Why Our Intuition About Sea-Level Rise Is Wrong | Hacker News Reader

Why Our Intuition About Sea-Level Rise Is Wrong

nautil.us

111 pointsdnetesn26 comments

Huh, yesterday's submission of the same thing got no attention at all:

https://news.ycombinator.com/item?id=11125050

Transcribing my comment from there:

I really want to see some papers, models or simulations to illustrate some of these effects - starting with the gravitational influence of ice sheets on sea level. This shouldn't be a difficult thing to illustrate numerically, but wow it would have a big impact on how (at least I) perceive the ice sheets.

So far, I haven't been able to dig anything up - there's some prose at [1] but nothing hard. A poster at [2]. Mitrovica's website [3] doesn't seem to have anything. It's late, so I'll have to postpone the search for now, but here's hoping other readers can help me :-).

[1]: http://sealevelstudy.org/sea-change-science/whats-in-a-numbe...

[2]: http://geo.orst.edu/files/geo/Mitrovica-2009-Science.pdf

[3]: http://environment.harvard.edu/about/faculty/jerry-x-mitrovi...

If you still have some FORTRAN compiler installed you can run some simulations yourself [1]. There are two difficulties in 'easily' solving this problem. First of all the high level of feedback: changes in the sea level anywhere influence all other sea levels (conserving mass!). So you cannot solve this problem locally. Secondly, solving problems on spheres is difficult and expensive, so most sea level models solve the problem in spherical harmonics which is faster. (It's like the Fourier Transform, but instead of decomposing 1D signals into sines/cosines, you transform spherical shapes in more fundamental 'blobs'. See the examples here: [2] , after the header 'Jouons un peu avec les coeeficients harmoniques sphériques de la topography Terrestre'. You can see the individual first 0-6 'blobs', which when added form the first image reconstruction de l = 0 à 6. Which kind of resembles the Earth's topography already.) Spherical harmonics have some desirable mathematical properties and allow for quick simulations, but obviously formulating a problem in a different domain masks the actual physics being modeled. So I doubt you'll become wiser by studying these models...

Maybe you can be more convinced through [3] . Paolo Stocchi developed for the SELEN FORTRAN program.

[1] http://www.fis.uniurb.it/spada/SELEN_minipage.html (nb. this requires GMT, nearly impossible to install on Windows, and needs to be built from source in Linux. But OS X's homebrew has got it easily.)

[2] http://www.geologie.ens.fr/~vigny/cours/chp-gphy-2.html

[3] http://www.tudelft.nl/en/current/latest-news/article/detail/...

There's a guy over at http://blogdredd.blogspot.com/ doing some modelling in C++. I have no idea how good or bad his modelling may be.

Apparently the "Ice Age Effect", which has caused rotation to quicken by making the planet more spherical, dominated over the last 20k yrs. More recently, "polar ice sheet/glacier melting" has moved water mass from the poles to the ocean, causing rotation to slow. It isn't clear from TFA how these phenomena can be differentiated: in both cases ice is melting in polar regions. However they seem to have opposite effects? I'm sure Mitrovica has a way to tease out these opposite effects, and I'm sure they're not exactly offsetting, but without those details TFA is just confusing.

yeah that's what jumped out at me too... Shouldn't it affect both?

The older effect is the earth getting less flat and more spherical. The newer effect is water moving from the poles to the equator.

TFA:

What is the Ice Age Effect?

The Earth is growing more spherical because 20,000 years ago we had a lot more ice at the poles. When ice sheets were at the poles they kind of squished the Earth from both poles and the Earth flattened a little bit. When those ice sheets melted, that flattening started to rebound and we’re becoming spherical, so our spin rate should be increasing, like a ballerina or a figure skater. The ice age correction is a speeding up of the rotation rate.

In both cases, polar ice is melting. Since it's claimed that this melting has two opposite effects, it's unclear how to separate one effect from the other.

I found this confusing too. Some ideas:

- The recent melts have only been of a relatively small mass of ice, and the two effects on rotation aren't linear,

- There hasn't been enough time for the Earth's shape to have changed in response to the recent ice melts.

I'll admit I'm a bit suspicious, though.

The timescale, I think. If the ice at the poles melts, the water immediately (days?) distributes itself along the geoid, the earth's moment of rotational inertia increases, it slows down. But, once the weight is taken off the poles, the crust rebounds over a period of thousands of years, making the earth less oblate, its moment decreases, it speeds up.

This is why I ask questions on HN. Thanks for this; I think you're probably right. ISTM decent editing would have put this explanation in TFA.

Agreed...

This article is fascinating...this man--Jerry Mitrovica--is obviously a genius...the techniques used to arrive at the conclusions he's reached...just wow...

My takeaway is that the gravitational attraction of ocean waters that the Arctic, Anarctic and Greenland ice sheets currently provide will diminish as those ice sheets melt...

This means that water previously attracted to those ares will move south...

The polar regions will experience lowered sea levels...the temperate regions will experience a rise well beyond what was earlier predicted...

Sobering news...

He's such a genius that he uses Newton's gravity instead of Einstein's.

In the area he's working here, it's totally fine. You only need relativity when you have extreme speeds or masses. Knowing which approximations are fine to use when is a skill too.

Can someone explain the physics behind how the Earth actually accelerates because of this?

Conservation of momentum

Specifically angular momentum. The Earth's moment of inertia changes as the mass distribution about the axis of rotation changes. Given a constant angular momentum, this redistribution causes it to gain or lose angular velocity.

Any competant geologist would tell you cant use local examples to illustrate global effects. The Roman costaline could be under the influence of local tectonic forces that far outweigh the global trend. That is why it takes a lot of effort to construct accurate global trends which people constantly debate.

And indeed there are well known "sunken cities" off the coast of Italy, which is a pretty geologically active area...

http://www.atlasobscura.com/places/the-sunken-city-of-baia

Heh, for a second I misread: "The Roman coastline could be under the influence is local Teutonic forces".

In Finland the land is still rebounding from the last ice age. Land is rising roughly 1-9 mm per year depending on where the ice melted last.

Rate of lithospheric uplift due to Postglacial Rebound: https://fi.wikipedia.org/wiki/Tiedosto:PGR_Paulson2007_Rate_...

Is he really saying that Greenland melting would lower the sea lever in the near surroundings by 30-50m, or was there a conversion mistake?

It's counterintuitive perhaps, but yes. The opposite also happened: during the Antarctic glaciation (build up of ice caps), globally sea levels dropped by 60 m, but in the Antarctic region the sea level rose with 150 meters [1]. Simply because of the huge gravitational pull towards the mass. Ice caps can be several kilometers high...by g=Gm/r^2 and being on a sphere the effect is quite local.

[1] http://www.tudelft.nl/en/current/latest-news/article/detail/...

Surprised the article had nothing to say about thermal expansion of the sea, thought to contribute at least as much to global sea level rises as meltwater, and something which would likely also affect local sea level dynamics.

Nice that he recognizes that land mass springs back after the ice sheet melts. But it looks like he ignores the effect the weight of all that additional water has on pushing the sea floor down. And would that increase the subsuming of plates, thus speed up continental drift and geo activity?

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