Wait. My intuition says that the mass of ice cannot possibly have a gravitational influence on nearby water big enough to create the effects described. Back of envelope calculation: surface area of Antarctica is on the order of 10M km^2, with an average thickness of 1 km (a high estimate), means 10M km^3 of ice; some further calculations from volume to mass multiply out to 10^19 kg of ice. (Note that we're assuming that 1kg of ice is 1 cubic deciliter, as with liquid water; this is definitely wrong but let's go with it since this is just an order-of-magnitude estimate). The mass of the Earth is on the order of 10^24 kg, so this is 10^-5 of the mass of the earth.
Can you get really get 50m of local displacement from 10^-5 Earth masses?
At a distance of 100 km, the gravitational acceleration caused by the greenland ice sheet is .019 m/s^2 or ~0.2% g. Doesn't sound like a lot but it's still 524 times higher than the effect of the moon's gravity which creates the tides. The ice sheet has the same gravitational effect as the moon at a distance of approximately 2300 km.
1) the center of the ice sheet can be much closer to the water than the center of the earth. Some water around greenland is at 600 km from the center of the ice sheet, and 6000 km from the center of the earth. This factor 10 difference is squared because gravity's strength drops off with the square of distance.
2) Both forces are orthogonal, so you take the ratio to get an idea of how much the ice sheet attraction is slanting the water surface. This might be a very small angle, but if you have a small angle sustained over hundreds of kilometers then you can arrive at a height difference of meters. E.g. if the ratio is 10^5, then you have a 1 meter height difference at 100km distance (ignoring that the ratio actually changes over that distance to simplify).
You’re missing one of the effects gravity and ice have. From the article:
> The second thing that happens is that this gravitational attraction that the ice sheet exerts on the surrounding water diminishes. As a consequence, water migrates away from the ice sheet. The third thing is, as the ice sheet melts, the land underneath the ice sheet pops up; it rebounds.
The land underneath the glacier or ice sheet (and it has to be on land, because ice displaces it’s melted volume when floating) pops up and increases in altitude (from the centre of earth) due to the drop in weight.
This popping up effect will of course affect surround land not under the ice because rock isn’t that flexible.
I'm not a physicist. I did get curious and went to Wikipedia. You're mostly spot on:
>It covers an area of almost 14 million square kilometres (5.4 million square miles) and contains 26.5 million cubic kilometres (6,400,000 cubic miles) of ice.[2] A cubic kilometer of ice weighs approximately one metric gigaton, meaning that the ice sheet weighs 26,500,000 gigatons.
26,500,000 gigatons is 2.65e+18 kg in scientific notation.
I compared this to the moon, which is 7.35 x 10^22 kg, or about 30,000 times as heavy. The moon does create quite some tidal effects, while at 384,400 km distance.
Since gravity is inversely proportional to the square of the distance Both the 50m of local displacement as well as the 2000km distance until it sufficiently cancelled out sound believable to me.
I found this Minute Physics video about "sea level" very enlightening [1], and I think it can help here (just replace mountains by "huge masses of ice").
If the Earth were perfectly round the center of mass would be in the exact center, but if it had lopsided bulge on one side the center of gravity would shift in that direction. The water, being liquid, will want to form a sphere around that new center of mass. That deviation from center means a corresponding increase in water around the surface in that direction. So intuitively, if you think about it this way, 30 something meters deviation from center doesn’t seem too counterintuitive.
The force vector from a polar ice sheet doesn't get cancelled out by the rest of the earth though (because they aren't pointing in opposite directions). Plus, you have to account for the force of the ice sheet, plus the water it attracts, plus the water that those combined attract, etc.
I'm no good with calculus so can't run that back-of-the-envelope for you, but it doesn't seem all that surprising to me.
G is 6.67e-11, so for 1kg of matter 1000km away from a center of that mass the force is (e-11 x e19 / e(6x2)), around e-4 newtons or e-5 kg of weight equivalent. Idk what’s that worth ocean-wise.
