• Question: Why don't the oceans freeze?

    Asked by byrneg to Claire, Kate, Matt, Rob, Sam on 21 Jun 2013.
    • Photo: Kate Husband

      Kate Husband answered on 21 Jun 2013:

      Mainly because it’s not cold enough!
      The oceans are salty which means they wouldn’t freeze at 0C but at -2C and this happens in places like the Artic and Antartic where there is some sea ice. This sea ice is not actually that salty (you could melt it and drink it) so if more of the sea froze then presumably the sea would get saltier and harder to freeze.

    • Photo: Robert Woolfson

      Robert Woolfson answered on 21 Jun 2013:

      What Kate said is absolutely right but there is also a second part to this answer. Ice is lighter than water, so it floats. When ice is on the ocean, it floats. This basically protects the ocean water underneath it from loosing heat and so the water below the ice can’t freeze.

      It sounds a little odd but it’s true. When rivers and lakes freeze over, all the water underneath is still a liquid, it’s just the water on the top that freezes.

    • Photo: Matthew Pankhurst

      Matthew Pankhurst answered on 22 Jun 2013:

      Actually the oceans do freeze! It’s what helps to drive the ocean circulation and during at least two times through geologic time (Earth’s history) the oceans have completely frozen over (yet there was still some liquid water underneath)
      http://en.wikipedia.org/wiki/Snowball_Earth
      When oceans freeze during winter at the north and south poles, freshwater ice is formed at the top, and it floats, and the remaining water gets much saltier, because the salt can’t be part of the ice. This very salty water is heavier than the rest of the ocean water and sinks. It pushes down to the bottom of the ocean floor and travels along, making currents. These currents stir up the ocean and keep it moving around. If it didn’t move around as much, we’d have even colder polar regions and warmer parts around the equator. It’s a complex system, but the sinking of dense water at the poles is a very important part.

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