My fear is not that it wont work; may fear is that it will. First, it is obvious that reducing emissions of greenhouse gases is expensive and difficult, and that it requires changes in life styles that many will find difficult; hence, if we can fix this some other way those changes will just not happen. Second, if it is difficult to reach international agreement on emission reductions, it will not be any easier to find international agreements on geoengineering. Third, assuming we by some miracle can reach those agreements, what happens in ten or twenty years from now when a major contributor decides to step away? We already have major influential actors that come and leave the IPCC and the Paris Agreement according election results every four years; why should that not happen with this?
Add to this the fact that solar and thermal radiation are both unevenly distributed across the Earth, thus changing one of these will not just alter the average energy flux; it will also change the distribution of net energy flux, with unexpected consequences many of which are likely to be adverse. If the last several decades should have taught us something, that should be that every time we tamper with the natural system the end effect is usually bad even when we have the best of intentions. So just don’t!