This isn't intuitive to me either, given that the ice sheet is less dense than the surrounding water. I mean, it floats. Never mind the effect of the surrounding landmass, which is WAY more dense. I'm not ready to call his theory "fact" like Nautilus is.
Not only that, but I think the whole idea here is just plain wrong. From the article:
> this gravitational attraction that the ice sheet exerts on the surrounding water diminishes. As a consequence, water migrates away from the ice sheet.
When an ice sheet melts, it doesn't create a void in its place. What was once ice becomes liquid water. That liquid water actually has a slightly higher density than it had when in ice form, but it will occupy slightly less volume in the ocean. Mass is conserved, and the net effect in terms of gravity is essentially zero.
If anything, you'd have slightly higher local gravity in that part of the ocean (due to a higher concentration of liquid water vs ice), but again, zero net change looking at the entire ocean.
I love counter-intuitive facts. They are a helpful reminder that things are more complicated than I think they are, and I don't understand the world as well I think I do.
"Gravity has a very strong effect. So what happens when an ice sheet melts is sea level falls in the vicinity of the melting ice sheet. That is counterintuitive. The question is, how far from the ice sheet do you have to go before the effects of diminished gravity and uplifting crust are small enough that you start to raise sea level? That’s also counterintuitive. It’s 2,000 kilometers away from the ice sheet. "
"What do Roman fish tanks tell us about sea levels?
"Wealthy Romans at the time of Augustus were building fish holding tanks. The fishermen would come in with the fish, they’d put them there so that the fish were fresh when they ate them—they wanted to keep them alive for a few days or weeks or whatever. The Romans were engineers, so they built these fish tanks at very precise levels relative to sea level at the time. You didn’t want the walls to be too low because at high tide the fish would swim out; you didn’t want it to be too high because you wanted tides to refresh the water within the tanks.
"Kurt Lambeck, a professor at the Australian National University, recognized that by looking at the present day elevation of those fish tanks, we could say something about how sea level had changed over the 2,500 years since then. If sea level over the last 2,500 years was going up at the rate that it went up in the 20th century, those fish tanks would be under 4 meters of water—12 feet of water—and I can assure you they’re not. You can see them. You can walk along the coast, they’re visible. What that tells you is that it is impossible that sea level went up by the rates that we saw in the 20th century for any extended period of time earlier than that. Sea level has not gone up over the last 2,500 years like it has in the 20th century."
Italy is tectonically active. The level of 2000 year old Roman constructions versus the sea may well have more to do with local ground rise or fall. This assertion needs significant supporting data.
The illustration seems misleading—showing increase in sea level starting right outside of a glacier. As discussed in the article, the area of diminishing sea level spans thousands of kilometers far from where a glacier used to be, depending on its mass.
The person interviewed claims that people usually don’t appreciate his points since they’re so counter-intuitive, well right there we have Nautilus not helping that at all.
As I understand it is gravity's inverse square law that makes the effect strongest closet to the glacier and tapering off over a couple thousand kilometers.
Good article. I was hoping that he would talk about the effect on tectonic plate stability with these massive bodies of weight shifting around a spinning sphere.
This is actually very concerning. We have significant evidence now to suggest that asteroids in the past have caused large earthquakes. Sometimes on the other side of the impact zone (the antipode).
More scary though is what comets can do. They have a highly elliptical orbit, so they are much harder to spot than asteroids as they come out of nowhere. The elliptical orbit gives them much greater speed to. Comets tends to generate enormous levels of heat too, they explode shorty before impact as one did in Tunguska in Russia in 1908. Anyway it now seems that these "bursts" have happened during ice ages, vaporizing massive ice sheets. We are talking perhaps the entire northern hemisphere in some cases like the Younger Dryas Impact Hypothesis.
The crust floats like water and is as gooey as pudding when suddenly trillions of tons of weight just "disappears" above it. We have no idea how this unfolds currently except to say it is destructive and chaotic at the least.
