Would it be possible for geophysicists/geoengineers to develop an artificial way of trapping carbon in the ocean? There's a mechanism by which the southern ocean sequesters carbon from the atmosphere. It happens when strong winds displace a large slab of surface water, accumulating in a specific region where the localised excess water in the surface layer gets injected downward into the ocean's interior. This makes the ocean's absorb about 25% of annual man-made $CO_2$ emissions.
Are any geophysicists and engineers working on developing a man-made mechanism by which we can make the ocean's absorb more carbon and therefore reduce climate change? Is it humanly possible?
NOTE: I read an article on the phenomenon in Physics World magazine, and it didn't mention anything about scientists looking to make it happen more.
 A: Another proposal involves putting large pipes into the middle of the oceans (tropical regions) and pumping cold water from the depths to the surface.  Here is one paper that analyzes it.
I will try to summarize.  This is a very multifaceted idea, so I'm going to try to keep to pure physics as well as I can.
Firstly, what is the engineering involved with this?  In order to pipe cold water from deep down to the surface we would have to pump it against the density gradient.  Hot water in this temperature range is less dense, so it naturally hangs around the surface.  This would take energy and very large pipes.  The good news, however, is that the conditions in the ocean make this a little easier in several ways.  The construction and placement of mile-long (or so) pipes wouldn't be that impractical, as you would have a floating thing holding them up and they would just hang there.  The pumping could actually be done by the waves themselves, and it moves the pipe up and down.  For this deployment, you would mainly require some one-way valve that allows the water to travel up the pipe but not down.  This wouldn't be all that difficult.  So we've established we can engineer and build these things that would make the ocean surface cooler.  Studies also seem to indicate that the capital cost wouldn't be prohibitive.  Ask it of your engineers and they should be able to make it.
Next, how would this affect climate?  Several ways.


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*The decreased surface temperature of the ocean would cause it to be a large Carbon sink.  The ocean has already demonstrated a measurable increase in pH and a decrease rate of CO2 absorption, which is a natural consequence of increasing concentration.  Decreasing the temperature would cause it to suck up more.

*The decreased surface temperature would reduce atmospheric temperature.  This is a pretty obvious impact, and the paper I reference notes that this could be the most major immediate effect.

*I would change the ocean life dramatically.  Obviously, since phytoplankton are one of the most major photosynthesizers in the world, changing their environment would change the rate of CO2 capture by the environment.  How?  I have no idea.


I'm not sure the context or motivations in which researchers propose and discuss such ideas, but I've heard some arguments for geo-engineering proposals in general, of which this proposal is one.  For one, the idea is entirely doable as I've argued.  It could decrease the temperature of the Earth and it could be done with our resources today.  If the situation on Earth became so perilous to human life, it's likely that politicians would order such a thing.  With that established, the motivations become awfully warped.  Many people argue that quantifying the effect of geo-engineering in the future could motivate current negotiations for emissions mitigation to take the necessary steps.  Almost all geo-engineering proposals have a temporary effect, so after implemented, they risk a return to an even hotter climate, and possibly deleterious effects due to the solution itself that could be even worse than global warming itself.  To add my own personal thinking, it's obvious that we're not reducing emissions today and won't in the conceivable future, so if one believes climate change will significantly impact life, then it's very likely that some form of this geo-engineering will be the future, whether we like it or not.
Here is a picture:
http://www.popsci.com/node/9798
(illustration credit to  Graham Murdoch of Popular Science, believed to be fair use)

A: It is possible for geoengineers to trap carbon dioxide in the oceans through a process called ocean fertilisation. The principle behind ocean fertilisation is that by dumping nutrients into the ocean we can encourage the growth of phytoplankton which absorb carbon dioxide in the atmosphere. When the phytoplankton die, they'll sink to the bottom of the ocean and lock up some of the carbon dioxide that was inside of them when they were alive, taking it out of the atmosphere. There are a couple of nutrients that could be used to encourage phytoplankton growth however the most favourable would be iron since this has been shown to produce large quantities of phytoplankton using only small amounts of iron.
Since the amount of carbon dioxide each phytoplankton absorbs is quite small, this process still takes thousands of years to have any effects on global carbon dioxide levels so there isn't much in the way of firm evidence that this process will work. We do, however, have evidence that fertilising oceans with iron does produce blooms in phytoplankton populations as several experiments have been performed where iron was dumped into the ocean and phytoplankton populations grew greatly in the area. A recent (illegal) experiment, for example, where 100 tonnes of iron sulphate was dumped into the Pacific Ocean, produced a plankton bloom around 10,000km2 in area. The problem with all of the experiments that have been performed to date is that they are very short-lived as the plankton either die out or are eaten by predators before any effects on carbon dioxide levels can be measured.
Assuming iron fertilisation is effective at reducing carbon dioxide levels, carrying out the process on a global scale will likely be impossible due to the cost of such a project. It would cost billions of dollars every year to provide the necessary nutrients to maintain a large population of phytoplankton and to replace the phytoplankton lost to predators. 
A: Look at Frank Savage's "Millenial Project" the section titled Aquarius
The basic idea there is to construct floating islands in equatorial waters (few big storms) They are powered by OTEC the difference in water temperature between abyssal water (floor of the ocean, 12,000 feet down on average) and surface water. (4 C vs 27 C)
 A side effect of bring nutrient rich water to the surface, is a vast plume of plankton. Savage surrounds his floating city with a series of sports field sized ‘lily pad’ aquaponics pools. The pools have two functions: Inflow water from the edge passes through a series of ponds and is warmed further, making the cold water engine more efficient. Outflow water is seeded with plankton, and passes through a series of fish tanks, both swimming and shellfish. The water is still rich in nutrients as it passes out of the series of ponds, and a plume of plankton supports a significant fishery down current. (An OTEC plant has to move a lot of water for each kWh generated.)
Much of the construction is done using seacrete -- a complex of calcium and magnesium carbonate precipitated from seawater.  This, by reducing the carbonates in the water may help drop pH in the ocean as a whole.
One of the side effects of an OTEC plant is the potential to produce a lot of fresh water.
Like many utopias, the devil is in the details. But it’s worth looking at.
OTEC is potentially real: Ocean thermal energy conversion - Wikipedia
Thermodynamics rear's it's ugly head:  You have to move a LOT of water per kWh.  I think I read somewhere that the recoverable energy per cubic meter of water is on the order with tidal pool generation.  
OTEC:  https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion
More information on the Millennial project: 
 https://en.wikipedia.org/wiki/The_Millennial_Project
Seacrete: https://en.wikipedia.org/wiki/Biorock
Details on the chemistry of seacrete:  https://www.intechopen.com/books/electrolysis/marine-electrolysis-for-building-materials-and-environmental-restoration  Yes it can be done in a CO2 negative way.
Potentially quite doable:


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*Open ocean isn't under national purview, so any company with deep pockets can play.

*The same tech used for floating oil platforms can be used for the core of the city.

*Producing a surplus of fish, shellfish, and possibly energy, and fresh water makes it potentially economically self sufficient.

