Stopping emissions at source and improving energy efficiency should be the top priorities. The laws of thermodynamics and atmospheric physics are strongly unfavorable towards after-the-fact cleanup attempts. Every large-scale engineering project has side-effects, and one of such massive proportions is likely to have proportionately massive side-effects.
Geoengineering has also been used by polluters as an excuse to continue polluting. This isn't a problem with the proposed solutions, but more with the political use to which those solutions are put.
