[helen tredgett]

helen tredgett  asked the Naked Scientists:
   
Hi,

I was led to believe that global warming will cause more ice to melt and hence raise sea levels.

But someone at work told me that this wasn't true and that the sea levels would remain the same.

So I conducted an experiment:

I put some water into a measuring jug with some ice cubes in and marked the level of the water. I covered the jug and let the ice cubes melt.

I remarked the water level and it was the same.

Please can you explain.

What do you think?

[graham.d]

Ice melting that is floating on the sea will not raise the sea level (Archimedes principle) but much of the ice on the earth is over land (e.g. Greenland or the Antarctic). When this melts it simply adds a lot of water to the sea. The ice in Greenland is over 1km thick on average, if I remember correctly, and if all this were to melt it would raise the sea level by around 7 metres.

[chemgeek]

Also, another factor that contributes to sea level rise is the thermal expansion of the sea due to rising temperatures.

[ccheric]

There are other secondary effects that probably won't make a big difference. When temperature rises, more water would evaporate into the atmosphere. Also, if you get longer warm growth seasons and more rain for plants, the ecosystem may keep more water above the sea level. But either way, when the Greenland ice sheet goes, Florida will go with it.

[Absalom]

(for non-scientists)
Helen, you can also think about the displacement of mass. Ice floats because it is less dense than liquid water (because water expands as it freezes), but it will, like anything, displace its mass. That's why the level didn't change in your experiment. So, ice on land doesn't even have to melt to raise sea levels, it just has to be moved into the sea. For instance if a chunk of ice broke off of Greenland or Antarctica (maybe because melted water running beneath it had caused a void) and fell into the sea, the sea level would rise, just as if you'd dropped another ice cube into your measuring cup.

[Joe L. Ogan]

The ice is in glaciers above the sea level.  When it falls into the sea and then melts, the water level will rise the amount of the melted ice.  Thanks for comments.  Joe L. Ogan