I think we as a humans on our only planet must change.

I don't think anyone who has seriously considered the issue will argue with that, or assert that geoengineering is a solution to climate change, or even particularly desirable all else being equal. Probably even the proponents of geoengineering schemes don't like their ideas.

However, any change process (or at least any that does not involve a sudden collapse) would take years to decades to implement, and the reversal of the climate changes already committed would take decades to centuries - probably longer.

There is no point in geoengineering unless humanity embarks on a sustainable path. But if this should occur, the question to my mind becomes: If it is within our power to limit the extent of climate change by geoengineering - or even if there is a certain likelihood of our being able to do so - don't we have a moral obligation to at least seriously study the options?

The fact is that what we're experiencing right now is a top-down disaster. -Paul Krugman

Perhaps we could couple it to mechanical air capture, increasing the carbon content of soils, and reforestation, depending upon how quickly we need to hit the break. Boyd says the following in the Q&A on ieee spectrum:

IEEE Spectrum: Q&A With: Ecologist and Geoengineering Expert Philip Boyd

Spectrum: Which schemes are the most promising and why?

PB: The one I favor the most is atmospheric carbon capture. That is the direct capture of carbon dioxide using some sort of scrubbing system. There have been proposals from U.S. researchers where they've described a medium-sized water-tower structure with a turbine system. As the wind blows through, there is a scrubbing device, which has a chemical absorbent that absorbs the carbon dioxide. Then the carbon dioxide can be converted into a solid and stored so that it can't impact climate. And with these towers, it is possible that they could be incorporated with wind turbines, so you're actually generating power to help drive the process.

The reason that one gets a vote is, although it's one of the more expensive ideas, it is much lower risk. In terms of an emergency stop, it can be shut down quickly, and you can verify how much carbon is being removed.

Unfortunately, the first tentative results I know of have been mixed and the energy needed to pump CO2 underground remains.

But it's nothing that a few rich countries could not do. The US is spending more on its wars of choice, alone.

Whitening clouds might be cheaper, still. Here's a fun piece of a Scientific American article on geoengineering:

Geoengineering: How to Cool Earth--At a Price: Scientific American

In Salter's concept, turbines spun by water moving past the ship would generate the electricity to keep the cylinders spinning and also to spray seawater out the stacks in 0.8-micron droplets. Salter and Latham estimate that 1,500 ships, each spraying eight gallons a second--and each costing $2 million, for a total of $3 billion--could offset the global warming caused by a doubling of CO2. Half the job could be done, according to modeling results from the Met Office Hadley Center for Climate Prediction and Research in Exeter, England, by deploying ships over just 4 percent of the ocean.

Still, no one has modeled how evenly the cooling would spread around the planet. "You could end up with a polka-dotted world, where there are really cold places and really hot places," Battisti says. Another concern is drought downwind of the spray vessels; clouds made of many small droplets last longer, which is desirable in a sunshade, but they also produce less rain.

Finally, just how much brighter the new clouds would be is not known. Existing climate models overestimate the effect: according to them, the aerosols in the atmosphere right now should be canceling global warming, which is manifestly not happening. Rasch has thus started modeling La­tham's idea. "This is one of the parts of climate that we understand most poorly," he says.

Still, as geoengineering schemes go, spraying seawater into the air from wind-powered vessels sounds pretty benign. If anything went wrong, Latham says, you could shut off the spray within days or, at most, a few weeks--whereas sulfuric acid in the stratosphere would stay aloft for years. "It's definitely worth looking into," Wigley says. But only a field test could answer some of the questions about the idea--and so far the only support Latham has received has been from the Discovery Channel. In need of good visuals for a documentary series on geoengineering, television producers funded the construction of a small Flettner ship.

On the SO2, it is currently tropospheric and more localised (the concentrations are mainly on a sub-continental scale, I think). The proposal of Crutzen et al is to have stratospheric SO2, which will probably have negative effects on the ozone layer, for one.