The interview is nicely done. The followup questions were exactly what I would have asked, probably to explain the theory to someone who doesn't know much about geology
I recall Augustus reigned on the order of 2000 years ago, not 2500? Normally this would be a trivial detail, but the calculations under discussion would seem to require exacting precision.
I suspect this is an error on the reporter's part, so it's good that Nautilus is online and can update its articles.
I wouldn't call it trivial at all; 500 years is the difference between the Renaissance and present day. 2500 years ago, Rome had just become a republic, while 2000 years ago (during Augustus' time) it became an empire and would soon reach its territorial peak.
The idea is interesting and I'm glad to know about it, but there is a glaring flaw in his reasoning which nobody else here has commented on so I'll do it:
"I was in Holland a few summers ago and was trying to convince the Dutch that if the Greenland ice sheet melts, they have less to worry about than the Antarctic ice sheet melting. But it doesn’t register."
because, is there a polar melting hypothesis that melts the Arctic without melting the Antarctic? It could be that his idea is that both poles have water stacked up from ice sheet gravity more than it would be than melted, but the reasoning in that quote doesn't make that point, it makes a different point that is not useful, and he is I guess saying that there would just be more flooding than expected near the equator?
this article doesnt do much to dispel the counterintuitive nature of the subject.
1) the mass of glacier sheets have gravity, so sea level is higher near glaciers
2) the weight of the glacier on the land its sitting on pushes the land into the water and raises sea level
3) when glaciers melt, they add water to the sea. however, the melting glacier removes gravitational mass and removes weight that was forcing the land it was sitting on into the water. the combination of these 3 effects is lowering the nearby sea level and raising the faraway sea level
4) the distance that makes a given spot nearby or faraway is dependent on the glacier sheet itself - how massive it is, its geometry, etc. the all-in, 3 dimensional accounting of where on the globe sea levels change for a given glacier melting is called its fingerprint.
the article claims that the major glacier fingerprints are unique such that global sea level data points can identify, simultaneously, what percentage of all glaciers are melting.
on a side note: the provided infographic is wrong - the yellow and green lines are touching at the edge of the glacier. its attempting to model pretty pictures in 3d with 2d lines, and its just not accurate. it sort of conveys the point, i guess, if you just squint and dont think about it.
The problem I have with this stuff is that I just can't imagine how bad it could possibly be if sea levels rose even three meters over the next hundred years.
Significant fractions of many coastal cities are under 3m above sea level. Even if they could put up walls, many seaside communities are also under 3m.
All the discussion about gravity is good, but I was equally interested in the slowing of the earth's rotation. 1.8ms per day? It seems like that's large enough to be noticed with modern clocks/astronomy outside of eclipse records. Is this confirmed outside of eclipse records?
Because when the ice melts, the water does not remain where the ice was, but rather enters the ocean. Water entering the ocean anywhere in the world will tend to raise the height of the ocean worldwide.
[+] [-] jaspax|5 years ago|reply
Can you get really get 50m of local displacement from 10^-5 Earth masses?
[+] [-] jjk166|5 years ago|reply
[+] [-] wcoenen|5 years ago|reply
2) Both forces are orthogonal, so you take the ratio to get an idea of how much the ice sheet attraction is slanting the water surface. This might be a very small angle, but if you have a small angle sustained over hundreds of kilometers then you can arrive at a height difference of meters. E.g. if the ratio is 10^5, then you have a 1 meter height difference at 100km distance (ignoring that the ratio actually changes over that distance to simplify).
[+] [-] avianlyric|5 years ago|reply
> The second thing that happens is that this gravitational attraction that the ice sheet exerts on the surrounding water diminishes. As a consequence, water migrates away from the ice sheet. The third thing is, as the ice sheet melts, the land underneath the ice sheet pops up; it rebounds.
The land underneath the glacier or ice sheet (and it has to be on land, because ice displaces it’s melted volume when floating) pops up and increases in altitude (from the centre of earth) due to the drop in weight.
This popping up effect will of course affect surround land not under the ice because rock isn’t that flexible.
