Humans won't change their behaviors
Though people and governments have known about the negative consequences of climate change for decades, they seem to have been reluctant to significantly alter their behaviors in ways that will avert climate catastrophes. Because of that, it is unrealistic to assume that people will alter their behaviors quickly enough and dramatically enough to avert catastrophe. Therefore, we need to invest in geoengineering, which would allow us the time to reduce carbon emissions at a more measured and reasonable pace. Though countries have signed agreements on climate change, such as the Paris Climate Accord, global emissions are still on the rise. Significantly decreasing our emissions would require massive lifestyle changes, ranging from how we travel around a city, to the food we eat, and how we power our homes. It is simply not feasible to expect people to enact those changes in time, especially when considering global trends. Geoengineering would buy us a few decades at a reasonably inexpensive cost, which would allow a slower and more reasonably paced transition to clean energy. 
Geoengineering solutions provide people with an easy way out of climate change, so they could ultimately be detrimental to the goal of reducing carbon emissions and changing lifestyles. People are already slow to alter their behaviors in ways that could reduce the effects of climate change, and they are not alarmed about the potential consequences of climate change. Introducing geoengineering as a way to solve climate change would reduce the incentive to alter behavior, which is what really needs to happen to solve climate change.
Rejecting the premises