Effects of Rising Ocean Levels

[zapatos]

As I recall parts of the North American continent have been 'rebounding' since the retreat of glaciers from the last ice age. As sea levels rise can we expect the ocean floor to sink due to the additional pressure caused by so much more water over it? If so would the sinking ocean floor in any meaningful way mitigate the risks due to rising sea levels? If the sea floor sinks would that result in any kind of corresponding lift of land not covered by ocean?

[JohnB]

The short answers are "No" and "Yes".

 

The crust is far more flexible than most people think. North america is still rising in areas due to the rebound. Southern England is sinking while Scotland is rising. Scandinavia is rising. (It was depressed some 800 metres at the LGM)

 

Meanwhile the sea floor is sinking due to the extra weight of water. On top of all this there is the usual rise and fall of the slowly moving continental plates.

 

The bottom line is that the Earths crust is a tremendously dynamic system and I doubt that anybody could give a more than WAG as to what effect all these factors will have on sea levels.

[michel123456]

The short answers are "No" and "Yes".

 

The crust is far more flexible than most people think. North america is still rising in areas due to the rebound. Southern England is sinking while Scotland is rising. Scandinavia is rising. (It was depressed some 800 metres at the LGM)

 

Meanwhile the sea floor is sinking due to the extra weight of water. On top of all this there is the usual rise and fall of the slowly moving continental plates.

 

The bottom line is that the Earths crust is a tremendously dynamic system and I doubt that anybody could give a more than WAG as to what effect all these factors will have on sea levels.

 

Why "extra weight of water"? Continents must have an average density higher than water, I suppose. IOW water is not "heavy" compared to anything else. If water was so heavy, the crust would be thicker under the oceans, which is not the case. I mean heavy are the continents, not the oceans. The rebound is connected to the loss of weight of ice upon continental masses, not about an increasing or decreasing depth of the seas.IMHO.

[swansont]

The rebound is connected to the loss of weight of ice upon continental masses, not about an increasing or decreasing depth of the seas.IMHO.

 

Yes, but there are two effects being mentioned here. The rebound confirms that the earth's crust is flexible (earth tides confirm this on a daily time scale). Which means that additional water should have an effect. The movement of water from the tides affect this, but there a larger change in depth involved than sea level rise.

[JohnB]

Why "extra weight of water"?

 

When we are talking about rebound we are talking about the difference between the weight during the last Ice Age and now. The land rebound is easy and obvious, take the ice away and the land goes up. But that is just the first order effect.

 

When land is depressed due to ice there are two distinct immediate effects. The first is the depression directly under the ice mass but the second is a bit odd. There is an upward bulge created outside the ice mass. This means that during the Ice Age Scotland was depressed due to the weight of ice, but southern England was artificially raised by the bulge. As these return to normal (whatever that means WRT long term geology) Scotland goes up and England goes down. BTW, I remember reading that some places in Canada are still rising at a rate of 2cm a year, which is massively fast when you think about it.

 

Obviously when the ice melted it went into the oceans and they rose. This rise wasn't any piddly little 18cm, the oceans rose nearly 150 metres. The extra weight of a million square miles of water 150 metres deep is enormous, so the crust under the oceans depresses. It's not as dramatic as the depression of land under ice, but it's still there. Think of it this way, if the ice could depress 1,000 square kms of land crust by 800 metres, then when it melts, the water would depress 1,000,000 square kms of ocean crust by 1,000 th of that amount or .8m. And we are talking about millions of square miles of ice, not mere thousands.

 

To put it another way. If we only consider the areas where the water is more than 150 metres deep, basically the places that were water during the Ice Age, then every single square kilometre of that ocean area now has an extra 150,000,000 tonnes of weight bearing down on the crust. It also helps to remember that the Earth has a pretty much constant volume under the crust, so if one bit goes up, another must go down.

[michel123456]

I thought it was more a phenomena to be related to the archimedes principle. Continents and oceanic crust are floating on the mantle like icebergs on the ocean. When you put more material on the land (ice in the circumstances), logic says that the continent goes down and that the regions without ice go up, as you said.

Because of the buoyancy, the downward displacement must be related to the average density of the whole continent compared to the density of the material the mantle is made of. Compared to rock, water (& ice) has the lowest possible density for a solid.

So IMHO and if the above is correct, a huge amount of ice should contribute to a very few downward displacement. When the ice melts, the rebound of continents must be balanced (I suppose) by the downward of something else, the ocean floor, because there must be equilibrium. I don't know if the weight of water has anything to do with it, because it is just like incorporating to the ocean crust a material of very low density: in fact you are reducing the average density of the material.

 

--------------------

While writing this, I have doubts. Don't shoot too hard.

Edited April 3, 2011 by michel123456

[JohnB]

Not going to shoot at all.

 

What you said about the movements is basically correct.

 

Think of the Earths surface as comprising of 3 things, the oceans as they were during the Ice Age, the land masses and an absolute sh*tload of water that can be ice or water. During an Ice Age the water is ice and is on the land, so all that weight is on the land and the land goes down. After the Ice Age the weight of the water is in the oceans so the ocean bottoms go down and the land goes up.

 

That the density of water is less than that of rock really doesn't come into it. We have a large gel filled ball called Earth and the weight of a vast amount of water is being moved from one place to another. Wherever the weight of the water is, the surface underneath will be depressed by some amount.

