1) Water expands when it freezes making it less dense than the water from which it freezes. In fact, its volume is a little over 9% greater (or density ~ 9% lower) than in the liquid state.
2) Therefore both ice shelves and icebergs float on the sea surface.
3) As they float, they displace the same volume of water that they contain.
4) So if they were to melt, the sea level would stay exactly the same.

Test it yourself - Expansion of water as it freezes

1. Half-fill a plastic bottle with water, screw the cap on tight and mark on the side where the water level comes up to.
2. Put it in the freezer overnight.
3. Check the bottle the next day to see how much more space the ice takes up than the water did!

This expansion is why your water pipes sometimes burst in the winter if they freeze!

Test it yourself - Melting ice doesn’t change water level

1. Put 3 ice-cubes into a glass or container of water and mark the level of the water on the side (you can do this with some sticky tape if you don’t wish to mark the glass with a pen).
2. Allow the ice-cubes to melt and see what happens to the level.

It shouldn’t move a millimetre!

Further Information
Just because melting ice shelves and icebergs have no effect on sea level doesn’t mean that there isn’t actually a problem. If global warming causes large areas of Antarctic ice-shelf to break off and melt, the sea level won’t be affected; but what could happen is ice and snow covering the continent will no longer have anything holding it in place. If this slides into the ocean and melts, then the sea level will rise. It has been estimated that if the Antarctic ice sheet melted, it would raise sea levels by around 65 to 70 metres! So global warming could have catastrophic effects!

Salinity & Freezing Point
At the bottom of the deep sea, the temperature can drop below zero (to -2oC), so why is it that this water isn’t frozen? The answer lies in the salt content (salinity) of sea water. Salt actually lowers the freezing point of the water; the more salt, the lower the freezing point. Normal seawater freezes at about -2oC; that’s 2oC colder than pure water.

Test it yourself - salt content changes freezing point of water

1. Add some water to a jug from the tap.
2. Take an ice cube tray and fill the first space with this pure tap water (1).
3. Now, in the next space - add half a teaspoon of table salt. Then add water from the jug and stir gently to dissolve the salt.
4. Repeat this in the next space in the tray, but this time use a whole teaspoon of salt.
5. Put the tray in the freezer and leave for about 30 to 40 minutes.
6. Take the tray out and see which cubes have started to freeze.
7. Return the tray to the freezer and check on the progress every 20 minutes or so.

You should see that (1) freezes first, followed by (2) and then (3).

This is why we put salt on icy roads in the winter. The salt lowers the freezing point of the water and keeps it melted.

Temperature and density
In the Open Ocean module, you saw that as warm water reaches the polar regions, it cools and sinks (known as ‘thermohaline circulation’), thereby setting up the ‘Global Conveyor Belt’ current. But why does this happen? The answer is that cold water is more dense (heavier) than fresh water, so as the ocean loses heat in the poles, the water gets colder and colder. Finally it gets so cold and dense it sinks to the bottom. It flows along the bottom and gradually comes up in different parts of the ocean to complete the conveyor belt circulation.

Test it yourself - cold water is heavier than warm water

1. Pour 50ml water into a container, add some drops of food colouring and mix well.
2. Pour this water into an ice cube tray and leave overnight in the freezer.
3. Fill a glass container with warm water from the tap and add your coloured ice cubes to the top.
4. Watch to see what happens!

You should see the cold, coloured water from the melting ice cubes sinking down through the warmer water and forming a layer on the bottom.

Salinity and density

Another reason that cold water sinks at the poles is because of its salt content. When water freezes at the polar regions, salt is rejected from the ice as it forms, and is released into the water beneath. This means that the ice is less salty than the seawater it froze from! The water below the ice becomes saltier and denser (heavier), and starts to sink.

Test it yourself - salty water is heavier than fresh water

You’ll need the following:
A container that you can see through the sides (e.g. plastic sandwich box).
A piece of cardboard.
Scissors.
Water.
Salt.
Food colouring.

1) Take the container and cut the cardboard so that you can divide the container in two across its width.
2) Make sure the cardboard is as good a seal as you can get it.
3) Measure out two equal amounts of water.
4) In one, add the food colouring and add several teaspoons of salt. Mix until the salt has dissolved.
5) Pour the coloured salt water into one side of the container and clear fresh water into the other side. You must pour the two lots of water at the SAME TIME, otherwise the cardboard divider may dislodge.
6) You should then see the salt water on one side of your clear container and the normal water on the other.
7) Remove the cardboard divider slowly. Be careful at this point!

As the two water bodies meet, you should see the coloured salt water slump to the bottom and the clear fresh water rise on top. There will be some mixing between the two but this is most likely due to the removal of the cardboard.

What does this tell us?

This is actually one of the first documented oceanographic experiments EVER and was conducted in the 16th Century. What it shows clearly is that the salt water is more dense than fresh water.

It the salt water is colder too then it becomes even more dense. This is why water from the poles, which is enriched with salt and is colder, sinks to the bottom of the ocean.