
Why Our Intuition About Sea-Level Rise Is Wrong - dnetesn
http://oceans.nautil.us/feature/564/why-our-intuition-about-sea-level-rise-is-wrong
======
jaspax
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?

~~~
jjk166
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.

~~~
curiousgeorgio
> 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.

Contrary to common belief, tides are not caused by the direct influence of the
moon's gravity (it's far too weak to have any effect)[1]. The tidal forces are
caused by the gravitational _gradient_ from the moon (and the "centrifugal"
forces from our path around the earth-moon barycenter), and I don't believe
you'd get the same effects from a gravity source on the surface of the earth.

Even a lot of very respectable scientists and textbooks get this wrong.

[1] See
[https://www.youtube.com/watch?v=pwChk4S99i4](https://www.youtube.com/watch?v=pwChk4S99i4)
for a pretty good explanation

~~~
pge
Just to add to this answer and perhaps save you the click, what he is
referring to is that the force pulling the water upward at high tide is not
the direct gravitational pull of the moon. It is that the gravitational force
from the moon at the edge of the earth is greater than the force at the center
(because it is closer to the moon). Similarly the force on the opposite side
of the earth is less (because that side is further from the moon than the
earth's center). So the water molecules are drawn away from the center of the
earth (near side because they are being pulled slightly harder than the center
of the earth, and far side because they are being pulled slightly less hard
than the center of the earth). Hence the high tides on both sides of the
earth, not just the side closer to moon.

~~~
mcguire
Or, to put it another way, the surfaces of the Earth closest to and farthest
away from the Moon are traveling at the same orbital velocity of the Earth.
However, they should be in different orbits; the point closest to the Moon is
too slow for the orbit it is in and the point farthest is too fast. The former
wants to into a lower orbit while the latter wants to go into a higher orbit.

~~~
mcguire
In a further translation from gibberish:

Or, to put it another way, the surfaces of the Earth closest to and farthest
away from the Moon are traveling at the same orbital velocity around the
center of the Earth/Moon system. However, they should be in different orbits;
the point closest to the Moon is too slow for the orbit it is in and the point
farthest away is too fast. The former wants to into a lower orbit while the
latter wants to go into a higher orbit.

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air7
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. "

~~~
seesawtron
Also Simpson's paradox is so counterintutive. [0]

[0]
[https://en.wikipedia.org/wiki/Simpson%27s_paradox#UC_Berkele...](https://en.wikipedia.org/wiki/Simpson%27s_paradox#UC_Berkeley_gender_bias)

~~~
barking
Yes, it sure is, took me a while to _get_ it. I don't have any paradoxes but
here are my two favourite surprising facts.

1: NY city on the East coast of N. America lies further west than Santiago the
capital of Chile on the West coast of S. America

2: The Atlantic opening of the Panama canal lies further west than the Pacific
opening.

~~~
VBprogrammer
One that always surprises me, Edmonton (Canada's most northern city) is about
the same latitude as Manchester in the UK.

~~~
eloff
Here's another one: the majority of Canada's population actually lives south
of the 49 parallel, which for the most part is the US border.

The reason is most of the population is in Ontario and Quebec, and the
majority of that population is south of the 49th. The southernmost part of
Canada at Niagara falls is actually at the same latitude as northern
California. Which is why it is a big wine growing region.

~~~
ddlatham
As a native to the US west coast, I was very surprised to realize that
Toronto, Ontario is further south than Portland, Oregon.

~~~
naveens
Distortions of the Mercator Projection...

~~~
ddlatham
I'm not sure how that would affect perceptions of relative latitude.

~~~
mec31
Here’s one of my favorites: What state in the US is closest to Africa?

~~~
barking
That's a really good one (I googled the answer), never would have expected it.

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mcguire
" _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.

------
strogonoff
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.

~~~
fritzo
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.

~~~
strogonoff
My understanding as well. The image, though, doesn't convey sea level dipping
for many kilometers around former glacier edge and rising only further away
(possibly in another hemisphere entirely)—instead it shows sea level rising
immediately at the edge, which doesn't seem to track.

~~~
dangirsh
I think the image is trying to convey that there's a rising immediately near
the glacier due to the tectonic plate lifting up. Then there's a falling due
to reduced gravitation. What's not shown would be (much further away) a rising
due to increased water volume.

I agree that the image is a bit confusing.

