Here are reasons why geoengineering should be opposed:
Firstly, it does not work. There is no proven track record for any of the technologies, and there are many unknowns and significant risks associated with them. In certain cases, the results could be clearly disastrous.
Secondly, it is or will be another tool of weaponization. Computer simulations demonstrate that geoengineering interventions can produce winners and losers on a regional scale; geoengineering will unavoidably become a weapon to the degree that it successfully modifies climate patterns in a predictable manner.
Thirdly, it detracts from real solutions. By promising an immediate fix, geoengineering raises the possibility of delaying the adoption of any healthy solution or of diverting money and resources from effective climate solutions. Huge energy requirements for some geoengineering plans translate into less energy that is climate-friendly for everyone else.
Lastly, a lot of geoengineering plans call for the extensive and intensive use of large tracts of land—BECCS, for example, is twice the size of India. Millions of people would unavoidably be displaced by those projects, and entire ecosystems might possibly be destroyed.
In summary, geoengineering methods are ineffective at addressing the underlying causes of climate change, and available data suggests that they are more likely to worsen the situation than to improve it—perhaps in a disastrous way.