As I have written elsewhere I do NOT think the issues of geo are primarily ethical (in any sense different from ethical issues in rest of science and technology) as much as they are methodological.
The methodological issues arise to the extent intervention is planetary wide– be it albedo geo or carbon capture – AND sub-scale experimentation also has to be planetary wide as well.
Under these circumstances the potential costs of “unknown unknowns” are quite different from lab or limited “field” experiments.
As such, the burden of proof is for researchers to show that we know enough about the atmospheric (and ocean) systems to delimit the range of risk OR that we know enough to project from sub-scale intervention to full scale intervention.
I am not saying such arguments can’t be made – but they have not been laid on the table as yet. I suspect we do know enough about both the atmosphere and the ocean (based on theory, history and volcanic activity) to have high confidence that there would be no large scale catastrophic weird non-linearities.
As such, if that were right one could make an extrapolation from sub-scale to scale in strength or extrapolate in time.
That said, none of this covers issues of social, economic, cultural or agricultural effects. But I doubt these are areas in which the worst case scenarios would be deal breakers – at least when it comes to sub-scale experimentation.
Of course, to the extent that there are non planetary wide geo options they are to be vastly preferred because one can limit risks associated with unknowns in experimental stages.
On moral hazard – this is something about which there is research, we ought not to proceed on the basis of intuition or anecdote. That said, from a policy point of view, I think it is as important to find out if sulfur injection works technically just to take it off the table for policy makers as a fix as to leave it on.
Finally, it is a mistake to view geo as the first time we have engaged in an intentional planetary wide intervention. For example, killing off smallpox was just such an intervention and done deliberately. If you think the later was justified (and most do), you need something more to make the argument that geo is morally impermissible merely in virtue of our hubris.
We are at a fork in the road in which we need to de-carbonize. Can we do that with less risk and damage if we do it with geo or without? That is the crucial comparison class.