the dawn of multi-cellular life. It's possible that the extreme difficulty of survival favoured cells that cooperated with other cells ....

There's a positive feedback mechanism. Ice is highly reflective. So as polar galciers spread, they cause the planet to cool. If they reach a certain critical extent, the cooling effect runs away and the entire planet's surface freezes. What unfreezes it? CO2 from volcanoes accumulates in the atmosphere, because there's no plant life on the surface to absorb it. CO2 is of course a greenhouse gas, so the planet warms until the ice melts and plantlife once again starts photosynthesizing using the CO2 as food.

On land it would be far worse than all rivers and streams freezing. It would be like Antarctica, everywhere: ice hundreds or thousands of feet thick. The last places to not freeze would be the ocean deeps and the places under the glaciers that are volcanically heated. However, it could only happen if the North pole became covered by a continent, before Antarctica moved away from the South pole. That will take a good few million years. Also it might not be possible today. The sun has become warmer over the last 600M years. Runaway global warming is probably a greater threat than runaway global cooling.

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  • $\begingroup$ Are you talking about the snowball Earth? $\endgroup$ – HDE 226868 Oct 13 '15 at 23:26
  • $\begingroup$ This is more speculative, so if you don't want to answer, or if it's too "discussion based," you don't have to. Would a human society that's largly medieval, be able to survive if such temperatures were to be had? Sure fires could be made/maintained. Wells could be dug for groundwater that's still liquid because of the Earth's Core temp. Hmm it'd still be an issue for other fauna and flora that cannot survive in the harsh temperatures/lack of freely available water. $\endgroup$ – Matt Woodspirit Oct 13 '15 at 23:50
  • $\begingroup$ Yes, snowball Earth. No, I don't think mediaeval humans could survive. There would be no food chain left to support them once the ice over the ocean was too thick for sunlight to penetrate. We'd survive an ordinary ice age, though. Humanity evolved during a series of ice ages. $\endgroup$ – nigel222 Oct 14 '15 at 7:56
  • $\begingroup$ "the planet warms until the ice melts and plantlife once again starts photosynthesizing using the CO2 as food". This optimistically assumes plantlife survives the freeze. Wouldn't it be nice if there some kind of automatically fully dormant photosynthesizing lifeform that went "online" as soon as it thawed? Like a guaranteed surviving species if the earth got too cold (from ice)? Not sure what the equivalent would be for too hot. Would it be safe too assume that too hot meant more and more water? $\endgroup$ – Anthony Jul 7 '18 at 1:31
  • $\begingroup$ @Anthony Cyanobacteria (a.k.a. blue-green algae) are the oldest form of photosynthesizing life, and they did survive the snowball Earth event. But you are right, multi-cellular plant life would not. $\endgroup$ – nigel222 Jul 10 '18 at 14:56
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I'll deal with the questions in reverse order, because your later questions are easier to answer.

If the oceans froze several feet deep (let's make the math easy and call it a meter - about 3.3 feet, fits the criteria), it would have no effect on tectonic plate movement. Let's think about the Americas moving west, into the Pacific Ocean. The Pacific Ocean is 165.2 trillion square meters, so we have 165.2 trillion cubic meters of ice in the way of the continents. This means we have a mass of about 1.5 quadrillion (1.5 * 10^15) kg of ice (note: I used the density of pure ice because the density of salt water ice depends on the level of salinity, so could change place to place). Which seems like a lot. But a ballpark estimate of the mass of a tectonic plate is on the order of 40 * 10^22 kg (http://www.answers.com/Q/How_much_does_a_tectonic_plate_weigh <-- not the greatest source, I know, but we're talking estimates here, and the math checks out). So the plates are still over a million times more massive than the ice. They're not slowing down.

As far as rivers are concerned, they'll freeze too. The oceans have a lot of water in them, and all of that water has thermal energy. As rivers have much less volume than oceans, they'll freeze way before the oceans do. How it would affect the overall ecosystem, I'll get to that in a second. Spoiler alert: it won't be good.

So, the million dollar question. Can it actually happen. Yes, though it's not much fun for anyone involved. It's similar to the Great Lakes freezing over (which has happened for individual lakes, but not the system as a whole - the record for the whole system is about 95.5% ice cover). You'd basically need temperatures to go below freezing, globally, for long enough that the water in the deeper parts of the oceans stop effectively warming the surface. Once heat transfer in the oceans slows down, the surface will start freezing (note: the coasts will freeze before the middle because of this, as there is less water near the coasts than in the middle of the ocean). Unfortunately, going back to the effect on the ecosystem, the temperature drop will freeze a lot of land water as well - not just the rivers, the water in dirt and such. So if it went on that long, the entire ecosystem would start to collapse - flora can't get enough water to grow, fauna can't get enough water to avoid dehydration, and eventually the food chain crumbles. And that's not even talking about marine life that suddenly has to get used to life in way colder, darker conditions. It won't be pretty.

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  • $\begingroup$ Thought I made a comment before. I guess it didn't post. Just wanted to thank you for taking the time to answer. Edit: The question: Would medieval humans be able to survive? They could dig wells for water that would be warmed by the Earths core, yes? And fires could be maintained. $\endgroup$ – Matt Woodspirit Oct 13 '15 at 23:47
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    $\begingroup$ They would be able to keep warm, yes. For a little while, at least. They could make fires, they could wear furs.... Basically like they're living in Siberia. Only the problem is that the rest of the food chain below them isn't doing so well. Trees have been known to explode when it gets cold enough that the water in them freezes and expands -- imagine what that would do to smaller flora that are used as food. And flora are the base of the food chain - if the flora start dying, the herbivores start dying, and then the carnivores start dying. Humans probably wouldn't last many years like that. $\endgroup$ – John Robinson Oct 14 '15 at 0:02
  • $\begingroup$ Yeah, that's what I was thinking once I saw your original comment. Just wanted to throw that out there. I think I need to head back to the drawing board and work out just how the logistics of this would work. $\endgroup$ – Matt Woodspirit Oct 14 '15 at 0:17
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Yes, all the oceans on the planet could freeze on the surface if it would become cold enough like it happens in the Arctic.

For the water to freeze, you need temperatures below 0°C, even on the equator. If the temperatures are cold enough for the ocean to freeze, all the other water bodies will also be trapped in ice. On the surface, it will become hard to find liquid water. You will need to dig under the ice to find some. Very quickly, much of the fauna will die, except creatures living underwater. On a longer period of time, even the fauna will die, leaving very few lifeforms alive on the surface.

Eventually, this will affect the food chain in a way that could also affect underwater species. They might not all survive.

Lastly, it will have no impact on the tectonic plates.

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Yes they can and they have too, in the past.

Such a period of massive glaciation is known as an Ice Age. Their history goes back at least to 2.2 billion years ago from now. It is not fully understood what triggers an ice age, but it certainly has something to do with carbon cycle, volcanic activity and the level of greenhouse gases in the atmosphere.

Ecosystems are shaken to the core. Previous genera (specially of mammals) are wiped out and new types of animals evolve which are (usually) larger than the normal type and are suited for icy climates. Plant life is affected likewise.

And no, ice ages (as far as we know) don't affect tectonic activity. During the last ice age, continental plates continued to drift.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Oct 14 '15 at 19:46

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