Why will the ice caps melting cause the sea level to rise?

March 4, 2006 2:52 PM

Why will the ice caps melting cause the sea level to rise?

If you put an ice cube in a glass of water, then allow the ice to melt, the water level stays the same - the ice displaces exactly as much water as is contained within it once melted.

Why, then, are sea levels expected to rise if the ice caps melt?

posted by Mwongozi to science & nature (21 comments total)

Because a whole lot of the ice is above sea level.
posted by borkingchikapa at 2:58 PM on March 4, 2006

Sure, but a whole lot of my ice cube is above the surface of the water. That's the point - it only displaces as much water as the volume of the ice will be, once it has melted. The water level in my glass doesn't rise when it melts.
posted by Mwongozi at 3:05 PM on March 4, 2006

Actually the level of water in the glass goes down. Water expands when it freezes.

But the previous poster is right.

This is just a little of the 'above water' ice that's out there

Ross Ice Shelf
posted by tiamat at 3:07 PM on March 4, 2006

Also, Antarctica is land with ice on top
posted by spork at 3:07 PM on March 4, 2006

Much of the South pole ice is supported by land. There's an actual continent underneath. Same with Greenland i think.

Otherwise you're right on the North pole which is primarily floating ice.
posted by vacapinta at 3:08 PM on March 4, 2006

To clarify, a lot of that ice is not floating either, it's at rest and built up from above by precipitation.
posted by tiamat at 3:09 PM on March 4, 2006

tiamat, the level of the water does not go down if the ice cube floats unrestricted on the glass. Indeed as it melts the water level remains the same. However you are correct in your assertion that ice is less dense than water. As such if you attach a piece of lead to the ice and have it sink to the bottom of a glass of water, once that ice melts, the level of the glass will indeed fall.

This doesn't apply to the geological ice we're talking about because much of it is supported by underlying earth and so the melting caps are not floating by gravity on water. Much of their mass is above water.
posted by drpynchon at 3:14 PM on March 4, 2006

Its a good question and the answer is - it depends, depends basically on where the ice is and what its sitting on.

Put simply, if the ice is sitting on top of a shelf of rock, ie: sitting in a pile above sea level, then when it melts the level of the sea will rise. Some of the icecap is like this. However, some of it isn't and like your ice cube there is a significant volume of ice below sea level. When this melts the level of the sea will fall.

Overall if I recall correctly only the Antarctic and Greenland sheets would cause a sea level rise if they melted (ie: the whole of the arctic could melt without affecting sea-level), but opinions differ ...

This account seems to suggest that even the Antarctic and Greenland sheets would not make much difference. Sea level rise is attributed to overall thermal expansion of the oceans and the melting of mountain glaciers.
posted by grahamwell at 3:17 PM on March 4, 2006

It's like adding more ice cubes to the glass. Much of that ice is sitting on solid ground, up above sea level. Antartica is a continent like any other. It is about 14 million km² big and almost all of it has around 2.5 kilometers of ice on top of it. Pretty big cube, eh? The Greenland Ice sheet is around 1,800,000 km². I don't know the average depth, but it hits 3 km (two miles deep) in spots. It fell there as snow or rain and accumulated over the millenia. When it melts it will go down into the sea, and this time New Orleans is really gonna' get it.
posted by Ken McE at 3:17 PM on March 4, 2006

By the way, the increase in sea level from the ice on Greenland and Antarctica melting is 77 meters. That's seventy-seven meters, or 252 feet.

The coastline of the world looks vastly different if it's 250 feet higher. The highest elevation in Florida, for example, is 345 feet. Basically all of Florida disappears under water.

But not all the ice will melt at once, you say. True enough. Even a 20 foot increase in sea level wipes out dozens of the world's largest cities, though. We have already put enough heat into the system to melt all that ice. The earth is now a glass of HOT water with ice cubes in it. The only question is how fast.

Washington Post, March 3, 2006, page A1: Antarctic Ice Sheet Is Melting Rapidly: New Study Warns Of Rising Sea Levels.
posted by jellicle at 4:22 PM on March 4, 2006

Has anyone done a map of revised coastlines post melt?
posted by b33j at 4:38 PM on March 4, 2006

Get me some of those 7' Wellies for trips to Venice.
posted by leafwoman at 4:59 PM on March 4, 2006

Has anyone done a map of revised coastlines post melt?

Don't buy beach property in Southern Florida.


posted by meehawl at 7:59 PM on March 4, 2006

Greenland is an archipelago (island chain) under all that ice, not a single solid body. There's still a massive glacier over the whole of its surface, though. Antarctica is a bona fide solid continent with a lot of ice on it.

Keep in mind that ice is (last I checked) about 7% less dense than water (give or take depending on exact temperature/pressure) so there's slightly less total volume being added to the oceans than your eyes would suggest.

Still enough to put all of us in deep shit.

The one that really gets me is the incredible amount of toxic shit that was poured into the world's water when New Orleans went under - now imagine the toxic additions of all the dozens of cities across the world that are less than 20m above sea level. What kind of effect is that going to have on the world's already tightening supply of drinkable water?
posted by Ryvar at 11:12 PM on March 4, 2006

ie: the whole of the arctic could melt without affecting sea-level)

If you can believe Google Earth's satellite photos, this has already happened. They show open ocean at the North Pole. Can this be true?
posted by Kirth Gerson at 4:25 AM on March 5, 2006

I think y'all are missing the point a bit. The primary mechanism by which global warming raises the water level is not the ice melting (although that contributes as well). Rather, as the ocean gets warmer, the same amount of water will occupy more volume, due to thermal expansion. A one degree temperature change translates to a whole lot of flooded coastal plains.
posted by blindcarboncopy at 11:37 AM on March 5, 2006

Getting into hot water – global warming and rising sea levels .
posted by wilful at 5:50 PM on March 5, 2006

Kirth, it's very true. It's still seasonal, though -- polar bears are now drowning more often due to fewer floes farther apart. In a couple of decades the Arctic and the Northwest Passage through Canada will be a viable year-round shipping lane.
posted by dhartung at 7:20 PM on March 5, 2006

Google Maps doesn't cover the North Pole. Google Earth doesn't, therefore, show you a satellite view of the north pole; it extrapolates from the furthest-north data it has, which is why the north pole looks like a vari-colored pinwheel in Google Earth.
posted by ikkyu2 at 12:02 AM on March 6, 2006

dhartung, your links do not support a yes answer to the "already happened" part of my question:
[1st link] This summer was the fourth in a row with the ice cap areas sharply below the long-term average, said Mark C. Serreze . . .
[2nd link] Some of the simulations, including those run on an advanced model at the National Center for Atmospheric Research in Boulder, Colo., show much of the summer ice disappearing by 2050, . . .

So, I guess Google Earth is a little premature, probably because of ikkyu's explanation.
posted by Kirth Gerson at 9:09 AM on March 6, 2006

Also, fresh and salt water have different densities, according to this Slashdot poster.

Fresh water also can disrupt the thermohaline, freezing Europe and the N.A. East Coast. Not a matter of sea levels, but still very bad.
posted by Yogurt at 1:46 PM on March 6, 2006