[+] [-] YetAnotherMatt|5 years ago|reply
>It covers an area of almost 14 million square kilometres (5.4 million square miles) and contains 26.5 million cubic kilometres (6,400,000 cubic miles) of ice.[2] A cubic kilometer of ice weighs approximately one metric gigaton, meaning that the ice sheet weighs 26,500,000 gigatons.
26,500,000 gigatons is 2.65e+18 kg in scientific notation.
I compared this to the moon, which is 7.35 x 10^22 kg, or about 30,000 times as heavy. The moon does create quite some tidal effects, while at 384,400 km distance.
Since gravity is inversely proportional to the square of the distance Both the 50m of local displacement as well as the 2000km distance until it sufficiently cancelled out sound believable to me.
[+] [-] kilburn|5 years ago|reply
[1] https://www.youtube.com/watch?v=q65O3qA0-n4
[+] [-] chrischen|5 years ago|reply
[+] [-] _petronius|5 years ago|reply
I'm no good with calculus so can't run that back-of-the-envelope for you, but it doesn't seem all that surprising to me.
[+] [-] mikro2nd|5 years ago|reply
Wonder which one I'd put money on.
[+] [-] PopeDotNinja|5 years ago|reply
https://youtu.be/MbucRPiL92Q
[+] [-] wruza|5 years ago|reply
[+] [-] larrik|5 years ago|reply
That said, I'm not a geophysicist.
[+] [-] curiousgeorgio|5 years ago|reply
> this gravitational attraction that the ice sheet exerts on the surrounding water diminishes. As a consequence, water migrates away from the ice sheet.
When an ice sheet melts, it doesn't create a void in its place. What was once ice becomes liquid water. That liquid water actually has a slightly higher density than it had when in ice form, but it will occupy slightly less volume in the ocean. Mass is conserved, and the net effect in terms of gravity is essentially zero.
If anything, you'd have slightly higher local gravity in that part of the ocean (due to a higher concentration of liquid water vs ice), but again, zero net change looking at the entire ocean.
[+] [-] air7|5 years ago|reply
"Gravity has a very strong effect. So what happens when an ice sheet melts is sea level falls in the vicinity of the melting ice sheet. That is counterintuitive. The question is, how far from the ice sheet do you have to go before the effects of diminished gravity and uplifting crust are small enough that you start to raise sea level? That’s also counterintuitive. It’s 2,000 kilometers away from the ice sheet. "
[+] [-] seesawtron|5 years ago|reply
[0] https://en.wikipedia.org/wiki/Simpson%27s_paradox#UC_Berkele...
[+] [-] mcguire|5 years ago|reply
"Wealthy Romans at the time of Augustus were building fish holding tanks. The fishermen would come in with the fish, they’d put them there so that the fish were fresh when they ate them—they wanted to keep them alive for a few days or weeks or whatever. The Romans were engineers, so they built these fish tanks at very precise levels relative to sea level at the time. You didn’t want the walls to be too low because at high tide the fish would swim out; you didn’t want it to be too high because you wanted tides to refresh the water within the tanks.
"Kurt Lambeck, a professor at the Australian National University, recognized that by looking at the present day elevation of those fish tanks, we could say something about how sea level had changed over the 2,500 years since then. If sea level over the last 2,500 years was going up at the rate that it went up in the 20th century, those fish tanks would be under 4 meters of water—12 feet of water—and I can assure you they’re not. You can see them. You can walk along the coast, they’re visible. What that tells you is that it is impossible that sea level went up by the rates that we saw in the 20th century for any extended period of time earlier than that. Sea level has not gone up over the last 2,500 years like it has in the 20th century."
Italy is tectonically active. The level of 2000 year old Roman constructions versus the sea may well have more to do with local ground rise or fall. This assertion needs significant supporting data.
[+] [-] strogonoff|5 years ago|reply
The person interviewed claims that people usually don’t appreciate his points since they’re so counter-intuitive, well right there we have Nautilus not helping that at all.
[+] [-] mikro2nd|5 years ago|reply
[+] [-] fritzo|5 years ago|reply
[+] [-] mentos|5 years ago|reply
Is there a chance it is beneficial to keep the next ice age at bay?
