The WackyWorld Of Physics

[TenEightyOne]

I think it's the weight of the object and of the displaced fluid that are equal, not the volumes.

No, a cube of concrete with a volume of 1000cm3 would displace 1000cm3 of water or of vegetable oil despite those volumes of fluid each having a different density (and therefore weight mass) - and each of those displaced amounts of fluid would have a different density (and therefore mass) to the concrete.

Archimedes measured the exact volume of the golden crown using the displacement method and then measured the density of a measured volume of known gold in order to have a benchmark for comparison.

why would melting ice caps result in a rise in sea levels, when the frozen ice is already displacing the amount of water which already exists in the iceberg, and so the melting water would only replace the displacement.

Frozen water actually has a lower density than liquid water, so ice takes up slightly more room than it does as water (hence burst pipes in winter). The problem is all the ice that's above sea level which isn't currently occupying "water space".

 

[Imari]

No, a cube of concrete with a volume of 1000cm3 would displace 1000cm3 of water or of vegetable oil despite those volumes of fluid each having a different density (and therefore weight mass) - and each of those displaced amounts of fluid would have a different density (and therefore mass) to the concrete.

Archimedes measured the exact volume of the golden crown using the displacement method and then measured the density of a measured volume of known gold in order to have a benchmark for comparison.

OK, there's kind of two different regimes here and you're both talking about separate ones..

For things that are heavier than the liquid, you can calculate the total volume of the solid from the displacement of the liquid. For things that are lighter than the liquid (ie. they float), you can calculate the total weight of the solid from the displacement of the liquid. These are based on the same principle but obviously give different information.

Frozen water actually has a lower density than liquid water, so ice takes up slightly more room than it does as water (hence burst pipes in winter). The problem is all the ice that's above sea level which isn't currently occupying "water space".

The problem is mostly all the ice that's on land. Floating ice doesn't change the level of the water it's in (notwithstanding @Northstar point that freshwater ice melting in seawater does change the level due to the differing liquid densities). You can try this with ice in a glass of water; mark the side of the glass and watch what happens to the water level as it melts.

 

[Touring Mars]

As has already been said, melting ice caps can contribute to sea level increases when the ice is on land, while melting sea ice will not significantly change sea level per se... but there is also the albedo effect. Sea ice such as in the Arctic reflects sun light and reduces the effect of the sun on the oceans and hence also the atmosphere, which in turn affects the ice on land. The net effect of less sea ice in the Arctic is a small but significant decrease in the Earth’s ability to reflect energy back into space which in turn may contribute to other kinds of ice loss, such as the Greenland ice sheet. Lower Arctic sea ice creates a feedback loop whereby the underlying water gets warmer and makes sea ice recovery slower, and also contributes to a feedback loop in Greenland whereby the more ice Greenland loses, the more susceptible it becomes to warming etc.

 

[Rallywagon]

Also to note, as the ice caps melts, the land underneath will begin to rise due to the fact that weight is, well, melting away. This land rise also causes the the sea level to rise, albeit, not as much as the land bound ice caps melting, but it does contribute.

 

[Dotini]

Global sea level is also affected by vertical crustal movements, changes in Earth's rotation rate, large-scale changes in continental margins and changes in the spreading rate of the ocean floor.

When the term relative is used in context with sea level change, the implication is that both eustasy and isostasy are at work, or that the author does not know which cause to invoke.

Post-glacial rebound can also be a cause of rising sea levels. When the sea floor rises, which it continues to do in parts of the northern hemisphere, water is displaced and has to go elsewhere.

https://en.wikipedia.org/wiki/Isostasy

 

[Imari]

Also to note, as the ice caps melts, the land underneath will begin to rise due to the fact that weight is, well, melting away. This land rise also causes the the sea level to rise, albeit, not as much as the land bound ice caps melting, but it does contribute.

Sorry, what? That doesn't make sense to me. If the land rises it ends up higher than it was before, it's then further above sea level, no?

Maybe it's just me having a senior moment or maybe I've had one too many drinkies tonight, but you might have to break the steps on this one down a bit further because I'm not seeing the logical chain from "land rises" to "sea level higher".

 

[Rallywagon]

Sorry, what? That doesn't make sense to me. If the land rises it ends up higher than it was before, it's then further above sea level, no?

