FWIW, Hansen is currently calling for a reduction in the atmospheric CO2 concentration to 350ppm:

If humanity wishes to preserve a planet similar to that on which civilization developed, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm.

Given that the past trend (aside from seasonal variation) is nothing but up, that's going to be one heck of a tough challenge.

Given that the past trend (aside from seasonal variation) is nothing but up....

That's not quite true.  If you look at the Keeling curve, there was about a 2-year freeze right after the eruption of Mt. Pinatubo; it appears that reduced temperatures helped sequester carbon through greater ocean uptake, reduced water stress on plants, or other factors.

Global dimming wouldn't help us for long; we need to get working on carbon-negative agricultural and energy systems.

There is a discssion of this preprint starting at realclimate.org
http://www.realclimate.org/index.php/archives/2008/04/target-co2/#more-5...

Chris