Geoengineering refers to deliberate large-scale manipulation of the planetary environment to counteract human-caused climate change, most notably global warming. Some scientists look at geoengineering as one of the few remaining solutions to tackle climate change but there are many things that could go wrong with this approach. Let me mention the most important ones:
1) The most obvious is of course the climate catastrophe. Computer models and simulations, despite taking into account countless factors that contribute to our climate are still incomplete because science is yet to fully grasp everything that goes behind the climate meaning that our still limited understanding of the world's climate system could result in major climate catastrophe, even worse than the one we tried to fix.
2) Various political issues would also occur because if changes in the climate affect various parts of the world, the question of who should control the global thermostat also raises issues, and one must ask whether one or few nations really have the right to decide what is best for the rest of the world.
As some middle solution, there were scientific proposals called a "soft-geoengineering" approach, in which changes we make are still widespread, but are reversible and predictable, though whether we actually can predict all factors and guarantee 100% reversibility and predictability, still remains a question.
Geoengineering should really be the last weapon against climate change, and even then it should be taken only in small doses because sometimes even the best possible intentions can go horribly wrong. The world should rather focus on reducing the carbon emissions by shift to clean energy sources. That's much safer solution to our planet and life on Earth in general.