1: Cloud seeding: The basic idea here is that clouds reflect sunlight more than land or water, thus cooling the earth. The reason I like this concept in principle is that if we had sufficient know how, we could do this in a way that mitigated droughts or other unwanted precipitation evens as well as cooled the climate. Though it would likely be expensive, it would generally be better than benign and could have a significant impact on global warming.
2: Ocean fertilization: The idea here is that much of the ocean is essentially a dead-zone due to a lack of iron in the water, which is necessary for life. Iron salt solutions would be attached to ships (probably already going about other business) and poured into the water in strategic places. This spawns plankton blooms (confirmed) which then presumably works up the food chain and ultimately sequesters carbon. Unfortunately, any carbon sequestration appears to be small and speculative, but on the other hand, if done well, we could use this to increase the biological capacity of the oceans, both increasing the amount we could harvest AND increasing the amount of life in the ocean. There is modest promise in this idea that is stupendously cheap, and it should be pursued.
3: Biochar: The basic idea here is to char plant material and use it as a soil amendment. Generally the idea is pretty sound and I don't see a whole bunch of downsides other than economic issues and a limited capacity, if managed properly.
4: Space mirrors: I divide these into two types - dumb mirrors whose only object is to block light, and smart ones which have good control over when and where the light is delivered. The first is possible with current technology but expensive. The latter is beyond us for the foreseeable future. Dumb mirrors, like any light-blocking scheme, have a huge downside - less light causes less photosynthesis, which means less life and lower crop yields. Also, it has no effect on ocean acidification and could disrupt ocean and weather patterns we rely on. To use any uncontrolled light-blocking scheme is an act of desperation. Controlled light blocking with smart mirrors, however, is something I have a hard time imagining our distant descendants not doing. Imagine moving light from the equator to the northern latitudes, making the former cooler and the later inhabitable. Imagine, using cloud seeding and the smart mirrors, turning Antarctica into an even huger block of ultra-cold ice, in order to offset rising oceans. Imagine using the smart mirrors to manipulate local weather patterns in order to ward off extremes. This is all possible in principle, but is not really relevant to solving climate change because it is still something that is far beyond our technological capabilities.
5: Light management with sulfates: This is basically the poor-man's version of dumb mirrors. It's cheap, but now you are making the air even filthier. Also, there is no meaningful method of controlling where the sulfate blocks light, so the technique cannot really evolve or improve towards being smart, as mirrors could.
6: Clean coal / carbon sequestration: Mostly a political farce. It's technically possible, but too expensive, and the sequestration is almost impossible to guarantee over long time frames. A limited amount of CO2 is and will be pumped underground in order to force out gas and oil, but the amount of demand here is trivial and by using the CO2 to extract fossil fuels, it makes the problem worse, not better. In the end, it is just thermodynamics. A coal plant would have to use a quarter of its output (at minimum in theory...in practice even more) just to compress the CO2 and put it back underground. Unless there is a pre-existing demand for CO2 nearby, which is rare, there is simply no way that this process will make financial sense vs wind, solar, or just about anything else. Also, since the plant would have to burn extra coal in order to compress and pump the CO2, it would release that much more soot, SOx, NOx, PAH's, particulates, heavy metals and all the other junk that continually comes out the smokestacks.