If the ice caps melt and the weight of all that water gets redistributed about the planet given that the planet’s just a ball of molten stuff underneath a relatively thin crust will the shape of the Earth change when all the ice goes?
It depends on how long a time scale you are looking at. Over a few tens of years then probably not, the mantle below the crust behaves pretty much as a solid over periods of tens of years. But if you look over thousands or millions of years, then yes the areas where the ice has melted from will lift slightly and the areas the water has moved to will sink. During the last ice age Scotland was pushed down a few metres by the 2km of ice sitting on top of it, and since the ice melted, 10 000 years ago, the crust has been floating back up again in a process called isostatic rebound.
This affect is very small though in comparison with the size of the planet though.
Sheryl Hohle asked the Naked Scientists: I love your show and listen to it while I muck stalls. I have been pondering a question about climate change/global warming. If the polar icecaps melt and the weight of all that water gets redistributed around the planet, and given that the planet is actually just a ball of molten stuff underneath a relatively thin crust, will the shape of the earth change? Will it bulge more in the middle? Will it spin differently or will the axis change? Or is the weight of the oceans insignificant relative to the earth? What do you think? Sheryl Hohle, Sun, 15th Jun 2008
The areas, such as Greenland and Antarctica will be subject to isostatic rebound, i.e., the land will rise in the mantle - the land will actually gain altitude above sea level. There is little probability it will change the spin, axis or the overall shape of the earth, which is a pear shape withe the larger part in the southern hemisphere. It only amounts to a few feet but does make it a pear! JimBob, Tue, 17th Jun 2008
I'm not so sure that I agree; the earth is in essence a giant gyroscope spinning through space; what would be the result on such a gyroscope of a fairly large distribution of mass from the poles to an even distribution around the earth? would this not be a potential trigger to inversion of poles? I'd love to hear from someone who knows more than me (should not be too difficult).