Maybe it's just me having a senior moment or maybe I've had one too many drinkies tonight, but you might have to break the steps on this one down a bit further because I'm not seeing the logical chain from "land rises" to "sea level higher".

Right, the land, like Antarctica or the arctic rises, raising it's own beaches. Locally, that land obviously goes above sea level, but globally, it effectively raises the sea level because land that was under water have now risen out of the water.
Think of it kinda like a deflated balloon in the bottom of a bowl. As you blow up the balloon, you push the water up in the bowl, raising the water level. As ice melts on the land, the weight is relieved from the land, which causes it to rise, including the portions under water, displacing that amount of water. Again, locally moonrise above sea level, but globally, that causes sea levels to also rise.

 

[polysmut]

No, a cube of concrete with a volume of 1000cm3 would displace 1000cm3 of water or of vegetable oil despite those volumes of fluid each having a different density (and therefore weight mass) - and each of those displaced amounts of fluid would have a different density (and therefore mass) to the concrete.

If the object sinks in the water, yes.

When the object is floating in equilibrium, the upward force exerted on the object by the water is equal to the weight of the object. Archimedes' principle tells us that the weight of the displaced water is also equal to this upward force.

 

[Imari]

Right, the land, like Antarctica or the arctic rises, raising it's own beaches. Locally, that land obviously goes above sea level, but globally, it effectively raises the sea level because land that was under water have now risen out of the water.

I'm still not so sure about this. Is this a personal idea that you're working on or something that has scientific backing?

Even assuming that a section of earth behaves as an elastic mass (which is possible but not immediately intuitive given that it's largely made of minerals and salts), removing weight from something like Antarctica does not cause all land on earth to rise. "Land" is not a single contiguous mass, even counting the underwater bits. There are 7 major tectonic plates, and dozens of smaller ones. At best you might cause a plate to rise, but the first sign would probably be massive earthquakes due to the increased relative movement between plates.

But let's assume that your plate is a sufficiently elastic mass for this to work. It's then not almost certainly not stiff enough for it to "raise" meaningfully in places where the weight on it didn't change. If I take the cat off my foam mattress, yes, the local area where the cat was is now higher. Some of the areas close by are also higher because they're not being dragged down, but the mattress in general hasn't gotten thicker or thinner overall. And the bit with the dog on it is still being squashed by dog. Generalise tectonic plates to a bunch of very stiff foam mattresses floating around on the surface of the earth and you start to see why local changes like this are unlikely to have major impacts on something like the sea, which covers ~70% of the earth's surface.

Think of it kinda like a deflated balloon in the bottom of a bowl. As you blow up the balloon, you push the water up in the bowl, raising the water level. As ice melts on the land, the weight is relieved from the land, which causes it to rise, including the portions under water, displacing that amount of water. Again, locally moonrise above sea level, but globally, that causes sea levels to also rise.

Causes sea levels to rise relative to the bowl. But on earth there is no bowl, we're on a sphere. Think of it like a floating sphere of water in zero-g with a deflated balloon inside it, and that becomes more accurate. So the meaningful point of reference for us is the balloon, because that's the surrogate for the land that we live on. For the balloon, the sea level has dropped because it was underwater and now it's not.

Analogies are always good to check your intuition and understanding, but finding the right analogy can sometimes be difficult when you're talking about relative changes. You're so used to thinking about the level of liquids in containers that it's not immediately obvious that it's not the correct measure here.

 

[Rallywagon]

Nope, not my idea at all. I'm just not a teacher so I do a piss poor job of explaining it. Here, try this instead.
Basically, as the ice melts, the land rises. Locally it appears the sea level drops (that is, the land rises, the sea level doesnt drop though.) But, by that land locally rising, you are displacing water just the same as if land bound glaciers were running off into the ocean. As I said, it's not a huge effect. But it does have an effect. It's still displacing water as more land is shifting and pushing the water around.

Also, interestingly, is the effect of gravity. As glaciers form they add gravity to the area they occupy which in turn causes water to gravitate to glaciers. The opposite happens when they melt, relaxing the gravitational pull and letting the water flow out to the rest of the oceans.

https://www.theguardian.com/environ...ctic-ice-sheet-sea-level-rise-science-climate

I think, also, it's important to remember, oceans aren't land locked, the land is ocean locked, what effects sea levels in the Antarctic or the arctic, it's going to effect sea levels globally.