Lots and lots of snow?
Lol jking, with less sunlight we could create a mass die off of vegetation and completely change the wind patterns and ocean currents, as the planet seeks to rebalance heat between the poles and tropics. The thing is when climate changes naturally, this happens very slowly and everything stabilizes and equlibriates and nature has millions of years to adjust. When human beings tamper with the system via climate change, everything occurs MUCH faster and there is no guarantee there won't be a lot of destruction before everything stabilizes and equilibriates-- and the new equilibrium point may not be where we want it to be. In the case of geoengineering, the changes occur even faster than anthropogenic climate change so you can guess there is even more of a chance things could go awry and create a chain reaction runaway effect. This has actually happened before and resulted in mass extinction events (via volcanoes).....the planet took a long time to equilibriate and when stability was finally reached 90% of life was killed off in the largest of these mass extinction events (the Permian mass extinction-- which also resulted in our best fossil fuel deposits.) We've already started the 6th mass extinction in the planet's history, this would accelerate that exponentially.
It's probably something that if it does ever get done, it has to be thoroughly tested first and needs to be approached with extreme caution.