[+] [-] trynumber9|5 years ago|reply
[+] [-] mberning|5 years ago|reply
[+] [-] codecamper|5 years ago|reply
[+] [-] rshnotsecure|5 years ago|reply
More scary though is what comets can do. They have a highly elliptical orbit, so they are much harder to spot than asteroids as they come out of nowhere. The elliptical orbit gives them much greater speed to. Comets tends to generate enormous levels of heat too, they explode shorty before impact as one did in Tunguska in Russia in 1908. Anyway it now seems that these "bursts" have happened during ice ages, vaporizing massive ice sheets. We are talking perhaps the entire northern hemisphere in some cases like the Younger Dryas Impact Hypothesis.
The crust floats like water and is as gooey as pudding when suddenly trillions of tons of weight just "disappears" above it. We have no idea how this unfolds currently except to say it is destructive and chaotic at the least.
[+] [-] Super_Jambo|5 years ago|reply
Intuitively seems like it should... But then this is an article about how bad intuition is for complex problems.
[+] [-] praveen9920|5 years ago|reply
[+] [-] jessaustin|5 years ago|reply
I suspect this is an error on the reporter's part, so it's good that Nautilus is online and can update its articles.
[+] [-] keiferski|5 years ago|reply
[+] [-] fsckboy|5 years ago|reply
"I was in Holland a few summers ago and was trying to convince the Dutch that if the Greenland ice sheet melts, they have less to worry about than the Antarctic ice sheet melting. But it doesn’t register."
because, is there a polar melting hypothesis that melts the Arctic without melting the Antarctic? It could be that his idea is that both poles have water stacked up from ice sheet gravity more than it would be than melted, but the reasoning in that quote doesn't make that point, it makes a different point that is not useful, and he is I guess saying that there would just be more flooding than expected near the equator?
[+] [-] barbegal|5 years ago|reply
[+] [-] kryogen1c|5 years ago|reply
1) the mass of glacier sheets have gravity, so sea level is higher near glaciers
2) the weight of the glacier on the land its sitting on pushes the land into the water and raises sea level
3) when glaciers melt, they add water to the sea. however, the melting glacier removes gravitational mass and removes weight that was forcing the land it was sitting on into the water. the combination of these 3 effects is lowering the nearby sea level and raising the faraway sea level
4) the distance that makes a given spot nearby or faraway is dependent on the glacier sheet itself - how massive it is, its geometry, etc. the all-in, 3 dimensional accounting of where on the globe sea levels change for a given glacier melting is called its fingerprint.
the article claims that the major glacier fingerprints are unique such that global sea level data points can identify, simultaneously, what percentage of all glaciers are melting.
on a side note: the provided infographic is wrong - the yellow and green lines are touching at the edge of the glacier. its attempting to model pretty pictures in 3d with 2d lines, and its just not accurate. it sort of conveys the point, i guess, if you just squint and dont think about it.
[+] [-] ppod|5 years ago|reply
[+] [-] gbear605|5 years ago|reply
[+] [-] unknown|5 years ago|reply
[deleted]
[+] [-] mgamache|5 years ago|reply
[+] [-] CabSauce|5 years ago|reply
[+] [-] vharuck|5 years ago|reply
[0] https://news.ycombinator.com/item?id=16656903
[+] [-] arnoooooo|5 years ago|reply
[+] [-] greendave|5 years ago|reply
This article has many such gems. Fascinating and well-written.
(Note that it was originally published in 2016).
[+] [-] puranjay|5 years ago|reply
https://www.harvardmagazine.com/2016/08/what-roman-ruins-rev...
[+] [-] eximius|5 years ago|reply
[+] [-] frank2|5 years ago|reply
[+] [-] BeKindAndLearn|5 years ago|reply
[+] [-] mrfusion|5 years ago|reply
[+] [-] supremeWhite|5 years ago|reply
[deleted]
[+] [-] Proven|5 years ago|reply
[deleted]