~~~
strogonoff
I think the green line is the sea level after the melting, since it is labeled
Final Sea Level. It does not show a fall, only a rise, next to the glacier.

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mentos
>If humans weren’t warming the climate, Earth might be poised to enter into
another Ice Age in the future.

Is there a chance it is beneficial to keep the next ice age at bay?

~~~
trynumber9
The agricultural regions of Europe and North America not being covered with
sheets of ice _may_ be a positive. Especially if you like having food.

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codecamper
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.

~~~
rshnotsecure
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.

~~~
arethuza
We do have some idea what happens - post-glacial rebound is very noticeable in
some parts of the world that were heavily glaciated in the last ice age:

[https://en.wikipedia.org/wiki/Post-
glacial_rebound](https://en.wikipedia.org/wiki/Post-glacial_rebound)

~~~
rshnotsecure
Just saw this. Fascinating.

------
praveen9920
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

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jessaustin
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.

~~~
keiferski
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.

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fsckboy
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?

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barbegal
If you want to get a better feel for this NASA has made a great visualization
tool [https://vesl.jpl.nasa.gov/sea-level/slr-
gfm/](https://vesl.jpl.nasa.gov/sea-level/slr-gfm/)

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kryogen1c
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.

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ppod
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.

~~~
gbear605
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.

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mgamache
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?

~~~
vharuck
This struck me, too. So days were 4 hours shorter 2500 years ago? The evidence
our ancestors worked fewer hours a day just keeps piling up [0]!

[0]
[https://news.ycombinator.com/item?id=16656903](https://news.ycombinator.com/item?id=16656903)

~~~
pdonis
_> So days were 4 hours shorter 2500 years ago?_

No. The cumulative clock difference between two clocks, one of which is adding
leap seconds due to the slowing of Earth's rotation, the other of which is
not, is not the same as the rate of slowing of the Earth's rotation.

The current rate of slowing is about 1.4 milliseconds per 100 years. That
means that, 100 years ago, the length of 1 day was 1.4 milliseconds shorter
than it is today. So let's suppose that 100 years ago, the day was exactly
86,400,000.0 milliseconds long. Then today would be 86,400,001.4 milliseconds
long.

Now add up all those days over the last 100 years, i.e., 36,525 of them, and
suppose the rate of increase in the day is linear. Then the average extra
milliseconds per day is half of 1.4, or 0.7 milliseconds, and the total number
of milliseconds in those 36,525 days is

3,155,760,025,567.5

instead of the

3,155,760,000,000.0

that it would have been if the length of the day had been constant. That's an
extra 25,567.5 millliseconds over 100 years, which is a lot bigger than 1.4.
That's the difference between the cumulative clock difference and the rate of
slowing.

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arnoooooo
I thought the main contributor to the rise was the thermal expansion of water
as it heats up. I don't see a single mention of this in the article ?

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greendave
> What do Roman fish tanks tell us about sea levels?

This article has many such gems. Fascinating and well-written.

(Note that it was originally published in 2016).

~~~
puranjay
More on Roman fish tanks if anyone wants to read about this topic. I thought
this was very insightful:

[https://www.harvardmagazine.com/2016/08/what-roman-ruins-
rev...](https://www.harvardmagazine.com/2016/08/what-roman-ruins-reveal)

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eximius
Ice is less dense than water. Why would ice's gravity be more influential than
the waters?

~~~
frank2
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.

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mrfusion
I’m trying to get as many people out to see Florida one last time before it’s
underwater.

------
rshnotsecure
This article is bizarre.

Some quotes that strikes me as just really really odd:

"Jerry Mitrovica has been overturning accepted wisdom for decades" \- no one
is this good.

"What he calls postmodern geology" \- this is very concerning. Postmodernism
is like Foucault and Derrida and about power and society. Foucault is actually
the most cited academic of all time statistically, so for a Professor to
knowingly use this word to describe the science of geology is just strange.

"Though a practiced public speaker" \- what is this? should he be ashamed?

"These are known as sea-level fingerprints, because each ice sheet has its own
geometry. Greenland produces one geometry of sea level change and the
Antarctic has its own. Mountain glaciers have their own fingerprint." \- what?

