I am generally skeptical of geo-engineering proposals to counteract global warming. But I agree with the Carnegie Institution's Ken Caldeira that research into such proposals is worthwhile, just in case. As he frames the question: "Which is the more environmentally sensitive thing to do: let the Greenland ice sheet collapse and polar bears become extinct, or throw a little sulfate in the stratosphere? The second option is at least worth looking into."
NOTE: Post edited to correct Caldeira's affiliation.