What are the risks of developing geoengineering?
We don't know, and that's why we have to develop it.
that's the thing about undeveloped sciences we don't know what many of the risks are, Its a case of knowing what we don't know. Which is one big reason to develop it, so we don't cause harm accidentally.
Lets be clear we already are geoengineering right now, by driving cars, changing the landscape, polluting the air,  we are just doing it blind. We have to research things to understand the risks, and geoengineering has barely be scratched.