When people talk about the Enhanced Greenhouse Effect, they often talk about sea levels rising. It's estimated that by the end of this century, the Enhanced Greenhouse Effect will cause the sea levels to rise by between 9 and 88 cm. Most people think the water will come from some of the ice melting at the North and South Poles. However, this isn't entirely correct. Although the ice at both poles will melt, the ice melting at the South Pole is a much bigger problem than ice melting at the North Pole. The difference is that the ice on the South Pole is sitting on land, but there is no land at the North Pole, so the ice there is floating. This week, we look at why it makes a difference:
You will need
2 glasses
Masking or "magic" tape
Pen
Some ice cubes
Some construction blocks.
Some Blue Tack or equivalent.
Water
This activity should be done in a bathroom, laundry or outside, as you may splash some water around.
My tower
Here's what to do:
Stick a bit of tape to the side of each glass. We will be using this to mark the water level.
Construct a little tower or platform using the blocks. Make it about two or three blocks high, small enough to fit inside the glass and with a flat top.
Put some blue tack on the base of the tower and stick in the bottom of one of the glasses.
Fill the glass with water until it is level with the height of the tower.
Place an ice cube on top of the tower.
Drop another ice cube into the other glass.
Pour water into this glass until it's at the same level as the first glass. The ice will float in the water, with a little bit sticking up out of the water.
Mark the height of the water in both glasses.
Come back after the ice has melted and look at the new water level.
Before and after the ice melted. If you roll your mouse over the "before" image, it will change to the "after" image, so you can see how much the water levels changed.
I found that the water level in the one with the tower had gone up, but the one where the ice was floating stayed the same.
The reason the water level in the one with the tower rose is simple: the ice melted and the water added to the water already in the glass. This is what the water at Antarctica does when it melts (and is usually compensated by more snow falling on the ice still there).
Some of the ice was sticking out of the water in the other glass as well, so didn't the same thing happen there too? It has to do with why things float and a weird property of water.
When you drop something into a liquid, it displaces some of the liquid, or it pushes it out of the way. This is why when you climb into a bath, the level of the water goes up. If the water that gets displaced weighs more than the object, the object will float. If the object is heavier than the water it displaces, it will sink. Objects floating on the surface have enough volume under the surface that the weight of the water it displaces is equal to the weight of the object.
The amount of water displaced by floating ice is the same as the amount of water produced when the ice melts, so there is no change in water level in the glass.
Most substances shrink when they freeze, but water is unusual because it expands, by about 10%. This is why ice floats. 1 kg of water takes up 1000 ml, but 1 kg of ice takes up 1090 ml. If you drop 1 kg of ice into water, it only has to displace 1000ml for it to float. This leaves 90ml of ice sticking up above the water. When the ice melts, it turns into 1000ml of water, so there is no change to the water level.
Like the ice in your glass, the ice at the North Pole is also floating in water. If you melted all the ice at the North Pole, you would annoy the polar bears but it would barely affects the sea levels at all. There would be a slight change, because the ice is fresh water and the sea is denser salt water. However, the effect is tiny. Melting 1 kg of floating ice increases the sea level as much as melting around 30g of ice on land
There is another way the Enhanced Greenhouse Effect may affect sea levels. Water expands slightly when it gets warmer. The change is tiny, only about 0.021% of its volume per degree, but there is so much water already in the oceans that it could cause more of a difference in sea levels than melting all the ice at the North Pole.
