Physics Professor, IPCC Lead Author
The so-called geoengineering approaches are really only a last resort, because all of them have potentially harmful environmental side effects, such as additional ozone depletion, and we're not at the point yet where we're desperate.
We don't understand the climate system very well and so trying to engineer a system that is probably unknowable and almost certainly uncontrollable is a very frightening thing. ... This may be a terrible idea but it might be better than the alternative, which is to let greenhouse-gas forming run away.