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Q & A: melting icecaps

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Most recent answer: 11/17/2007
Q:
When ice is placed in a glass and water poured in up to the top of the glass(almost overflowing) the ice will melt but the water level will fall because water is denser than ice. I recently asked my chemistry teacher why the same isn't true for the ice caps melting into the ocean. She said something about temperature but i am still confused. Is the temperature causing the water to expand?
- Steven Groves (age 15)
Kingston,Jamaica
A:

First, let’s deal with the glass and then with the very different ice cap problem.

Actually, the liquid water level in the glass will not fall, because part of the ice was sitting above the liquid. The volume of the melted ice is less than the volume of the ice, but exactly equal to the volume of the submerged part of the ice. So the liquid level won’t change.

The ice cap situation is quite a bit different. Large parts of the ice sheets are not floating on water but sitting on land. When they melt, water gets dumped into the seas, raising their levels. Also (and this is probably the point your teacher was making) , most of the ocean is warmer than 4°C. That means that nearly all of the water expands when it's warmed further. So global warming is raising the water level both by melting land-based ice and by expanding the water already in the seas.

Mike W.


(published on 10/22/2007)

Follow-Up #1: Melting Icecaps

Q:
Your wrote about the melting ice caps: Large parts of the ice sheets are not floating on water but sitting on land. When they melt, water gets dumped into the seas, raising their levels. Ice covers what percentage of land? land covers what percentage of the Earths surface? The amount of land ice if melted will have how much effect on the Ocean surface level? This is not as drastic as proposed. The truth is, if all the ice melted on the land it would not replace the ice melted in the water as far as surface water levels. Thank you
- Bill Dinwiddie
Garner, NC USA
A:

What counts is not the area covered but the total volume of ice which might melt and slide off land into the sea.
For a nice discussion of the contributions (and uncertainties) of various components to the sea level rise from global warming, you might check


In the short run, thermal expansion of the water due to rising ocean temperatures is the main contributor. In the medum run, melting of the Greenland ice pack would raise sea level seven meters. In the longer run, huge rises would result if much of the Antarctic pack melted. The rates of these melting processes are poorly understood, because it's not known how much under-ice water lubricates the ice flows.

I have no idea what you mean by "replace the ice melted in the water’. That ice is melting, not evaporating!

Mike W.
 


(published on 11/17/2007)

Follow-up on this answer.