On the nascent science of geoengineering

I'm no scientist. I'm just a guy. But I heard a scientist talking about geoengineering  on the radio the other day, and I'd like to say for the record that I do not agree. Geoengineering refers to proposals to manipulate the planet's climate in order to counteract global warming.  An example: spraying chemicals in clouds to make them more reflective thereby bouncing the sun's rays away from the earth. Another example: sucking carbon dioxide into big holes in the ground. Hmm.

Now, I like tools well enough. I appreciate well-thought-out and well-crafted technological solutions. Take hammers. Hammers are cool. Hammers do their job really well, and you can choose from a number of different designs depending on your work-style and end-purpose. Hammer designers have been working on better hammer designs for a long time, and at this stage, we'd have to say that hammer design and technology is pretty mature. And yet, we occasionally smash these modern, well-designed hammers into our thumbs and create problems for ourselves. Nobody can design all the risk out of our tools.

So my question is, how worried should we be that well-meaning scientists, as smart as they may be, are talking about designing planet-sized hammers to solve a global problem? Even if I believed that Scientists (that group of people who totally agree all the time on how things work ... right?) could understand all of the large-scale mechanisms at work in global climate patterns, which I don't really believe, I'm not sure I would want them trying their hand at a global solution. What if the hammer slips? We're not talking about a big toe here. We're talking about the Earth.