Part of the show Naked Science Questions and Answers
Richard in Essex asked:
If you get an ice cube and put it in a glass of water, when it melts it has exactly the same volume. This is the reason that when glaciers melt, there's absolutely no difference in sea level. So the people who say about global warming and sea level rises have got it wrong.
You're right for ice that's floating on water because it doesn't change the volume at all. But if you've got ice on Antarctica or the Greenland ice sheet where it's on rock and nowhere near the sea, when that melts, it will end up in the sea and increase the volume of the sea. Greenland is a massive ice sheet and there are tonnes and tonnes of water locked up as ice on land. The melting of ice on Greenland is raising the ocean depths by about half a centimetre every single year. If all that lot goes in the next 100 years, we could see a one metre rise in sea level.