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Melting Icebergs

This week's experiment comes from a report I recently heard on National Public Radio. Unfortunately, I was driving and could not write down the fellow's name, so I could give him proper credit. He was talking about the facts and fictions of global warming. One point that he mentioned was one that I had heard many times and had never thought all the way through. What would happen if the global temperature rose enough for the polar ice caps to melt? All of that extra water would cause worldwide flooding, right? Lets investigate. You will need:

  • a glass
  • water
  • ice cubes

Try to get a large lump of several ice cubes frozen together. You can place several ice cubes into a bowl and leave it in the freezer over night and they should freeze together. Place the ice cubes into a glass or bowl. Add enough water to fill the glass to the top. Add as much water as you can, until the glass will not hold any more without overflowing.

Now, look carefully at the glass, water and ice. There is quite a bit of ice sticking up above the glass. What will happen when the ice melts? Now that you have formed a hypothesis (a scientific guess), watch to see what happens. Be sure that the glass is not bumped or disturbed. As the ice melts, does the water overflow?

No, it does not. Even when all of the ice has melted, the glass is just a full as it was when you started. As water freezes, it expands. It still weighs the same, but it takes up more space. This means that it will float when you put it into water. As it floats, the part of the ice that is underwater takes up exactly as much space as the water that if formed from took up. When it melts, it will take up that amount of space again, and so the glass does not overflow.

Back to what would happen if the polar ice caps melted, there is a big difference between the two polar ice caps. The North polar ice is all ice, floating in water. If you could selectively melt just the northern ice cap, sea level would stay the same.

The southern polar ice cap is not floating. Instead, it sits on the continent of Antarctica. If it melted, then the sea level would rise. The same is true for glaciers, and other ice that sits on land instead of floating in water.

The fellow that was giving the information said that some of the figures for global flooding overstated the rise in sea level because they failed to take into account that the northern ice would not change sea level. I have not been able to verify that, but I will keep my ears open.