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December 11, 2011

Desalting The Oceans--The Next Forty Thousand Years

A guest post by Joseph Friedlander

Once in a while you read a statistic that gets you thinking. What I read was that the volume of salt in the oceans was the volume of the land of the African continent above the waterline.

That sounded amazing to me. I had to run the figures to see if it was true.

Volume of the oceans-- about 1.4 billion cubic kilometers. Estimates range from 1.3 to 1.5 billion cubic kilometers. A recent satellite based estimate is 1.332 billion km3

The total mass of the hydrosphere is about 1,400,000,000,000,000,000 metric tons (1.5×10^18 short tons) or 1.4×10e21 kg, about 0.023 percent of the Earth's total mass.

Percentage of the oceans that is salt--3.5 %




Pro-rated volume of the oceans that would be salt--46.62 million km3 (of the water-salt sea water mix)

This would be a “salt asteroid” sphere about 444 km diameter.

Actually the density of halite (rock salt) is around 1.5 so that mass of salt would be a third less volume --say 390 km diameter--but that is just the salt in the ocean neglecting massive deposits in the continents.

Salt deposits (and more so sodium and chlorine content) in the mass of the Earth almost certainly exceed the amount in the oceans. (If anyone has definite figures on whole-Earth salt content please cite them in the comments :))

Area of the African continent--30.2 million km²

Average elevation of the African continent say around 2200 feet (670 m)

So volume of Africa above sea level is .67 x 30.2 million km3= 20.234 million cubic kilometers.

So actually the estimate of volume of salt in the oceans was conservative-- really the salt is over TWICE the volume of Africa over the waterline.

Every year the sun desalts part of the ocean for free-- you know it as rain or snow when it falls.

Average ocean depth say 3800 meters.
Average evaporation on the surface say 1 meter per year.
(So if the vapor were magically transported off Earth, it would take 3800 years to dry up the ocean from evaporation alone.)

And of course it would leave a pile of salt twice the volume of Africa.

Each year however the precipitation falls as rain or other forms of hydrometeors (atmospheric water phenomena)

World wide rainfall has been estimated as 505000 km3, 398,000 of it in the oceans. Just to give you an idea how much fresh water this is, the Amazon, with one-fifth of the world's total river flow has an average discharge of around 7000 cubic kilometers a year. Lake Michigan’s volume of freshwater is around 4918 cubic kilometers. Lake Baikal contains 23,615.39 km3. So there is an enormous amount of water in the natural rain cycle per year. So enormous in fact that the mere mechanical energy of it falling from an average freeze out height of say 2500 meters would generate nearly 392 terawatts constantly (each billion tons of water falling from 2500 meters will generate say 6.81 billion kilowatt hours) For comparison we use about 16 terawatts in our world wide civilization today.

This is not terribly efficient-- 392 terawatts which might be tapped versus 174000 for the whole Earth insolation (power supplied to the dayside of the Earth 24/7 by the sun) so net efficiency at 100% hydroturbine efficiency would be only 1 part in 444.59 or .224 pct. Supposing a future civilization had the capability to totally capture all rainfall at 2.5 kilometers height before it hits the ground or sea would only yield and efficiency of 1/5% efficient of whole Sun input to Earth (but it would be sustainable and green and would enable a pollution free upper limit of say 24 times today's energy consumption without large pollution.).

If Man in the future can tap all the water supply of the air this way, his power needs may be met cleanly. Suppose for a second we could, and lets’ imagine a world with a way to not just capture all rainfall but which literally covers the continents with advanced Bolonkin style enclosure systems to avoid losses to all freshwater evaporation (this is a thousand years from now). After supplying the needs of the ecosphere it would be a waste to let all the fresh water run off into the ocean, so suppose we capture it in large water bags made from rock fiber and film produced on a scale inconceivable today such as postulated in--

http://nextbigfuture.com/2011/02/new-space-age-materials-for-new-space.html

http://nextbigfuture.com/2011/01/two-world-industrial-bootup-enabling.html

and immersed in the ocean. These, being filled with fresh water, would naturally float. They in fact might be the foundations of floating cities (as well as emergency fresh water supply)

Even a small bag system can hold a thousand times its’ mass. A large one might hold a good fraction of a million times its mass. So a cubic kilometer, a billion tons, of fresh water might be contained by several tens of thousands of tons of bags.

After say 3800 years of this constant fresh water capture, the oceans would be progressively more salty unless steps were taken to remove salt from other water--perhaps regular ocean water might be sequestered underneath the floating bags (thus protected from evaporation) and more concentrated brine might be preferentially evaporated on top leaving salt. The net effect would be constant saltiness in the salt water portion, a growing fresh water portion in bag reservoirs and elsewhere, and large amounts of salt needing disposal in space or elsewhere.


Simply for reasons of saltwater species ecosphere conservation the entire ocean could not be desalted-- so we might imagine a world wide network of giant floating water bags (constantly needing refurbishing each generation) but between them the ocean ecosystem would be intact though diminished (exactly as nature on the continents is reasonably intact but diminished in our more primitive age) The mass of the bags would be stupendous, easily that of a major asteroid. This would be the product of thousands of years of future industry and is so far beyond current capabilities that we merely outline the (remote) possibility.

