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.
- 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:
(illustration credit to Graham Murdoch of Popular Science, believed to be fair use)