 

It becomes clearer if you think of the water that made up the ice shields as something separate to the rest of the planet, an extra weight that is being moved from place to place.

 

Cheers.

[lemur]

Why isn't sea level just determined by how much or little land erodes into the oceans compared to how much is forced above sea level my mountain/land-generating plate movements?

[SMF]

The crust of the earth is less dense than the mantle, so it floats on top. Thicker continental regions of crust rock (largely granite) have a little higher altitude on top, but also depress the mantle on the bottom a little below the crust mean. Thinner regions of ocean crust rock (largely basalt) have a lower altitude on top where the ocean water collects. Crust rock is approximately 2.65 times as dense as water, so moving it around between the top of continental crust and ocean crust regions is not a zero sum game, but effect the ocean and continental altitudes differentially such that, for example, the ocean is relatively lower during glacial periods and higher during interglacials. Search for "Isostasy," which is the description for the study of these factors, and you will find research such as this- http://www.ualberta.ca/~dumberry/geoph440/papers/peltier_areaps04.pdf. SM

[lemur]

the ocean is relatively lower during glacial periods and higher during interglacials.

How much of the lower ocean level is due to precipitation freezing before it runs back into the ocean and how much is due to plate tectonics?

[SMF]

Lemur, I don't think that plate tectonics are affected much by surface temperature. SM

[lemur]

Lemur, I don't think that plate tectonics are affected much by surface temperature. SM

?!?! what did I say that suggested that plate tectonics are affected by surface temp?

[zapatos]

How much of the lower ocean level is due to precipitation freezing before it runs back into the ocean and how much is due to plate tectonics?

In what way does plate tectonics lower the ocean level?

[SMF]

Lemur. You either forgot what you wrote, or can't read what you wrote, in response to my comment about low ocean levels during glacial periods. SM

[JohnB]

How much of the lower ocean level is due to precipitation freezing before it runs back into the ocean and how much is due to plate tectonics?

 

If you mean "lower ocean levels relative to todays level", then most of the change was due to the water being locked up on land as ice.

 

As SMF said Isostasy is the study of this area. Continental drift does effect sea levels, but generally on a long term scale, IIRC in the very early days of the planet, most of the oceans were quite shallow and it was thought that there were very few mountains. Deep oceans and high land didn't happen until the plates started to move around. Once things got moving life got more interesting. Deep water got deeper due to the extra weight of water while mountains got higher. Higher mountains had more erosion which moved weight into the oceans which makes them deeper due to weight and shallower due to being filled with silt. Erosion also lightens the land and allows it to rise relative to the sea.

 

Ice Ages come and go, moving vast amounts of mass around the planet and adding to the deformation of the crust. Magma breaks through the crust creating new islands which displace water and leads to a rise in sea levels. However most of the rise and fall of sea level relative to the land due to geological forces is small for short time scales, say less than 20 million years, the big one is always the ice. Without considering the ice, sea levels only change slowly due to deformations of the underlying crust. Ice, land ice that is, directly effects the amount of water available to fill the ocean basins. During an Ice Age millions of cubic miles of water is being removed from the oceans and placed on the land. The Laurentide Ice Sheet in North America covered more than 4,000,000 square kilometres with ice over 3 kilometres thick, that's 12 million cubic kilometres of water that was removed from the oceans and placed on the land, making a huge difference to relative sea levels.

 

The big thing to remember is that WRT to sea levels, temperature and most other factors on the planet, there is no "normal". We tend to think of the levels of the last hundred years or so as "normal" sea levels, but 12,000 years ago it was "normal" to walk from Alaska to Russia and from England to France. The process is dynamic rather than static. Sea level is always relative to the land and a "sea level rise" could be due to more water and the sea rising or due to the land sinking.

 

Even on short time scales the difference in sea levels can be dramatic. Evidence suggests that a mere 8,000 years ago in areas like the Maldives sea level was 2 metres higher than it is today. This is a local effect and the reverse is also true. Beaumaris Castlein Wales was build around 1300 AD and had a sea dock on it's southern wall where ships of up to 40 tons could tie up, granted it was a tidal dock. That dock is now 200 yards from the sea. Another castle (the name of which eludes me ATM) was reinforced and supplied by sea during a siege, it is now more than a mile from the water. However silting from erosion is probably a factor in both of these cases, how much of a factor we just don't know. Silting isn't a factor in Tonsberg Norway where the old Viking docks are now several metres above sea level. Scandinavia also has some ports that had to be moved due to land rise, what were ports in the 11th and 12th Centuries are now 5-10 kilometres inland, quite a change for a few hundred years.

 

Similarly I've read about but have yet to confirm by original source documents a tactic of Drakes against the Spanish Armada. Apparently there was a rock shoal that submerged over a 60 odd year period in the English Channel. These submerged rocks were shown on English maps but not the Spanish ones. The idea was to try and force to Spanish into sailing into the area and grounding themselves. Did the sea level rise or did the land subside?

 

This is why satellites are superior to tide gauges when measuring sea level. If the land is subsiding while the sea level is rising it will appear that sea level is rising faster than it is. However if the land is rising faster than the sea level is, then the gauges will show the sea level dropping.

 

Like most things in planetary climate, sea level and its rise and fall is a very complex question with many interacting factors.