Science Questions

Why does melting ice cause sea level rise?

Sat, 23rd Sep 2006

Part of the shows Catalysts for Cleaner Environments and Future Energy and Best of the Naked Scientists 2

Question

Pom in Kent asked:

I was curious about the scaremongering we're hearing about the ice caps melting and flooding everywhere. I thought the Archimedes' principle was that a body immersed in water, given that the majority of icebergs are immersed in water, when the ice melts it will only displace its own weight in water.

Answer

I understand the point you're making and it's a good one. You're half right and half not right and the reason is as follows: the north pole is ice that's already floating. If that melts, you're quite right, it's never going to overflow because when it melts it will displace an equal volume of water as itself. As it's made of water it will just turn into water and never overflow. But here's the spanner in the works: Greenland for example and Antarctica are continents and there's land under there. The ice is not under water but sitting on land, so if that melts it's going to raise sea levels drastically. In fact there's enough ice locked up in Antarctica and Greenland to give us about 7 - 70 metres of sea level rise. There's a paper that was published in this week's edition of the journal Nature in which scientists have used two satellites in space. It's called GRACE but should be called BRACE because they work in concert with each other, and they work out how much mass there is on Earth underneath them by working out how fast one is accelerating because of gravity compared with the other one. What they've done is to watch Greenland for the last two years and they've found that Greenland has lost 248 cubic kilometres, plus or minus about 60, of water every single year in the past two to four years. It's gone up 250%, so it's a scary amount. That is enough to raise sea levels every single year by about 0.5 millimetres. That's Greenland on its own in one year. So if things really do take off with global warming, we're in trouble.

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