Needless to say the networked bags might be the foundation of an amazing mass of floating stuff (ie a super civilization) based in the ocean (enabling floating roadways and vacuum subways between continents for 1 hour around the world ballistic-in-vacuum undersea pipeline journeys) vast seasteading, and so on. There is the little matter of the Africa sized pile of salt but in fact the space industries of that far off era might welcome having an exportable salt supply sourced from Earth. (Sodium and Chlorine are quite useful chemicals for many purposes, and Chlorine (though not Sodium) is scarce in space. And molten salts have their own uses.) It would mean exporting around 12000 cubic kilometers of salt a year, beginning a century from now (the density of salt is about 1.5) or about 18 trillion tons a year. This itself would require about a gigawatt year per million tons (a terawatt year per billion tons) for export to escape velocity. This would require many times the power we just generated by capturing all the rainfall. However, another possibility exists, that of direct mass momentum exchange with space.



The rotovator is one but only one idea of this sort, and we suppose in the economy of a millennium from now such momentum exchange works are quite conceivable. (Why this is not practical today can be seen in this graph



--the materials available would support lunar escape velocity, not earth escape velocity --the multiplier factor at the bottom of the graph is times Vc, characteristic velocity of a given material, for example quartz fiber is 1.8 km sec, times a tip speed factor say of 1.5 to yield 2.7 km sec tip velocity--great for the moon, suborbital for Earth) But with suitable tether materials, a billion tons of incoming space industrial product (say, sea bags) or mineral wealth (e.g., magnesium oxide to absorb excess CO2) could be exchanged for momentum with a billion tons of outgoing salt for use in space. Needless to say if we had this technology we could also export garbage and toxics, and wastes of all kind for disposal and recyling with the unlimited energy of space.Theoretically it would be quite practical to desalt perhaps half the volume of the oceans within perhaps 4000 years with the right rain capturing equipment, then keep the rest as a nature preserve. )

Will this level of complete terraforming actually be done? I rather doubt it in its’ entirety, it’s just a vision of the possible, not one I necessarily advocate. If you asked 1000 years ago the best road system conceivable an educated monk might have guessed at a comprehensive restoration of the Roman roads, not superhighways, a train network or a world wide network of jumbo jets and airports. I think it quite likely that there will be huge capture of rainwater because of the enormous shortage upcoming by say 2025. (Google water+shortage+2025) More than enough fresh rain water in the world falls than needed to irrigate every desert; the problem is the huge cost and logistics of capturing and pumping it.

A minimalist version is quite possible too--capture say 1/1000th of rainwater (505 km^3 a year) and recharge aquifers Man’s carelessness has depleted. There are many deserts with deep water tables; probably a century of such water capture could be pumped into the ground with nothing but positive results.

But if the maximalist version were done what would be the results? Lower ocean levels.

Consider that the ice in the cryosphere if melted would raise sea levels 80 meters--

Ice sheets are the greatest potential source of global freshwater, holding approximately 77% of the global total. This corresponds to 80 m of world sea-level equivalent, with Antarctica accounting for 90% of this. Greenland accounts for most of the remaining 10%, with other ice bodies and glaciers accounting for less than 0.5%.

The Antarctic Ice Sheet contains 30 million cubic kilometers (7.2 million cubic miles) of ice.

If the Antarctic Ice Sheet melted, sea level would rise by about 60 meters (200 feet).

Taking out half the salt in the oceans would decrease the volume of the seas, but not by as much as a glacial meltdown would have them rise. If on the other hand the glaciers expanded, sea levels would drop again (and the northern continents would be in danger again)

But the key to glacial formation is fresh water falling as precipitation when its so cold it freezes out. If did have a nearly unlimited amount of fresh water we might be tempted to freeze some of it out in the winter to store it in the icecap (saving on the giant sea bags) If we doubled the volume of the ice (with some of that abundant power by spraying the surplus freshwater on cold areas (including some mountains) and exported the salt to space it is conceivable we might lower sea levels by 80 meters or more--possibly by as much as 150 meters, exposing the entire continental shelves of the continents.--possibly another 8 percent of land area. Most of this would be level plain, with plenty of fresh water available ideal for production of all kinds. For example, the far future version of China probably would favor such a project because the ~12% of land they have that is well watered productive plain could easily be doubled.


The argument which undoubtedly will be employed is that the melting of the glaciers (at the end of the Ice Ages) stole good lowland from China. (The area in light blue off the Chinese coast in this picture)



China will want that land back.
Any country with a coastline could make similar arguments and it is said that 38 percent of the world’s people live within 100 kilometers of a coast 44 percent of the world’s people live within 150 kilometers of a coast and nearly 50 percent of the world’s people live within 200 kilometers of a coast. So the political votes to make dramatic terraforming happen might be more easy to mobilize, in the far future, than many anticipate, from fear of loss (sea level rise) or prospect of gain.
And of course if the cryosphere (ice) melted we’d be looking at massive loss of some of the most productive land on Earth. See this map--everything not in dark green goes:



Even if anthropogenic global warming (long term the risk is not from CO2 but waste heat) does not happen, interglacial periods do. Lowering sea levels might look more attractive then.

My own position on this is caution: First do no harm. But then I don’t live in the far future; but the people of the future will. it is their voice that will count.

Basically an entire industrial revolution, or several has to happen to make what has been outlined in this article possible. But stranger futures have been postulated in science fiction.



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